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Wednesday, January 18th 2017

1:25 pm:

More than a century after the discovery of dissipationless charge transport in mercury below 4.2K, the search for room-temperature superconductivity continues. Record-high transition temperatures above 200K have been achieved but only under the extreme conditions of ultra-high pressure or, possibly, intense transient light fields. The leading candidate for realizing ambient superconductivity have remained strong electronic correlations, an established ingredient in unconventional high-temperature copper-based superconductivity. Iron-based compounds have emerged as entirely distinct species, in which superconductivity can be refined to a single atomic layer and enhanced to occur at 65K by interfacial interactions. Here, we will consider the influence of many-body effects on the low-energy charge transport and band structure in iron-based superconductors and parent compounds in order to shed light onto the microscopic mechanism of superconducting pairing in this novel class of materials.

Wednesday, January 25th 2017

1:25 pm:

In a normal magnet strong interactions tend to align the spins in a periodic fashion forming a ferromagnet or an antiferromagnet. However, sometimes when the interactions compete with each other, the spins cannot order into a single low-energy ground state and keep on fluctuating, resulting in highly degenerate ground states with very interesting emergent quantum behavior. Such a ground state is the Kitaev quantum spin liquid (KQSL) which occurs in a honeycomb semiconductor and predicts the emergence of Majorana Fermions and non-abelian anyons that can be used for topological quantum computation. It was proposed that certain d5 materials with a strong octahedral crystal field and a strong spin-orbit coupling in a low-spin ground state can realize a KQSL. In this talk, I will describe the synthesis, ground-state properties and the excitation spectrum of a graphene-like honeycomb magnet a-RuCl3 which complies with the requirements. Using neutron scattering we show that, despite a low-temperature long-range order, the excitation spectrum contains an unusual broad feature matching the predictions of Majorana Fermions arising from strong Kitaev interactions. Detailed theoretical analysis allows us to compare our spectrum with Kitaev exact solutions as well as extensions based on mean-field approximations. Finally, we will talk about our recent endeavors to extend this work using doping, thin-films and application of magnetic field. The later, most interestingly, suppresses the long-range order hopefully leading to a true spin-liquid state.

Wednesday, February 1st 2017

1:25 pm:

The wave functions of electronic states in solids typically respect the symmetries of the host material. However, interactions among electrons can give rise to a variety of exotic correlated phases characterized by broken symmetry. An intriguing example is the formation of electron fluids with wave functions that spontaneously break the symmetry of the underlying crystal lattice. These phases are quantum analogues of classical liquid crystals and have been studied in recent years across disparate platforms ranging from high-temperature superconductors to two-dimensional electron systems. In this talk, I will describe scanning tunneling microscope measurements that allow us to visualize such a quantum liquid in real space. We examine the quantum Hall states that arise at high magnetic field from anisotropic Fermi pockets on the surface of bismuth. Our measurements reveal that a combination of strain and exchange interactions lift the Landau level degeneracy and produce valley-polarized states. We image the resulting anisotropic wave functions and find that they have a different orientation for each broken-symmetry state, providing a direct spatial signature of a nematic electronic phase.

Wednesday, February 8th 2017

1:25 pm:

Superconducting artificial atoms are created by connecting Josephson junctions, which are nonlinear, non-dissipative elements, to simple electrical circuits. Individual artificial atoms can be coupled using this same toolbox of inductors, capacitors, and Josephson junctions to build novel quantum materials. In this talk, I will discuss prospects for using the fluxonium artificial atom as a building block for topological materials. Topological phases of matter have excitations with exotic quantum statistics and have been proposed as a platform for robust quantum computation. Building a topological material from the bottom-up, however, requires individual components with degenerate ground states and strong coupling between these components.

I will describe two circuits based on the fluxonium artificial atom that meet these requirements. The first circuit is an artificial molecule composed of two strongly-coupled fluxonium artificial atoms, which realizes a Hamiltonian with a dominant σ_{z} σ_{z}-type interaction between the individual atoms [1]. We find excellent agreement between the measured spectroscopy of the circuit and the theoretically-predicted level transitions, which highlights the suitability of superconducting circuits for implementing tailored quantum systems. Instead of the cosφ energy term characteristic of a Josephson junction, the second circuit realizes an unconventional cos2φ energy term, which results in a nearly-degenerate ground-excited state manifold. Taken together, these circuits fulfill the requirements for the building blocks of topological phases and we can thus start to realize topological materials using superconducting circuits.

[1] A. Kou, et al., arxiv:1610.01094 (2016).

Wednesday, February 15th 2017

1:25 pm:

The appearance of quantum mechanics on the macroscopic scale underlies some of the most fascinating phenomena in condensed matter physics: topological states arising from Berry phases in electronic bands, and superconductivity from many-body entanglement. Recently, advances in the control and coherence of microscopic quantum systems have enabled these fundamental quantum effects to be studied at the single or few-particle level, as well as be leveraged towards new paradigms for information processing. In this talk, I will focus on experiments using one such system, an atomic-scale defect in diamond known as the nitrogen-vacancy (NV) center. Behaving as a solid-state ‘trapped atom’, the NV center possesses spin and orbital levels, isolated within the host semiconductor’s bandgap, that can be readily manipulated with optical and microwave fields. Through resonant optical excitation, we demonstrate that the NV center spin can be adiabatically evolved to point along arbitrary loops and acquire a Berry phase after the cycle. Such precise quantum control sheds insights on the robustness of geometric phase. Moreover, we accelerate adiabatic dynamics and minimize decoherence by using engineered optical pulses to construct ‘shortcuts’ that absorb deleterious transitions. Finally, I will overview how combining optical and electrical techniques at the atomic level can contribute to applications of material systems in computing, energy, and sensing.

Wednesday, February 22nd 2017

1:25 pm:

Since the discovery of graphene via mechanical exfoliation, it has been shown that the electronic properties of solids can undergo dramatic change when the material thickness is reduced to the atomic limit. Recently, the quality of these 2-dimensional (2D) electronic systems has been significantly improved by hexagonal boron nitrides encapsulation, enabling the electron mean free path only limited by the size of the samples. However, mesoscopic transport studies in these systems are relatively unexplored due to the challenges in the device fabrication processes. Here we develop a robust procedure for making gated-defined nanostructures in 2D van der Waals materials without compromising their intrinsic 2DEG quality, providing versatile experimental platforms to explore various novel quantum phenomena in these systems. By confining and manipulating charge carriers [1][2], we demonstrate relativistic electron-optics, resonant quantum Hall (QH) tunneling spectroscopy, tunable optical trion lifetime and quantized mesoscopic transport in graphene and transition metal dichalcogenides [2]. Our results bode well for addressing many key problems in condensed matter physics, including Luttinger physics, high fidelity logic gates in Loss-DiVincenzo qubits, gate-controlled quantum optics, measurement of small fractional QH energy gaps, and functional quantum devices based on pseudospin manipulation and electron optics.

[1] Wang, et al, Phys. Rev. Lett. 111, 046801 (2013).

[2] Wang, et al, ArXiv:1610.02929 (2016).

Wednesday, March 1st 2017

1:25 pm:

Wednesday, March 8th 2017

1:25 pm:

I consider some of the ways in which the practice and even the definition of "condensed-matter physics" has evolved since its inception in the early twentieth century, with particular reference to its relationship to neighboring and even distant disciplines. I speculate on some possible directions in which the discipline may develop over the next few decades, emphasizing that there are still some very basic questions to which we currently have no satisfactory answers.

Wednesday, March 22nd 2017

1:25 pm:

An interesting topic in the theory of quantum phase transitions is that of the transition between a classically ordered state, such as a Néel state, and a quantum ordered state, such as a valence bond crystal. On the classically ordered side of the transition, we have a well developed understanding based on the spontaneous breaking of a continuous symmetry and the occurrence of associated Goldstone modes (spin waves). On the quantum ordered side, things are rather less clear. Even the description of the quantum ordered state itself is not straightforward, since it is not a saddle-point of the usual spin-coherent-state path integral.

In this talk, I shall present an approach designed to address this problem by constructing the path integral over matrix product states rather than spin coherent states. This allows both classically ordered states (which are matrix product states of bond dimension 1) and quantum ordered states (which are matrix product states of higher bond dimension) to be captured on an equal footing. I shall use this approach to show that, for at least one example of such a transition, a Landau-Ginzburg-Wilson-type description can be given, where the ‘order parameter’ is a field representing the nearest-neighbour entanglement of the spins.

The talk is designed to be entirely self-contained: in particular, no prior knowledge of matrix product states or quantum ordered phases is assumed. The work about which I shall speak was undertaken in collaboration with Andrew Green (University College London), Jonathan Keeling (St Andrews), and Steve Simon (Oxford), under the auspices of the TOPNES programme. Most of the details can be found at https://arxiv.org/abs/1607.01778.

Wednesday, March 29th 2017

1:30 pm:

Wednesday, April 5th 2017

1:30 pm:

Wednesday, April 12th 2017

1:25 pm:

In selectively-doped semiconductor structures, the electrons are spatially separated from the dopant atoms to reduce scattering by the ionized impurities. Thanks to the reduced disorder and scattering, such “clean” structures provide nearly ideal 2D systems for studies of electron-electron interaction phenomena, especially at low temperatures and high perpendicular magnetic fields where the thermal and kinetic energies of the electrons are quenched. The dominant electron interaction leads to various fascinating and exotic ground states such as the fractional quantum Hall state, Wigner crystal, and anisotropic (stripe) phases.

In my talk I’ll discuss our latest results in probing the intriguing properties of some of these phases. For example, in high-quality GaAs 2D electrons, a stripe phase is observed in the excited (N = 1) Landau level when a parallel magnetic field (B||) is applied. The stripes are typically oriented perpendicular to the B|| direction. Our experimental data reveals how a periodic density modulation, induced by a surface strain grating from strips of negative electron-beam resist, competes against the B||-induced orientational order of the stripe phase. Even a minute (≪ 1%) imposed density modulation is sufficient to reorient the stripes along the direction of the surface grating, if its period matches the (expected) period of the intrinsic stripes. The data thus suggest that the parallel and perpendicular orientations of the stripes must be energetically very close. I will also present experimental data on other 2D systems, e.g., 2D holes in GaAs or 2D electrons confined to AlAs quantum wells.

Wednesday, April 19th 2017

1:25 pm:

Wednesday, April 26th 2017

1:25 pm:

The process of materials design involves solving the inverse band structure problem to predict what stoichiometry and crystal structure give rise to desired properties. First principles methods, such as Density Functional Theory (DFT), are used extensively to predict/design novel materials, from ferroelectrics to high temperature superconductors. In this talk, I am going to discuss my materials design efforts using Dynamical Mean Field Theory (DFT+DMFT), which is the state of the art first principles method to approach on-site electronic correlations in transition metal systems. In particular, I am going to start with discussing a novel group of J=1/2 fluoro-iridates, and then move on to efforts of designing superior transparent conductors by taking advantage of the electronic correlations in the well known perovskite oxide strontium vanadate.

Wednesday, May 3rd 2017

1:25 pm:

For certain classes of insulating materials, it is possible to derive a very precise connection between properties of the bulk and properties of the surface. A connection of this kind is known as a bulk-boundary correspondence. While such correspondences can be very useful, unfortunately the only cases where they are understood in generality involve either non-interacting or low-dimensional systems. In this talk, I will discuss progress on the bulk-boundary correspondence for a large class of three-dimensional, interacting systems. Specifically, the systems I will discuss are known as symmetry-protected topological phases and can be thought of as generalizations of topological insulators and superconductors.

Thursday, May 4th 2017

11:00 am:

Tuesday, June 27th 2017

12:30 pm:

In this talk, Oleg will discuss:

(i) the p-B-T phase diagram.

(ii) logarithmic decay of the coupling constant ("asymptotic freedom").

(iii)Narrow paramagnons in magnetically disordered regime.

(iv) Bose condensation.

(v) Higgs mode and its decay width in different regimes.

(vi) Comparison with data on TlCuCl3.

Wednesday, September 6th 2017

1:30 pm:

Wednesday, September 13th 2017

1:25 pm:

Spin-triplet superconductor Sr2RuO4 was predicted to support exotic objects such as half-quantum vortices, which carry a magnetic flux half of the flux quantum Phi_0=hc/2e. We report electrical transport measurements on micron-sized, doubly connected cylinders of Sr2RuO4 single crystals with the cylinder axis along the c axis. Large amplitude magnetoresistance oscillations were observed, revealing unconventional Little-Parks effect dominated by vortex crossing. The free energy barrier that controls the vortex crossing was modulated by the magnetic flux enclosed in the cylinder, an in-plane field, measurement current, and factors related to sample geometry. Distinct features on magnetoresistance peaks were found consistent with the emergence of the half-quantum state in this material, only in samples for which the vortex crossing is confined at specific parts of the sample.

Wednesday, September 20th 2017

1:25 pm:

One of the simplest models representing lattice frustration in magnetic materials is the triangular lattice Heisenberg antiferromagnet. It was shown theoretically that it orders into 120 degree order. However, a large number of spin liquid candidates on triangular lattice are found among quasi-two-dimensional molecular- based Mott insulators. One explanation is that new charge degrees of freedom that can emerge in molecular-based Mott insulators resulting in a quantum dipole liquid state. Studies of the materials has been hampered by a lack of spectroscopic information, since neutron scattering has generally not been possible.

We use Raman scattering to detect spectrum of magnetic excitations in these magnetic compounds. At first I will demonstrate a Raman scattering study of the helical antiferromagnet α- -SrCr2O4 (T_N=42K). Further, we identify magnetic excitations in an antiferromagnetic compound k-(BEDT-TTF)2Cu[N(CN)2]Cl and a spin liquid candidate k-(BEDT-TTF)2Cu2(CN)3. We show that their spectrum of excitations is very different from that of another triangular lattice Mott insulator k-(BEDT-TTF)2Hg(SCN)2Br. Our data demonstrate the emergence of an on-site dipole degree of freedom in the latter material. Raman scattering allows us to detect dipole fluctuations both with vibrational molecular spectroscopy and through observation of a collective mode at about 8 meV. Heat capacity of k-(BEDT-TTF)2Hg(SCN)2Br demonstrates a linear term at low temperatures, supporting a scenario where composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest temperatures.

Wednesday, September 27th 2017

1:25 pm:

Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. But definitive evidence for a thermodynamically stable exciton condensate has never been achieved. In this talk I will describe our use of momentum-resolved electron energy-loss spectroscopy (M-EELS) to study the valence plasmon in the transition metal dichalcogenide semimetal, 1T‐TiSe2. Near the phase transition temperature, TC = 190 K, the plasmon energy falls to zero at nonzero momentum, indicating dynamical slowing down of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. At low temperature, the plasmon evolves into an amplitude mode of this electronic crystal. Our study represents the first observation of a soft plasmon in any material, the first definitive evidence for exciton condensation in a three-dimensional solid, and the discovery of a new form of matter, “excitonium.”

Wednesday, October 4th 2017

1:25 pm:

Topological phases offer diverse novel properties of matter. These are mostly determined by material properties which can not easily be changed. An alternative are driven topological systems such as Floquet topological insulators and charge pumps. In these cases application of an external time-periodic field allows for easy control of the topology of the system. In this talk I discuss general properties of slowly driven systems in the presence of interactions with the example of a charge pump. Intraband scattering thermalizes all particles within one band which makes topological properties of partly filled bands accessible. On the other hand interactions also cause excitations to the other band with opposite topological index. I will show that for a wide class of systems there exists a prethermal state in which one band is fully thermalized while the other band remains unoccupied. During this time window the topological properties are experimentally accessible.

Monday, October 16th 2017

2:30 pm:

It is usually believed that in clean metals, at finite temperature, the electron-electron umklapp scattering is proportional to the inelastic scattering rate, 1/\tau_{ee}. In this talk it will be shown that, for three dimensional systems, when \hbar/\tau_{ee} is of order or larger than the band splitting energy, the umklapp scattering rate saturates to a value which is independent of both \tau_{ee} and the temperature, T. This phenomenon sheds new light on the old problem of the resistivity saturation at high temperature.

Wednesday, October 18th 2017

1:25 pm:

The second-order nonlinear response, defined by the relation, J^((2)) (ω±ω)=σ^((2) ) (ω)E_ω^2, is allowed only in media without a center of inversion. A new class of inversion breaking materials, “Weyl semimetals,” have been synthesized and are currently under intense investigation. Recently we reported that the first of this class of materials to be discovered, the transition metal monopnictides such as TaAs, and NbAs, exhibit the largest σ^((2) ) (ω)E_ω^2 of any known crystal. Large values of σ^((2) ) (ω) are of interest for applications involving frequency generation and conversion of light to electrical current. The observation of a “giant” response raises two related questions for which I will attempt to provide partial answers: what is special about TaAs, and is there an upper bound on σ^((2) ) (ω) of inversion-breaking crystal?

Wednesday, October 25th 2017

1:25 pm:

Wednesday, November 1st 2017

1:25 pm:

Wednesday, November 8th 2017

1:25 pm:

Wednesday, November 15th 2017

1:25 pm:

Wednesday, December 6th 2017

1:25 pm:

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