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Wednesday, January 20th 2016

1:25 pm:

The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without external magnetic field, which can be realized by time reversal symmetry breaking in a topologically non-trivial system. A QAH system carries spin-polarized dissipationless chiral edge transport channels without external energy input, hence may have huge impact on future electronic and spintronic device applications with ultralow-power consumption. The many decades’ quest for the experimental realization of QAH effect received a boost in 2006 with the discovery of topological insulators (TIs). In 2013, the QAH effect was observed in thin films of Cr-doped TI for the first time [1]. In 2015, a near ideal system in V-doped TI, contrary to the expectation of first principle calculations, was demonstrated to show extremely high-precision QAH quantization with more robust magnetization and higher Curie temperature [2]. In this talk, I will introduce the route to the experimental observation of the QAH effect in above-mentioned two systems [1,2], and discuss the zero magnetic field dissipationless edge mode and the origin of the dissipative channels in the high-precision QAH state [3]. Finally I will outline the prospects and the barriers of a viable QAH-based device.

References

[1] Cui-Zu Chang et al, Science 340, 167(2013).

[2] Cui-Zu Chang et al, Nature Materials 14, 473(2015).

[3] Cui-Zu Chang et al, Physics Review Letters 115, 057206 (2015).

Thursday, January 21st 2016

3:35 pm:

In most systems that exhibit order at low temperatures, the order occurs in the elementary degrees of freedom such as spin or charge. Prominent examples are magnetic or superconducting states of matter. In contrast, emergent order describes the phenomenon where composite objects such as higher order correlation functions exhibit longer range correlations. This can appear even though the elementary objects remain short-range ordered. One specific example are frustrated magnets, where long-range discrete order in the relative orientation of spins may occur in the absence of magnetic order. This can induce other phase transitions as is the case for the nematic transition in the iron pnictides. In my talk, I will focus on algebraic "critical" correlations of an emergent Z6 clock order parameter in an isotropic Heisenberg antiferromagnet on the windmill lattice, which consists of interpenetrating honeycomb and triangular lattices. This is surprising as the interaction of the Goldstone modes present in Heisenberg systems usually causes the spin-wave stiffness to renormalize to zero on long scales. Here it occurs due to the decoupling of an emergent collective degree of freedom given by the relative phase of spins on different sublattices. We establish this result and the formation of an extended critical phase at intermediate temperatures both using an analytical renormalization group analysis based on the Ricci flow and large-scale classical Monte-Carlo simulations. Our results also reveal that both phase transitions, which bracket the critical phase, lie in the Berezinskii-Kosterlitz-Thouless universality class.

Wednesday, January 27th 2016

1:25 pm:

Recently, there has been growing interest in electronic systems that exhibit both strong spin-orbit coupling and strong electron correlations. These systems combine two central threads of modern quantum materials research: correlated electron physics that underlies phenomena such as high-Tc superconductivity, and spin-orbit physics that describes systems such as topological insulators. When these two interactions compete at the same energy scale in a material, fundamentally new quantum phenomena have been predicted to emerge. Recently, an experimental platform with this rare interaction regime has been realized for the first time in 5d transition metal oxides such as iridium oxides. In this talk, I will focus on one specific member of this family Sr2IrO4 that has received much attention due its striking similarities to the cuprate high-Tc superconductors. I will describe a new nonlinear optical spectroscopy technique that enabled us to find an unusual hidden magnetic ordered phase in the Sr2IrO4 system. I will discuss the potential relevance of this novel phase to superconductivity in both iridates and cuprates.

Tuesday, February 2nd 2016

12:35 pm:

We present a controlled microscopic study of hole dynamics in both a gapped and a gapless spin liquid. Our approach is complementary to previous phenomenological works as we introduce mobile holes into the spin liquid ground state of the exactly solvable Kitaev honeycomb model. In the gapped phase of the model, we address the single-particle properties of individual holes (such as their particle statistics and hopping properties) and the multi-particle ground state at a finite density of holes. Our main result is that the holes possess internal degrees of freedom as they can bind the fractional excitations of the spin liquid and that the resulting composite holes with different excitations bound are distinct fractional particles with fundamentally different single-particle properties and different experimental signatures in the multi-particle ground state. In the gapless phase of the model, we consider a single hole and address the possibility of a coherent quasiparticle description by investigating its spectral function. We employ a variational treatment and also study a simplified one-dimensional problem to argue that a mobile hole has a finite quasiparticle weight which vanishes in the stationary limit.

Thursday, February 11th 2016

11:15 am:

Low dimensional systems, such as atomically thin materials and material interfaces, offer a rich ground to discover new types of electronic states. Spatially resolved electrical probes provide direct access to these states on the mesoscopic scale, complementing conventional transport techniques. In this talk, I will present two comprehensive studies on 2D electronic states employing Microwave Impedance Microscopy (MIM), a scanning probe technique that senses materials’ capacitance and conductivity on the nanoscal. The first study investigates magnetic domain walls in a unique all-in-all-out magnetic insulator, Nd2Ir2O7. Through a combined study of MIM, transport, and X-ray micro-diffraction, we conclusively show that metallic states emerge at the magnetic domain walls when the all-in-all-out magnetic order forms with a concomitant metal-insulator transition occurring in the bulk. This represents a new type of interface electronic states in a both chemically and structurally homogeneous material. The second study examines the canonical “edge state” picture of the quantum Hall (QH) effect in graphene. To our surprise, we find an unconventional edge-bulk correlation in graphene devices: the QH transport plateaus occur before the bulk Landau levels are completely filled. This result has implications in both QH transport analysis as well as understanding of general edge transport behaviors in other 2D topological systems.

Monday, February 15th 2016

1:25 pm:

A central theme of modern condensed matter physics is to discover and understand emergent phenomena in quantum materials. These phenomena emerge only through the collective behavior of electrons due to their mutual interactions as well as interactions with other degrees of freedom inside the materials. In this talk, I will give two examples of the emergent phenomena which I have been studying with angle-resolved photoemission spectroscopy in recent years. The first one is the negative electronic compressibility. I will present our evidence for the first experimental case of such phenomenon in a bulk material—the electron-doped iridate—and discuss the obtained insights into its microscopic origin, which heralds an uncharted territory of negative compressibility research that potentially features a whole variety of bulk quantum materials. The second example is the pseudogap phenomena. I will show how our view about the nature of these phenomena in hole-doped cuprate superconductors has evolved in the past two decades and been finally converging into one that they signal a phase of matter which is fundamentally distinct from superconductivity and characterized by multiple broken symmetries. I will discuss our results contributing to this development as well as their implications particularly regarding the broken translation symmetry aspect of the still-mysterious pseudogap phase.

Wednesday, February 17th 2016

1:25 pm:

Confined spins in semiconductors are a versatile platform for exploring quantum information processing and condensed matter physics. Individual spins can have coherence times exceeding seconds in some cases, making them promising quantum bits, or qubits, and also highly sensitive probes of their local electric and magnetic environments. I will discuss recent work exploiting the joint spin-state of two electrons in a GaAs double quantum dot as a “singlet-triplet” qubit. We perform high-fidelity single- and two-qubit gates with this architecture. We also use the qubit as a sensor to precisely measure its magnetic environment, which results from the statistically fluctuating nuclear spins in the semiconductor crystal. Using these measurement techniques, we extend the qubit coherence time by more than two orders of magnitude through adaptive control, and we uncover the surprisingly strong effect of spin-orbit coupling on electron-nuclear dynamics in GaAs.

Monday, February 22nd 2016

1:25 pm:

A handy way to tell what‘s inside a black box is to give it a good shake. Similarly it is often a good idea to bring a system far from equilibrium to understand its inner workings.

This talk mainly focuses on the results of ultrafast optical spectroscopy on Na2IrO3, a frustrated Mott insulator with strong spin-orbit coupling. Our results indicate that there is a distinct change in the non-equilibrium behavior of excitations as the system becomes magnetically ordered at low temperatures. Specifically, we observe that in the disordered phase the transient response is due to both bound “Hubbard excitons” and unpaired single particles, whereas in the ordered phase the single particle contribution becomes strongly suppressed. This is an indication of an increase of binding energy of Hubbard excitons which we argue is due to unique interplay between the strong frustrated Kitaev term and the weak Heisenberg-type ordering term in the Hamiltonian. In this regime magnetic ordering gives rise to an effective attraction between charged excitations which grows with distance causing them to become trapped within the excitons in analogy with quark confinement inside hadrons.

Wednesday, February 24th 2016

1:25 pm:

In cuprate high-temperature superconductors, an antiferromagnetic Mott insulating state can be destabilized toward unconventional superconductivity by either hole or electron doping. Besides these two phases, in hole-doped cuprates a periodic distribution of the electronic density, or charge order (CO), was recently detected in the Y-based family [1], and echoed the long-known presence of stripe order in the La-based cuprates [2]. However, at that point, the universality of the CO phenomenon to the cuprates remained to be determined.

In this talk I will first discuss a novel combined scanning tunneling microscopy (STM) and resonant X ray scattering (RXS) experimental approach that established the formation of CO in the high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212) [3]. The CO in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity, therefore establishing its commonality to hole-doped cuprates. Still, the universality of charge order remained in question, with several experiments and theories pointing to hole-doping as a critical ingredient to its formation.

Here I will also present RXS measurements that demonstrated for the first time the presence of charge order in the electron-type cuprate Nd2-xCexCuO4 (NCCO) [4]. A comprehensive study of CO in NCCO as a function of doping, temperature, and magnetic fields, shows that CO does not require a pseudogap precursor state. We also find that while CO is universal to all cuprates, its interplay with superconductivity and antiferromagnetism is not. Finally, open questions in the field, as well as prospects for future experiments, will also be discussed.

[1] G. Ghiringhelli, et al. Science 337, 821 (2012).

[2] J. M. Tranquada, et al. Nature 375, 561 (1995).

[3] E. H. da Silva Neto, et al. Science 343, 393 (2014).

[4] E. H. da Silva Neto, et al. Science 347, 282 (2015).

[5] E. H. da Silva Neto, et al. in preparation (2016).

Tuesday, March 1st 2016

3:35 pm:

"Motivated by the persistent doping problems plaguing the seminal topological "insulator" Bi_2Se_3, we have developed an apparatus capable of thin film growth combined with in situ, real time transport measurements, which has provided insight into gaining ready access to the topological regime. We have extended this success to studying the topological Dirac semimetal Na_3Bi, whose reactivity to ambient prohibits the use of conventional sample preparation techniques. Our thin film samples have low temperature mobilities in excess of 6,000 cm^2/Vs. Perpendicular magnetoresistance up to 1T shows unusually large quadratic behavior with weak anti-localization at low field. I will discuss our latest efforts to understand our results in terms of spatial charge inhomogeneity."

Wednesday, March 2nd 2016

1:25 pm:

Wednesday, March 9th 2016

1:25 pm:

Intriguing phase behaviors of correlated electrons have been a subject of major research interest over the past half a century, and they are all realized in a framework in which the electrons constantly interact with their hosting crystal lattice. As a result, not only does electrons’ footprint on the lattice help us on the detection of various electronic phases, but the explicit consideration of electron-phonon interactions may be necessary in order for us to thoroughly understand the collective behavior of the electrons.

In this talk, I will present a series of studies aiming to elucidate the role of electron-phonon interactions in electron correlations, using Raman scattering as the primary tool in conjunction with complementary techniques. I will first discuss how electron-phonon interactions, primarily in the form of dynamic magnetoelastic coupling, can be utilized to elucidate the genuine phase boundary and physical origin of the electronic nematic phase in iron-based superconductors. Then I will take a detour to a conventional charge-density-wave material and show that the momentum dependence of electron-phonon interactions is crucial for a quantitative understanding of charge ordering behavior even in the simplest case. This brings up the idea whether it is possible to enhance the interactions between conduction electrons and phonons in particularly important momentum regions, if the material also possesses local-moment magnetism on the back side of the same coin from the Fermi-liquid quasiparticles. Indeed we have found that in a structurally simple spiral magnet, the interaction between phonons and local-moment magnetism gives rise to hybrid magnetoelastic excitations, or “magnetophonons”, at the dispersion intersections between phonons and magnons. How such emerging excitations might affect conduction electrons or even mediate Cooper pairing in unconventional superconductors will be discussed.

Thursday, March 10th 2016

2:00 pm:

After an introduction, in the first part of the talk, I discuss a new fermionic functional renormalization group variant for two-dimensional lattice models that combines a physically appealing truncation of the wavevector dependence of the running interactions with certain numerical advantages. This allows for an efficient parallelized evaluation in which, e.g., the convergence of the expansion can be checked. In the second part of talk, I present new results on a model for iron superconductors that show that nematic orbital ordering occurs in different forms as competitor but also concomitant ordering tendency to the more conventional antiferromagnetic ordering and spin-fluctuation-induced pairing at low energies.

Wednesday, March 16th 2016

1:25 pm:

Wednesday, March 23rd 2016

1:25 pm:

We consider the two-dimensional electron gas confined laterally to a narrow channel. As the Zeeman splitting matches the inter-subband splitting due to the geometrical quantization, the non-local spin polarization develops a minimum as reported by Frolov et al. [Nature (London) 458, 868 (2009)]. This phenomenon termed Ballistic Spin Resonance is due to the degeneracy between the nearest oppositely polarized sub-bands lifted by spin-orbit coupling. The resonance survives the weak and short-range interaction. The latter detunes it and as a result shifts the Zeeman splitting at which the minimum in spin polarization occurs. The shift is attributed to the absence of Kohn theorem for the spin sloshing collective mode. We characterized the shift due to weak interaction quantitatively by analyzing the spin sloshing mode phenomenologically.

Wednesday, March 30th 2016

1:25 pm:

I will discuss scanning tunneling microscopy (STM) measurements of electronic nematicity in the iron pnictide superconductors, focusing on the compound NaFeAs. I will show that a clear signal of the electronic nematicity can be seen in the local density of states as measured by STM. Questions that can be answered from the STM measurements include (a) do magnetic or structural degrees of freedom drive nematicity (b) how can we distinguish between true long range order and a strong susceptibility using STM measurements (c) where in the phase diagram are nematic order and fluctuations observed (d) what is the energy scale associated with nematicity at different points in the phase diagram (e) what is the relationship between nematicity (either order or fluctuations) and superconductivity. I will discuss answers to all of these questions, and along the way discuss new STM techniques that we have developed for this purpose that are broadly applicable to other quantum materials.

Wednesday, April 6th 2016

1:25 pm:

In the thirty years since the discovery of quasicrystals by Dan Shechtman there has been tremendous progress in our understanding of the structure of quasicrystals and aperiodic systems in general. Indeed, the question first asked by Per Bak soon after Shechtman’s discovery, “Where are the atoms?”, can largely be answered for at least one class of quasicrystals, the i-YbCd5.7 icosahedral phase. Progress in our understanding of the consequences of aperiodicity for physical phenomena such as the electronic, magnetic, and optical properties has lagged somewhat but we have recently seen a surge of activity and new results. On the magnetism front, the discovery of a new family of magnetic quasicrystals and their closely related crystalline approximants, has allowed for a direct comparison of the impact of aperiodicity on magnetic interactions in compounds that have similar local structures.

Examples of stable binary icosahedral quasicrystals are quite rare and, before the discovery of icosahedral quasicrystals in the i-R-Cd system (R = Gd to Tm, Y), there were no known examples that featured localized magnetic moments. Local-moment-bearing binary quasicrystals represent the compositionally simplest system for the study of magnetic interactions in aperiodic compounds and, therefore, the new R-Cd quasicrystal family will play a key role in these studies, offering non-magnetic, Y, Heisenberg-like, Gd, and non-Heisenberg (CEF split) Tb to Tm members, in addition to the structural and compositional simplicity of a binary phase. Furthermore, the existence of a corresponding set of cubic approximants, RCd6, to the icosahedral phase allows for direct comparisons between the low- temperature magnetic states of crystalline and quasicrystalline phases with fundamentally similar local structures. RCd6 may be described as a body-centered cubic packing of the same clusters of atoms as found in the newly discovered icosahedral phase. Using x-ray resonant magnetic scattering we have shown that the RCd6 approximants manifest long-range magnetic order at low temperatures, whereas the related icosahedral phase exhibits only spin-glass-like freezing at low temperatures. In order to understand the reason for the absence of long-range magnetic ordering in the quasicrystalline phase, we have recently completed a full structural refinement of the i-R-Cd system (R = Gd, Dy) series and are in the midst of both elastic and inelastic neutron scattering measurements of i-Tb-Cd. Our results, to date, will be described and discussed.

Tuesday, April 12th 2016

3:00 pm:

Superconducting circuits offer a compelling platform for investigating light-matter interactions inaccessible to conventional atomic systems. Aided by the low-dimensionality and low loss of such circuits, here we experimentally investigate how changing the properties of the electromagnetic vacuum modifies atomic fluorescence. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution using a near-quantum-limited parametric amplifier. We observe a dramatic dependence of the spectrum of resonance fluorescence on the relative phase of the driving and squeezed vacuum fields and observe subnatural radiative linewidths that indicate up to 3.1 dB of squeezing below the ordinary vacuum level in the artificial atom’s environment [1]. Our results both validate the canonical predictions for resonance fluorescence in squeezed vacuum and provide a circuit architecture for investigating high-precision superconducting qubit measurement with squeezed input light.

[1] D.M. Toyli, A.W Eddins, et al., arXiv:1602.03240

Wednesday, April 13th 2016

1:25 pm:

Two key symmetries underlie the formation of superconductivity: parity and time reversal. These symmetries lead to the usual description of superconductivity in terms of pseudo-spin 1/2 fermions. Strong spin-orbit coupling plays an essential role in defining pseudo-spin and, we show, also allows for the intriguing possibility of pseudo-spin 3/2 fermions. In this talk, after a discussion on the role of symmetry on superconducting pairs, I will contrast the physics for pseudo-spin 1/2 and pseudo-spin 3/2 pairing with an emphasis on the topological half-heusler superconductors YPtBi and LuPtBi. These materials have recently been found to exhibit gapless excitations in the superconducting state. These materials also lack parity symmetry, and the general consequences of this for both pseudo-spin 1/2 and pseudo-spin 3/2 pairing will be highlighted throughout the talk.

Thursday, April 14th 2016

2:00 pm:

Majorana fermions (non-Abelian anyons) appear as edge states in the topological phase of the Kitaev wire, and are naturally immune to static disorder. I discuss the effects of a classical noise ("time-dependent disorder") on the Majorana edge correlations, and on the fidelity of braiding operations for both global and local noise in the chemical potential. While in general noise will induce heating and dephasing, it is still possible to have long-lived quantum correlations in the presence of fast noise due to motional narrowing, even when the noise drives the system rapidly between the topological and non-topological phases.

Wednesday, April 20th 2016

1:25 pm:

Wednesday, April 27th 2016

1:25 pm:

Wednesday, May 4th 2016

1:25 pm:

Wednesday, May 25th 2016

1:25 pm:

I will explore electronic band structure of various group III-V semiconductor nanowires using electrical transport and photocurrent spectroscopy on photo-lithographically contacted single nanowire devices. The nanowires were grown by MOCVD technique where the growth was initiated by a 50-100 nm gold catalyst which determines the diameter of the nanowire. Typically these nanowires are 4-6 µm long. Further, I will introduce a more complex GaAs/AlGaAs core-multishell heterosturucres, quantum well tube (QWT), where a thin GaAs layer was embedded inside a thick AlGaAs shell surrounding a 50 nm diameter GaAs nanowire core. Here I will present the nature of exciton localization in single GaAs/AlGaAs QWT nanowire devices using photocurrent (PC) spectroscopy combined with simultaneous photoluminescence (PL) and photoluminescence excitation (PLE) measurements at 10 K . Excitons confined to GaAs quantum well tubes of 8 nm and 4 nm widths embedded into an AlGaAs barrier are seen to ionize at high bias levels. Spectroscopic signatures of the ground and excited states confined to the QWT seen in PL, PLE, and PC data are consistent with numerical calculations. The demonstration of good electrical contact with the QWTs enables the study of Stark effect shifts in sharp emission lines of excitons localized to quantum dot-like states within the QWT. Atomic resolution cross-sectional TEM measurements and an analysis of the quantum confined Stark effect of these dots provide insights into the nature of the exciton localization in these nanostructures.

Wednesday, June 8th 2016

1:25 pm:

Wednesday, August 24th 2016

1:30 pm:

Recently, strain engineering of graphene’s electronic properties has attracted significant attention [1]. I will report transport studies in graphene sheets with linearly-shaped strain regions created by nm-scale wide folds. We find that these strain regions act as quantum wires and waveguides. We attribute this to strain-induced pseudomagnetic fields acting as confining barriers. I will also discuss transport studies on coupled massive and massless electron systems, realized using twisted monolayer graphene/natural bilayer graphene stacks. Due to the interlayer screening, we observe a nonlinear monolayer gate capacitance. Moreover, in a perpendicular magnetic field, we observe a distinct pattern of gate-tunable Landau level crossings that enable the mass and Fermi velocity in the layers to be determined. We find different values than those of isolated layers, indicating that the interlayer interactions renormalize the band structure parameters. Additionally, novel physics in graphene under different external conditions, for example under one-dimensional (1D) periodic potentials [2] and aligned to boron nitride (BN) substrates, will also be reported. Our recent studies on natural few-layer graphene, including quantum Hall effect, Landau level transitions and new Dirac points [3] in ABA-stacked trilayer graphene will also be introduced.

**References:**

[1] Nikolai N. Klimov, Suyong Jung, Shuze Zhu, Teng Li, C. Alan Wright, Santiago D. Solares, David B. Newell, Nikolai B. Zhitenev, and Joseph A. Stroscio. Electromechanical properties of graphene drumheads. Science, **336**(6088):1557–1561, 2012.

[2] Cheol-Hwan Park, Young-Woo Son, Li Yang, Marvin L. Cohen, and Steven G. Louie. Electron beam supercollimation in graphene superlattices. Nano letters, **8**(9):2920–2924, 2008.

[3] Maksym Serbyn and Dmitry A. Abanin. New dirac points and multiple Landau level crossings in biased trilayer graphene. Physical Review B,

**87**(11):115422, 2013.

Wednesday, September 7th 2016

10:00 am:

We are all familiar with spontaneously broken symmetry: the north pole of a magnetic needle will be at one end or the other, not a superposition of the two. Conversely, can a system possess at low energies a symmetry that the bare Hamiltonian does not? I'll discuss a case where such a symmetry emerges in measurements of a nanopatterned system of electrons: a double quantum dot (Ref. 1).

Then I will tell how a similar system with no obvious symmetry can be tuned using voltage on nanoelectrodes to a quantum critical point with an exact theoretical description even at finite temperature (Ref. 2). The excitations of nearby mobile electrons at the critical point are collective and look nothing like individual electrons: this is a non-Fermi liquid. Tuning across the critical point, the crossover from one phase to the other through the quantum critical region turns out to have surprising universal properties (Ref. 3).

I hope that this approach to many-body systems and quantum phase transitions -- engineering and building an artificial realization of a well-defined Hamiltonian, then probing its properties experimentally -- will drive theoretical and computational efforts, and ultimately will help us understand the richness of electronic materials such as heavy fermion metals.

1. A. J. Keller, S. Amasha, I. Weymann, C. P. Moca, I. G. Rau, J. A. Katine, Hadas Shtrikman, G. Zaránd and D. Goldhaber-Gordon, "Emergent SU(4) Kondo physics in a spin-charge-entangled double quantum dot" Nature Physics 10, 145 (2014).

2. R. M. Potok, I. G. Rau, H. Shtrikman, Y. Oreg, and D. Goldhaber-Gordon, "Observation of the two-channel Kondo effect" , Nature 446, 167-171 (2007).

3. A. J. Keller, L. Peeters, C. P. Moca, I. Weymann, D. Mahalu, V. Umansky, G. Zaránd & D. Goldhaber-Gordon, "Universal Fermi liquid crossover and quantum criticality in a mesoscopic system," Nature 526, 237–240 ( 2015).

2:30 pm:

It is strongly believed that the quantum Hall fluid at filling factor \nu = ½ realizes the composite fermion liquid - an exotic phase of matter with emergent excitations - composite fermions - forming a Fermi-surface. The original theory of this state proposed by Halperin, Lee and Read (HLR) in 1993 is well supported by experiments. However, one aspect of HLR theory has continued to puzzle theorists for the past 20 years: an apparent lack of particle-hole symmetry expected to emerge in the lowest Landau level. Recently, D. Son has conjectured a surprising resolution of this puzzle, proposing that the composite fermion is a Dirac fermion. I will give a derivation of this conjecture by making a connection between the physics of a half-filled Landau level and the surface of a 3d topological insulator (TI). The derivation will proceed via a dual theory of the single Dirac cone on the TI surface, given by quantum electrodynamics (QED3) with a single dual Dirac fermion coupled to a fluctuating gauge field. Finally, I will present smoking-gun numerical evidence for the Dirac nature of the composite fermion coming from DMRG simulations of quantum Hall fluid at \nu = ½.

Wednesday, September 14th 2016

2:30 pm:

Symmetry protected topological (SPT) phases are generalizations of topological band insulators; they are quantum phases of matter with a bulk energy gap and characteristic edge or surface properties. Over the past few years, exciting progress has been made in the theory of SPT phases with strong interactions, and, separately, SPT phases with crystalline symmetry. The intersection of these two directions — strongly interacting crystalline SPT phases — has potential experimental relevance but remains rather poorly understood. In this talk, I will present a general framework to classify and characterize SPT phases protected by crystalline point group symmetry. The basic insight is that all such SPT phases can be reduced to lower-dimensional topological phases with internal symmetry. This leads to a physically transparent approach that is generally applicable to bosonic or fermionic systems in any spatial dimension, without regard to the strength of interactions. I will illustrate the key ideas via discussion of a few interesting examples.

Wednesday, September 21st 2016

2:30 pm:

Recently, strongly-correlated transition metal oxides, that exhibit first order metal-insulator transitions, have received renewed attention because they can be controlled and manipulated at the nanoscale to develop unique properties. Although the existence of these materials has been known for some time, the physical origin of many of these phenomena remains a very controversial issue. In addition, hybrid heterostructures allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. By properly designing hybrid ferromagnet/oxides new magnetic properties arise unlike any known magnetic materials. I will describe the static and dynamical properties of strongly correlated nanostructured oxides, which exhibit metal-insulator transitions. These materials when reduced to the nanoscale exhibit interesting properties such as avalanches, unique response to disorder and critical slowing down in their fast time dependence. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization and dynamical properties are clearly modified by the proximity effect and give raise to interesting perhaps useful properties. The oxide work supported by the US-AFOSR and the magnetism aspects by the US-DOE. Work done in collaboration with many young researchers who will be individually credited.

Wednesday, September 28th 2016

2:30 pm:

The revolution started by the discovery of topological insulators a few years ago has turned out to be the proverbial tip of the much larger iceberg of exotic phases of quantum matter driven by spin-orbit coupling effects. Consideration of electronic states protected by time-reversal, crystalline and particle-hole symmetries has led to the prediction of many novel materials, which can support Weyl, Dirac and Majorana fermions, and to new types of insulators such as topological crystalline insulators and topological Kondo insulators, as well as quantum spin Hall insulators with large band gaps capable of surviving room temperature thermal excitations. [1] I will discuss our recent theoretical work aimed at predicting topological materials and identify cases where robust experimental evidence has been obtained toward their successful materials realization. [2-10] I will also comment on potential of topological materials as next generation platforms for manipulating spin and charge transport and other applications.

[1] Bansil, Lin and Das, Reviews of Modern Physics 88, 021004 (2016).

[2] Chang et al, Science Advances 2, e1600295 (2016).

[3] Huang et al., Proc. National Academy of Sciences 113, 1180 (2016).

[4] Zheng et al., ACS Nano 10, 1378 (2016).

[5] Xu et al., Science 349, 613 (2015).

[6] Zeljkovic et al., Nature Materials 14, 318 (2015).

[7] He et al., Nature Materials 14, 577 (2015).

[8] Xu et al., Nature Physics 11, 748 (2015).

[9] Crisostomo et al., Nano Letters 15, 6568 (2015).

[10] Xu et al., Science Advances 1, e1501092 (2015).

Wednesday, October 5th 2016

2:30 pm:

In several two-dimensional films that exhibit a magnetic field-tuned superconductor to insulator transition (SIT), stable metallic phases have been observed. An influential theory of the SIT involves disorderd bosons (Cooper pairs and vortices) in a magnetic field. Building on this `dirty boson' description of the SIT, we suggest that the observed metallic behavior near the SIT is analogous to the composite Fermi liquid observed about half-filled Landau levels of the two-dimensional electron gas. The composite fermions here represent composites of vortices and Cooper pairs. We describe several experimental consequences stemming from these fermionized vortices.

Wednesday, October 12th 2016

2:30 pm:

Wednesday, October 19th 2016

2:30 pm:

In a system of free electrons, both the Coulomb repulsion and quantum kinetic energies diminish as the electron density is decreased. Since the kinetic energy diminishes faster than the Coulomb energy, it becomes energetically favorable for electrons to localize into a crystal known as a “Wigner Crystal". In the case of 2D systems, applying a quantizing magnetic field favors crystal formation by further freezing out the kinetic energy into Landau levels. Theory predicts that a Wigner crystal of quasiparticles in a Landau level exists near integer quantum Hall states as an insulating phase with an expected transition temperature in the range of a few hundred millikelvin or below. As the state in insulating, it is very difficult to probe it. Using a refined pulsed tunneling method, capable of probing insulating phases, we are able to measure tunneling current directly into the electronic crystal. I will present high-resolution tunneling measurements that reveal very sharp structure arising from the vibrational spectrum of the spatially ordered electronic structure. This observation conclusively demonstrates the existence of a Wigner Crystal with long correlation length and opens the door to using tunneling to probe and detect a wide variety of ordered electronic phases. Finally, I will also show results from a related tunneling method that we have developed that permits quantitative determination of spectral function of the 2D electron system as a function of both energy and momentum.

Wednesday, October 26th 2016

2:30 pm:

Conventional electronic transport methods have been tremendously powerful in exploring systems with low lying charge excitations. In recent years there is growing interest in similar transport properties but of the spin degree of freedom, however, an analogous set of tools for spin transport currently does not exist. A grand challenge is therefore to develop spin analogues to voltage and current sources and meters which will allow us to explore both stationary and transient spin transport phenomena in a broad range of quantum materials. In this talk I will report on some of our efforts to develop such measurement capabilities using NV centers in diamond. Specifically I will report on recent measurements we have performed that explore magnetic textures in thin magnetic films and the spin chemical potential in magnetic insulators.

Wednesday, November 2nd 2016

2:30 pm:

I will discuss quantum order-by-disorder effect and will present an evidence that the non-linear terms in the anisotropic kagome-lattice antiferromagnets can yield a rare example of the ground state that is different from the one favored by thermal fluctuations. The corresponding order selection will be shown to be generated by the topologically non-trivial tunneling processes, yielding a new energy scale in the system.

I will also discuss the effect of the non-linear terms in the spectra of the kagome-lattice systems and will provide an analysis of the spectral properties of realistic kagome-lattice antiferromagnets such as Fe-jarosite, for which a remarkable wipe-out effect for a significant portion of the spectrum should exist due to a resonant-like decay processes involving two flat modes.

Recent result concerning the spectrum of the kagome-lattice ferromagnets will also be presented.

Wednesday, November 9th 2016

2:30 pm:

I will show that Mott insulators with strong spin-orbit coupling and bond-dependent interactions host yet another strongly correlated regime, besides the well-known Kitaev quantum spin liquid [1]. This regime is governed by a classical spin liquid instability and unconventional spin-spin correlations along closed or open strings. The key predictions are common for all available 2D and 3D tri-coordinated materials with bond-directional anisotropy, and provide a consistent interpretation of the suppression of the x-ray magnetic circular dichroism signal reported recently [2] in β-Li2IrO3 under pressure.

[1] I. Rousochatzakis and N. B. Perkins, arXiv:1610.08463v1

https://arxiv.org/abs/1610.08463

[2] T. Takayama et al., Phys. Rev. Lett. 114, 077202 (2015).

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.077202

Wednesday, November 16th 2016

2:30 pm:

The Hubbard model is one of the fundamental models of correlated electron physics. In two dimensions it exhibits various superconducting, Mott insulating, magnetically ordered, pseudogapped, and metallic phases. On a square lattice and for repulsive interactions comparable to the bandwidth, the model is believed to capture much of the low-energy physics of the cuprate high-temperature superconductors, while in three dimensions it can be realized experimentally in ultracold atomic gases. In this talk I will show results from `numerical experiment’ on this model and compare them to experimentally obtained response functions and theoretical calculations, illustrating how simulations can be used to further our understanding of correlated electron systems.

Wednesday, November 23rd 2016

2:30 pm:

Monday, November 28th 2016

12:30 pm:

We consider superconductivity suppression in homogeneously disordered thin films. Anderson’s theorem stating that the critical temperature is insensitive to the degree of disorder is violated in the vicinity of the Anderson localization transition. For strongly disordered films, the interplay between disorder and interaction effectively suppresses the BCS coupling constant, thereby reducing the critical temperature. For strictly 2D films, superconductivity suppression is coming from large scales (similar to the 2D localization), and summation of the leading logarithms can be performed with the help of Finkelstein’s renormalization group. For thicker and sufficiently dirty films, there exists an additional effect originating from small scales (similar to the 3D localization). We calculate the corresponding contribution to the shift of the critical temperature and discuss its importance in the context of experimental situation.

Wednesday, November 30th 2016

Recently, epitaxially connected at facets semiconductor

nanocrystals have been introduced to fascilitate the electron transport

between nanocrystals. To fully deploy their potential a better understanding

of the exciton transfer between connected NCs is needed. We go beyond the two

well-known transfer mechanisms suggested by Forster and Dexter and propose a

third mechanism of exciton tandem tunneling. The tandem tunnelling occurs

through the intermediate state in which electron and hole are in different

NCs. The corresponding rate for exciton hops between epitaxially connected

at small facets NCs is larger than the Dexter rate and can be comparable with

the Forster one.

Wednesday, December 7th 2016

2:30 pm:

Chiral-spin modes in a 2D Fermi-Liquid with spin-orbit coupling(SOC) are oscillations of magnetization in zero magnetic field resulting from a many-body effect. To study them, we develop a scheme to incorporate many-body effects in SOC systems. As a result, we discover a number of interesting features associated with SOC (of Rashba and Dresselahaus type). Since collective modes usually dominate the response to any probe they couple to, it is necessary to correctly formulate a theory that can express these modes in terms of the material parameters. I will show how our scheme does precisely this and highlight some unique features introduced by SOC, namely, a linear-in-q dispersion of a massive collective mode in an externally applied in-plane magnetic field, and a characteristic anisotropy of the mass with rotation of the in-plane field. Our interpretation is consistent with several observed features in a series of Raman experiments in CdMnTe quantum well (which were earlier interpreted as an indication of a strong renormalization of SOC by electron-electron interaction). We also offer predictions for other observables with/without the externally applied field.

Wednesday, December 14th 2016

Quantum vortices in weakly coupled superfluids have a large healing length, so that many particles reside within the vortex core. They are characterized by topologically protected singular points, which in principal should keep their core structure rigid. I will describe how, in practice, the point singularity of a vortex deforms into a line singularity, in proportion with the Magnus force experienced by the vortex. The vortex structure is described by weak solutions of the Gross-Pitaevskii equation, similar to shock waves in hydrodynamics. I will discuss how the core deformation significantly affects many aspects of vortex dynamics. A striking example I will describe is the instability of the Abrikosov vortex lattice in the weak-coupling limit. All vortex singularities in the lattice spontaneously deform into finite cuts, which then order into superstructures.

Thursday, December 15th 2016

4:00 pm:

We analyze the superconducting instabilities in the vicinity of the quantum-critical point of an inversion symmetry breaking order. We first show that the fluctuations of the inversion symmetry breaking order lead to two degenerate superconducting (SC) instabilities, one in the s-wave channel, and the other in a time-reversal invariant odd-parity pairing channel (the simplest case being the same as the of 3He-B phase). Remarkably, we find that unlike many well-known examples, the selection of the pairing symmetry of the condensate is independent of the momentum-space structure of the collective mode that mediates the pairing interaction. We found that this degeneracy is a result of the existence of a conserved fermionic helicity, χ, and the two degenerate channels correspond to even and odd combinations of SC order parameters with χ = ±1. As a result, the system has an enlarged symmetry U(1) × U(1), with each U(1) corresponding to one value of the helicity χ. We discuss how the enlarged symmetry can be lifted by small perturbations, such as the Coulomb interaction or Fermi surface splitting in the presence of broken inversion symmetry, and we show that the resulting superconducting state can be topological or trivial depending on parameters. We present a global phase diagram of the superconducting states and discuss possible experimental implications.

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