Condensed Matter Seminar

semester, 2015

Thursday, January 22nd 2015
12:00 pm:
Speaker: Dmitry Reznik, University of Colorado-Boulder
Subject: Anomalous Phonons and High Temperature Superconductivity in Copper Oxides
Note: change of time and location for seminar, this week only.

It is well known that electron-phonon coupling is responsible for superconductivity in conventional superconductors, but the prevailing view is that it is not important in high temperature superconductivity. Yet, evidence that electron-phonon coupling is very strong for certain phonons in the copper oxides has been building. In particular, Cu-O bond-stretching phonons at 65-85meV in La2−xSrxCuO4 are known to show anomalously large broadening and softening near the reduced wavevector q=(0.3,0,0). Recently we systematically investigated spectral functions of these phonons by inelastic neutron and x-ray scattering and measured dispersions of electrons to which these phonons should be coupled by angle resolved photoemission (ARPES). These electronic dispersions have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode (which could be a phonon) that mediates superconductivity. Remarkably, we found that the kinks remain strong in the heavily overdoped region of the doping phase diagram of La2−xSrxCuO4, even when the superconductivity completely disappears. We also found that doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. In fact, while the Cu-O bond stretching phonons show giant electron-phonon effects, there are no features in the electronic dispersions of the same samples that can be attributed to these phonons. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity. I will also discuss earlier results on a copper oxide with a very high Tc, YBa2Cu3O7, where a similar phonon anomaly becomes greatly enhanced in the superconducting state.

Faculty Host: Martin Greven

Thursday, January 29th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Deborah Jin (JILA, University of Colorado)
Subject: Universal Dynamics of a Degenerate Unitary Bose Gas

Ultracold quantum gases of atoms are model systems that provide access to strongly correlated many-body physics as well as to intriguing few-body physics phenomena. It is particularly interesting to explore the behavior of quantum gases with infinitely strong interactions (the so-called unitary gas). This regime has been experimentally studied for Fermi gases of atoms but is difficult to access for an ultracold gas of bosons. I will discuss a recent experiment where we quickly took a Bose-Einstein condensate to this regime of strong interactions and looked at the ensuing dynamics in this non-equilibrium system.

Faculty Host: Rafael Fernandes

Thursday, February 5th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Rebecca Flint, Iowa State
Subject: Hidden order in URu2Si2: hybridization with a twist

The development of collective long-range order occurs by the spontaneous breaking of fundamental symmetries, but the broken symmetry that develops below 17.5K in the heavy fermion material URu2Si2 has eluded identification for over twenty five years – while there is clear mean-field-like specific heat anomaly, the absence of any large observable order parameter has given the problem the name "hidden order." In this talk, I will show how the recent observation of heavy Ising quasiparticles in the hidden order phase provides the missing puzzle piece. To form Ising quasiparticles, the conduction electrons must hybridize with a local Ising moment - a 5f2 state of the uranium atom with integer spin. As the hybridization mixes states of integer and half-integer spin, it is itself a spinor and this ``hastatic'' (hasta: [Latin] spear) order parameter therefore breaks both time-reversal and double time-reversal symmetries. A microscopic theory of hastatic order naturally unites a number of disparate experimental results from the large entropy of condensation to the spin rotational symmetry breaking seen in torque magnetometry. Hastatic order also has a number of experimental consequences, most notably a tiny transverse magnetic moment in the conduction electrons.

Faculty Host: Fiona Burnell

Thursday, February 12th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Ian Fisher, Stanford University
Subject: Probing electronic nematicity via elastoresistance measurements

In recent years, anisotropic electronic phases have been discovered in a variety of strongly correlated quantum materials. Borrowing language from the field of soft condensed matter physics, such phases are referred to as electronic nematic phases when they are driven by electron correlations and break a discrete rotational symmetry of the crystal lattice without further breaking translational symmetry. In this talk I'll outline a new technique that we have developed based on elastoresistance measurements, which probes an associated quantity, the nematic susceptibility. Measurements of this quantity directly reveal the presence of an electronic nematic phase transition in underdoped Fe-based superconductors, and an associated quantum phase transition near optimal doping (i.e. the doping that yields the maximum critical temperature of the superconductor). I'll explain the possible significance of this observation. I'll also discuss the case of ferroquadrupolar order in 4f intermetallic systems.

Faculty Host: Rafael Fernandes

Thursday, February 19th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Victor Gurare, University of Colorado - Boulder
Subject: Unconventional Anderson localization in high dimensions and Dirac materials

Hamiltonians describing particles moving in a random potential often have eigenstates which have finite spatial extend, a well-known phenomenon called Anderson localization. Above two dimensional space, localized wave functions all correspond to energies below a critical energy usually called the mobility edge. The size of the localized wave functions diverges as mobility edge is approached, a phenomenon called Anderson transition. It is generally believed that the details of Anderson transition depend on the dimensionality of space only, which is usually referred to as universality of the Anderson transition. We argue that in sufficiently high dimensions a second type of Anderson transition develops if the disorder strength is close to some critical value, distinct from the conventional transition, with a number of unusual features. For a conventional Schrodinger equation with a random potential one has to be above four dimensional space to see this new transition, thus it is not straightforward, although not impossible, to observe it. In electronic systems with a Dirac-like spectrum, one only has to be above two dimensions to observe this transition. We discuss the consequences of the existence of this transition for disordered materials with Dirac-like electronic spectra.

Faculty Host: Alex Kamenev

Thursday, February 26th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Brian Andersen, University of Copenhagen
Subject: Recent developments of cuprates and pnictides; pairing symmetry, competing order, and nematicity

In this talk I will present some of our recent theoretical efforts to understand the fascinating interplay between magnetic, charge, and superconducting order in cuprates and Fe-based superconductors. The talk will include a general discussion of spin fluctuations in iron pnictides, including the evidence and modelling of nematic (anisotropic) spin fluctuations. These can have profound influence on the transport properties of these systems both through inelastic scattering and emergent new static defect states significantly contributing to anisotropies in the measured quantities. This is true even above the magnetic transition where the anisotropic spin fluctuations can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials that scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. From the calculated scattering potentials, relaxation rates, and conductivity in this region we conclude that such emergent defect states are essential for the transport anisotropy observed in experiments. I will end this part of the talk by presenting a general scenario for the transport anisotropy throughout the whole phase diagram.

Next, I will turn to a discussion of competing magnetic phases in the pnictides relevant to recent experiments finding magnetic order in a tetragonal crystal lattice. This points to the existence of other so-called C4 symmetric magnetic phases with, for example, non-collinear moments. I will present a theoretical microscopic study of these phases and their general electronic properties. A discussion will be included on the role of superconductivity and disorder in destroying and stabilizing these novel magnetic C4 states, respectively. Finally, if time allows, I will also discuss some recent studies of the doping dependence of the pairing symmetry of the cuprates in the presence of spin-density wave order.

Faculty Host: Rafael Fernandes

Friday, February 27th 2015
12:00 pm:
Condensed Matter Seminar in in Physics 435
Speaker: Paula Melado, School of Engineering and Applied Sciences, Adolfo Ibanez University, Santiago, Chile

Low-energy states of quantum spin liquids are thought to involve partons
living in a gauge-field background. We study the spectrum of Majorana
fermions of Kitaev's honeycomb model on spherical clusters. The gauge field
endows the partons with half-integer orbital angular momenta. As a consequence, the
multiplicities reflect not the point-group symmetries of the cluster, but
rather its projective symmetries, operations combining physical and gauge
transformations. The projective symmetry group of the ground state is the
double cover of the point group [1].

[1] Phys. Rev. B 91, 041103(R) (2015).

Faculty Host: Andrey Chubukov

Thursday, March 5th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
There will be no seminar this week due to APS March Meeting

Thursday, March 12th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Ioannis Rousochatzakis, UMN
Subject: Multiscale theory of Cu2OSeO3: helimagnetism, skyrmions, and beyond

Thursday, March 19th 2015
1:25 pm:
Speaker: Eugene Levin, Iowa State
Subject: Recent findings for the Seebeck effect in thermoelectric tellurides

The Seebeck effect, known since 1821 and utilizing in thermoelectric materials to convert thermal to electrical energy, have been intensively studied and used in various applications, but there are indications that its fundamental understanding is still lacking. In order to better understand the Seebeck effect, we have used tellurides as model systems, common experimental methods (XRD, Seebeck coefficient, electrical resistivity, Hall effect, thermal conductivity, SEM, and EDS), and advanced method (125Te NMR to obtain the carrier concentration in tellurides via spin-lattice relaxation measurements). Several intriguing relations between the Seebeck coefficient, carrier concentration, composition, and rhombohedral lattice distortion have been established for GeTe alloyed with Ag and Sb (well-known high thermoelectric efficiency TAGS-m series). The Seebeck coefficient in some tellurides can be described by a common model with energy independent carrier scattering, but in some not. The latter can be explained by the effect from additional mechanism, likely energy filtering produced by [Ag+Sb] pairs in GeTe matrix. Our findings demonstrate that the Seebeck effect still holds surprises, and innovative research in the area of thermoelectric materials may help to elucidate and better utilize thermoelectric phenomena.

Faculty Host: Boris Shklovskii

Thursday, March 26th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Michael Schuett, University of Minnesota
Subject: Electronically-induced resistivity anisotropy in underdoped cuprates and iron pnictides

Monday, March 30th 2015
Speaker: Greg Fuchs, Cornell
Subject: Coherent control over diamond nitrogen-vacancy center spins with a mechanical resonator (and other recent results)

Thursday, April 2nd 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: James Analytis, UC Berkeley
Subject: Magnetoresistance near a quantum critical point

The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high temperature superconductivity.
Near a quantum critical point(QCP)a new kind of metal emerges, whose thermodynamic and transport properties do not fit into the unified phenomenology with which we understand conventional metals - the
Landau Fermi liquid(FL)theory - characterized by a low temperature limiting T-linear specific heat and a T^2 resistivity.Studying the evolution of the T­ dependence of these observables as a function of a control parameter leads to the identification both of the presence and the nature
of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe2(As1-xPx)2,at T by suppressing superconductivity with high magnetic fields. We find an anomalous magnetic field dependence that suggests that not only does magnetic field directly affect the scattering rate (which is unusual for metals), but it does so in a way that is identical to temperature. We suggest that there is a universal phenomenology of scattering near a quantum critical point.

Faculty Host: Andrey Chubukov

Friday, April 3rd 2015
12:00 pm:
Condensed Matter Seminar in Physics 435
Speaker: Dmitrii Maslov, University of Florida
Subject: "Transport in strongly correlated electron systems"
Faculty Host: Andrey Chubukov

Thursday, April 9th 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Steve Kivelson, Stanford University
Subject: Enhancement of superconductivity near a nematic quantum critical point
Please note: change of time for seminar, in effect for rest of semester.

Two topics that have attracted intense theoretical study over the past decade are the nature of quantum critical phenomena in metallic systems and what, if anything, such critical points have to do with an unconventional mechanism of superconducting pairing. The still un-mastered subtleties of the first problem have precluded convincing resolution of the second. For the model problem of a weakly interacting metal in proximity to a nematic quantum critical point (NQCP), we identify a broad regime of parameters in which the nature of the induced superconductivity can be understood in a theoretically well controlled manner without needing to resolve the deep, unsolved issues of metallic criticality. We show that: 1) a BCS-Eliashberg treatment remains valid outside of a parametrically narrow interval about the NQCP; 2) the symmetry of the superconducting state (d-wave, s-wave, p-wave) is typically determined by the non-critical interactions, but Tc is enhanced by the nematic fluctuations in all channels; 3) in 2D, this enhancement grows upon approach to criticality up to the point at which the weak coupling approach breaks-down, but in 3D the enhancement is much weaker. Preliminary results of determinental quantum Monte-Carlo studies of the true critical regime will also be presented.

Faculty Host: Rafael Fernandes

Thursday, April 16th 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: David Hsieh, Institute for Quantum Information and Matter, CIT
Subject: Subtle structural distortions and a hidden magnetic phase in Sr2IrO4 revealed using nonlinear optical measurements
Please note: change of time for seminar, in effect for rest of semester.

Iridium oxides are predicted to host a variety of exotic electronic phases emerging from the interplay of strong electron correlations and spin-orbit coupling. There is particular interest in the perovskite iridate Sr2IrO4 owing to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La2CuO4. However, despite theoretical predictions for unconventional superconductivity and recent observations of Fermi arcs with a pseudogap behavior in doped Sr2IrO4, no superconductivity has been observed in this compound so far. In this talk I will describe the nonlinear optical spectroscopy and wide field microscopy techniques that we have recently developed to resolve the symmetries of both lattice and electronic multipolar ordered phases on single domain length scales. I will show our results on the undoped Mott insulator Sr2IrO4 that reveal a subtle structural distortion and provide evidence for a hidden loop-current ordered magnetic phase that has previously eluded other experimental probes. The significance of these novel orders to magneto-elastic coupling and the pseudogap phase in Sr2IrO4 will be discussed.

Faculty Host: Natalia Perkins

Thursday, April 23rd 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Alan Goldman, Iowa State University and Ames Laboratory
The Condensed Matter Seminar has been cancelled this week

Thursday, April 30th 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: James Kakalios, UMN
Subject: Is the Conductivity in Amorphous Semiconductors Thermally Activated?
Please note: change of time for seminar, in effect for rest of semester.

Thursday, May 7th 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Premala Chandra, Rutgers University
Subject: Frustration, Unexpected Order and Criticality in 2D Heisenberg Antiferromagnets
Please note: change of time for seminar, in effect for rest of semester.

Despite having finite spin correlation lengths at nonzero temperatures, frustrated two-dimensional Heisenberg magnets can develop long-range discrete order driven by short-range thermal spin fluctuations. Indeed this "order from disorder" phenomenon can lead to a finite-temperature Ising or Potts transition; it has recently been realized experimentally in the iron-based superconductors where it is responsible for the high-temperature nematic phase. We can also ask whether such a mechanism can lead to an emergent critical phase in an isotropic Heisenbergm magnet and we present a model where this is indeed the case. Our results are supported by both Wilson-Polyakov scaling and by Friedan's geometrical approach to nonlinear sigma models. We also discuss recent computational studies that support our results and discuss possible experimental realizations. We end with a more general discussion of the relation between Friedan scaling and 2D antiferromagnetism, and the possibility of using it to simulate generalized surgery-free Ricci flows of topological manifolds of broader mathematical interest.
Work done in collaboration with P. Orth, R. Fernandes, B. Jeevanesan, J. Schmalian, P. Coleman and A.I. Larkin.

Faculty Host: Rafael Fernandes

Thursday, May 14th 2015
1:15 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Gia-Wei Chern, LANL
Subject: Stiffness from Disorder in Frustrated Quasi-2D Magnets
Please note: change of time for seminar, in effect for rest of semester.

Frustrated magnetism has become an extremely active field of research. The concept of geometrical frustration dates back to Wannier’s 1950 study of Ising antiferromagnet on the triangular lattice. This simple system illustrates many defining characteristics of a highly frustrated magnet, including a macroscopic ground-state degeneracy and the appearance of power-law correlations without criticality. In this talk I will discuss a simple generalization of the triangular Ising model, namely, a finite number of vertically stacked triangular layers. Our extensive numerical simulations reveal a low temperature reentrance of two Berezinskii-Kosterlitz-Thouless transitions. In particular, I will discuss how short-distance spin-spin correlations can be enhanced by thermal fluctuations, a phenomenon we termed stiffness from disorder. This is a generalization of the well-known order-by-disorder mechanism in frustrated systems. I will also present an effective field theory that quantitatively describes the low-temperature physics of the multilayer triangular Ising antiferromagnet.

Faculty Host: Natalia Perkins

Tuesday, July 7th 2015
Speaker: Kyunghoon Lee, University of Michigan
Subject: Electron transport in two-dimensional materials: Graphene and beyond

Owing to their unique energy band structure and the ease of material synthesis, two dimensional nanomaterials, such as graphene, have become the ideal platform for observing novel electron transport phenomena. In particular, low energy quasiparticles in monolayer graphene behave like massless Dirac fermions, which have led to observations of many interesting phenomena, including Klein tunneling, anomalous Quantum Hall effect, etc. In contrast to the monolayer graphene, quasiparticles in bilayer graphene (BLG) are massive chiral fermions due to its parabolic band structure. Thus, BLG also gives a number of intriguing properties which are very different from those of monolayer graphene, including tunable band gap opening and anti-Klein tunneling, arising from chiral characteristics of charge carriers. However, unlike SLG, experimental works on chiral electron transport in BLG have received less attention.

In addition, other two-dimensional atomic layer crystals, such as atomically thin layered transition metal dichalcogenides (TMDCs), are also attractive material platform with unique electronic and optical properties, including indirect to direct band gap transition, and valley polarized carrier transport. However, study of the low temperature electron transport in atomic thin layered TMDCs is still in its infancy. One of the major hurdles for electron transport study lies in the large metal/semiconductor junction barrier for carrier injection, which leads to the contact resistance dominated charge transport in short channel nanoscale devices.

In this talk, I will show our first demonstrated successful synthesis of wafer scale BLG with high-homogeneity by low-pressure chemical vapor deposition (CVD). Next, I’ll discuss about the importance of chiral electron transport in BLG. I’ll present the signature of electronic cloaking effect with anti-Klein effect as a manifestation of chirality by probing phase coherent transport behavior in CVD bilayer graphene nanostructure. Finally, I’ll talk on the electron transport in two-dimensional TMDCs. I successfully fabricated monolayer MoS2 single electron transistors using low work function metal for the contact electrodes, and observed Coulomb blockade phenomena attributed to single electron charging on a fairly clean quantum dot.

Faculty Host: Vlad Pribiag

Monday, August 3rd 2015
11:00 am:
Thesis Defense in 110 PAN
Speaker: Abdul Malmi Kakkada, University of Minnesota
Subject: Role of Disorder in Quantum Crystals
This is the public portion of Mr. Kakkada's thesis defense

Wednesday, September 9th 2015
1:25 pm:
Speaker: Joseph Falson, MPI, Stuttgart
Subject: Electron correlation physics revealed in high mobility MgZnO/ZnO heterostructures

The two-dimensional electron system (2DES) hosted at the interface of MgZnO/ZnO now displays electron mobilities exceeding 1,000,000 cm2/Vs and electron scattering times comparable to the best AlGaAs/GaAs 2DES. In this talk I will discuss the growth technology used to create such high quality devices and introduce the physical phenomena they host at low temperatures. These themes include interaction-induced renormalization of the spin susceptibility of carriers and how it reveals the new facets of the fractional quantum Hall effect, along with non-equilibrium phenomena, such as microwave-induced resistance oscillations.

Faculty Host: Michael Zudov

Wednesday, September 16th 2015
1:25 pm:
Speaker: Jeremy Levy, University of Pittsburgh
Subject: Electron Pairing Without Superconductivity

Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor. It exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to that of high-temperature superconductors—two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowire-based single-electron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of two-electron conductance resonances—paired electron states—that bifurcate above a critical pairing field Bp of about 1–4 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes real-space electron pairing as a precursor to superconductivity.

Faculty Host: Vlad Pribiag

Wednesday, September 23rd 2015
1:25 pm:
Speaker: Ivar Martin, Argonne
Subject: Mean field theory of (quasi)crystallization

Crystallization is one of the most familiar phase transitions; despite that, its theoretical understanding remains a major challenge. Why some crystal structures are more common than others? What selects between different close-packed orderings? Why do atoms sometimes prefer to arrange themselves quasi-periodically? While there is probably not a unique answer to these questions, in metallic alloys, I will argue that the driving physics may not be very different from that of various density (spin, charge, etc) instabilities within crystalline states.

Faculty Host: Rafael Fernandes

Wednesday, September 30th 2015
1:25 pm:
Note change of day and time to Friday at 11:00 a.m., this week only.

Friday, October 2nd 2015
11:00 am:
Speaker: D. Basov, UCSD
Subject: Interacting polaritons in van der Waals atomic layered materials
Please note change of day for the CM seminar, this week only.

Layered van der Waals (vdW) crystals consist of individual atomic planes weakly coupled by vdW interaction, similar to graphene monolayers in bulk graphite. These materials reveal diverse classes of light-matter modes (polaritons) including: surface plasmon polaritons in graphene, hyperbolic phonon polaritons in boron nitride, exciton polaritons in MoS2, cooper pair plasmon polaritons in high-Tc cuprates, topological plasmon polaritons and many others. I will overview recent nano-optical investigations conducted in our group aimed at probing interactions between different types of polaritons in artificial structures comprised of dissimilar vdW atomic layers.

Fei et al. Nature 487, 82 (2012)
Dai et al. Science 343, 1125 (2014),
Basov et al. Reviews of Modern Physics 86, 959 (2014),
Post et al. PRL 115, 116804 (2015);
Dai et al. Nature Nanotechnology 10, 682 (2015),
Ni et al. Nature Materials October (2015).

Faculty Host: Andrey Chubukov

Wednesday, October 7th 2015
1:25 pm:
Speaker: Eugene Kolomeisky - University of Virginia
Subject: Optimal number of terms in QED series and its consequence in condensed matter implementations of QED

In 1952 Dyson put forward a simple and powerful argument indicating that the perturbative expansions of QED are asymptotic. His argument can be related to Chandrasekhar's limit on the mass of a star for stability against gravitational collapse. Combining these two arguments we estimate the optimal number of terms of the QED series to be 3.1(137)^{3/2}\approx5000 . For condensed matter manifestations of QED in narrow band-gap semiconductors and Weyl semimetals the optimal number of terms is around 80 while in graphene the utility of the perturbation theory is severely limited.

Faculty Host: Boris Shklovskii

Wednesday, October 14th 2015
1:25 pm:
Speaker: Oleg Tchernyashov, Johns Hopkins
Subject: Anyons in solvable models of quantum magnets

Following Anderson's 1973 conjecture of a resonating valence-bond (RVB) state, theorists have been actively exploring quantum spin liquids – states of magnets without long-range order. In the last 15 years we have progressed from knowing what quantum spin liquids are not (states with local order) to understanding what they are. Several solvable models of spin liquids have been shown to be instances of lattice gauge theories (Kitaev). An alternative perspective is a picture of fluctuating closed strings (Wen). An open string contains two "fundamental" particles on its ends, which can be bosons, fermions, or anyons, depending on the model. Although these models, featuring multi-spin interactions, look contrived, more realistic models (e.g., Kitaev's honeycomb) share many of their exotic features. I will show how certain lattice defects of Kitaev's honeycomb model can bind Majorana zero modes.

Faculty Host: Natalia Perkins

Wednesday, October 21st 2015
1:25 pm:
Speaker: Jian Kang - University of Minnesota
Subject: Robustness of quantum critical pairing aginst disorder

Unconventional superconductivity is found in close proximity to a putative magnetic quantum critical point in several materials, such as heavy fermions, organics, cuprates, and iron pnictides. This led to the proposal that, in contrast to conventional electron-phonon superconductors, critical magnetic fluctuations promote the binding of the electrons in Cooper pairs. Experiments in many of these materials revealed that their superconducting transition temperature Tc is remarkably robust against disorder, particularly when compared to the predictions from the conventional Abrikosov-Gor'kov theory of dirty superconductors. In this talk, we investigate the impact of weak impurity scattering on the onset of the pairing state mediated by quantum critical magnetic fluctuations.We find that both the build-up of incoherent electronic spectral weight near the magnetic quantum phase transition, as well as the changes in the pairing interaction caused by disorder, lead to a significant reduction in the suppression rate of Tc with disorder, as compared to the Abrikosov-Gor'kov theory. Both effects are unique to the problem of electronically-mediated pairing, shedding new light on the understanding of unconventional superconductivity in correlated materials, where disorder is always present.

Faculty Host: Rafael Fernandes

Wednesday, October 28th 2015
1:25 pm:
Speaker:  Xiang Cheng, University of Minnesota, Department of Chemical Engineering and Material Science
Subject: Impact response of granular materials: From the origin of the universe to catastrophic asteroid strikes

Granular materials are large conglomerations of discrete macroscopic particles. Examples include seeds, sand, coals, powder of pharmacy, etc. Though simple, they show unique properties different from other familiar forms of matter. The unusual behaviors of granular materials are clearly illustrated in various impact processes, where the impact-induced fast deformation of granular materials leads to emergent flow patterns revealing distinctive granular physics. Here, we explored the impact response of granular materials in two specific experiments:

First, we performed the granular analog to “water bell” experiments. When a wide jet of granular material impacts on a fixed cylindrical target, it deforms into a sharply-defined sheet or cone with a shape mimicking a liquid of zero surface tension. The jets' particulate nature appears when the number of particles in the beam cross-section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment reveals a universal fluid structure arising from the collision of discrete particles, which has a counterpart in the behavior of quark-gluon plasmas created by colliding heavy ions at the Relativistic Heavy Ion Colliders.

Second, we investigated impact cratering in granular media induced by the strike of liquid drops—a ubiquitous phenomenon relevant to many important environmental, agricultural and industrial processes. Surprisingly, we found that granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we develop a simple model that quantitatively describes various features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals an interesting analogy between familiar phenomena of raining and catastrophic asteroid strikes.

Faculty Host: Vlad Pribiag

Wednesday, November 4th 2015
1:25 pm:
Speaker: Xiaoyu Wang, University of Minnesota
Subject: Crisscrossed stripe CDW in cuprate superconductors
The Sack Lunch and the CM Seminar will swap places, this week only.

Friday, November 6th 2015
12:20 pm:
Speaker: Adam Kaminski - Ames Laboratory
Subject: Interaction of electrons with collective excitations in conventional and unconventional superconductors
Please note the date and time change this week.

Interaction of electrons with collective modes plays a central role in theory of conventional superconductors and many models of unconventional superconductors. In cuprates for example, presence of such interaction was discovered almost two decades ago [1, 2], however there is no consensus as to its origin [2-5]. It is not even known whether it is responsible for driving the pairing or a mere spectator of the superconducting transition. One of the reasons for this situation is lack of momentum resolved data from classical superconductors that would serve as a baseline for identifying the physical nature of the coupling interaction and its role in the pairing mechanism. In this talk we will present such data from classical BCS superconductor MgB2 [6, 7] and review the old and new properties of the collective mode present in cuprates. In the classical superconductor, the coupling of the electrons to the phonon mode responsible for pairing remains almost unaffected across the critical temperature. This is unlike the situation in cuprates, where the signature of coupling to the collective mode vanishes above Tc. We also discovered new low energy spectral feature in MgB2 that is most likely a signature of coupling to the Leggett mode – a relative phase excitation of the two suprefluids present in this material.

[1] M. R. Norman et al., Phys. Rev. Lett. 79, 3506 (1997).
[2] M. R. Norman and H. Ding, Phys. Rev. B 57, 11089 (1998).
[3] A. Kaminski et al., Phys. Rev. Lett. 86, 1070 (2001).
[4] A. Lanzara et al., Nature 412, 510 (2001).
[5] G. H. Gweon, Nature 430, 187 (2004).
[6] D. Mou et al., Phys. Rev. B 91, 140502 (2015).
[7] D. Mou et al., Phys. Rev. B 91, 214519 (2015).

Faculty Host: Andrey Chubukov

Wednesday, November 11th 2015
1:25 pm:
No seminar this week.

Wednesday, November 18th 2015
1:25 pm:
Speaker: Norman O. Birge, Michigan State University
Subject: Spin-triplet supercurrents in ferromagnetic Josephson junctions

When a superconductor (S) and a ferromagnet (F) are put into contact with each other, the combined S/F hybrid system exhibits altogether new properties. There is a proximity effect where electron pair correlations from S penetrate into F, but the pair correlations oscillate rapidly and decay over a very short distance due to the large exchange splitting between the spin-up and spin-down electron bands in F. In the presence of non-collinear magnetization, Bergeret et al. predicted that spin-triplet pair correlations are generated, which are immune to the exchange field and hence persist over much longer distances in F [1]. Furthermore, these triplet pair correlations satisfy the Pauli Exclusion Principle in a new and strange way: they are odd in frequency or time. Several groups have now observed convincing evidence for such spin-triplet correlations in a variety of S/F and S/F/S systems. Our approach is based on measuring the supercurrent in Josephson junctions of the form S/F’/F/F’’/S, with non-collinear magnetizations in adjacent ferromagnetic layers [2]. I will discuss our latest results toward controlling the supercurrent in these junctions [3], as well as how various types of ferromagnetic junctions could be used as memory elements in a fully superconducting random-access memory [4].

Work supported by the US DOE under grant DE-FG02-06ER46341, by Northrop Grumman Corporation, and by IARPA via U.S. Army Research Office contract W911NF-14-C-0115.

[1] F.S. Bergeret, A.F. Volkov, and K.B. Efetov, Phys. Rev. Lett., 86, 4096 (2001).
[2] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., and N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010); C. Klose et al, Phys. Rev. Lett. 108, 127002 (2012).
[3] W. Martinez, W.P. Pratt, Jr., and N.O. Birge, arXiv:1510.02144.
[4] E.C. Gingrich et al., arXiv:1509.05368.

Faculty Host: Vlad Pribiag

Wednesday, November 25th 2015
1:25 pm:
Thanksgiving - There will be no seminar this week.

Wednesday, December 2nd 2015
1:25 pm:
Speaker: Alex Levchenko, University of Wisconsin
Subject: Anomalous Hall Effect Generalities - The Case of Kondo Topological Insulators

We calculate the anomalous Hall conductivity of surface states on three dimensional topological Kondo insulators with cubic symmetry and multiple Dirac cones. We treat a generic model in which the Fermi velocity, the Fermi momentum and the Zeeman energy in different pockets may be unequal and in which the microscopic impurity potential is short ranged on the scale of the smallest Fermi wavelength. Our calculation of AHE to the zeroth (i.e. leading) order in impurity concentration is based on the Kubo-Smrcka-Streda diagrammatic approach. It also includes certain extrinsic skew scattering contributions with a single cross of impurity lines, which are of the same order in impurity concentration. We discuss various special cases of our result and the experimental relevance of our study in the context of recent hysteretic magnetotransport data in SmB6 samples.

Faculty Host: Vlad Pribiag

Wednesday, December 9th 2015
1:25 pm:
Speaker: Konstantin Reich, UMN
Subject: How many electrons make a semiconductor nanocrystal film metallic

For films of semiconductor nanocrystals to achieve their potential as novel,
low-cost electronic materials, a better understanding of their doping to tune
their conductivity is required. So far, it not known how many dopants will
turn a nanocrystal film from semiconducting to metallic. In bulk
semiconductors, the critical concentration of electrons at the metal-
insulator transition is described by the famous Mott criterion. We show
theoretically that in a dense NC film, where NCs touch each other by small
facets, the concentration of electrons N at the metal-insulator transition
satisfies the condition: N r^3 = 0.3, where r is a radius of contact facets.
In the accompanying experiments, we investigate the conduction mechanism in
films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest
electron concentration achieved in our samples, which is half the predicted
N, we find that the localization length of hopping electrons is close to three
times the nanocrystals diameter, indicating that the film approaches the
metal-insulator transition.

Thursday, December 10th 2015
Speaker: Ohad Shpielberg, Technion
Subject: Le Chatelier principle for out of equilibrium and boundary driven systems : application to dynamical phase transitions.

A stability analysis of out of equilibrium and boundary driven systems is presented. It is performed in the framework of the hydrodynamic macroscopic fluctuation theory and assuming the additivity principle whose interpretation is discussed with the help of a Hamiltonian description. An extension of Le Chatelier principle for out of equilibrium situations is presented which allows to formulate the conditions of validity of the additivity principle. Examples of application of these results in the realm of classical and quantum systems are provided.

Wednesday, December 16th 2015
1:25 pm:
Speaker: Nadya Mason, University of Illinois
Subject: The Emergence of Superconductivity in Inhomogeneous, Mesoscopic Systems

Although low-dimensional, inhomogeneous superconductors have been intensely studied, the nature of the onset of superconductivity in these systems is still largely unknown. In this talk I will present transport measurements on mesoscopic disks of granular, inhomogeneous Nb, where the superconducting transition temperature is determined as a function of disk diameter. We observe an unexpected suppression of superconductivity at micron diameters, length scales that are considerably longer than the coherence length of Nb. This suppression does not appear in large-scale films, and cannot be explained by single-grain small-size effects. By considering the diameter-dependence of the transition, as well as observations of strong fluctuations in the transition temperature as disk diameters decrease, we are able to explain this long length scale dependence by an extremal-grain model, where superconducting order first appears in unusually large grains and, due to proximity coupling, spreads to other grains. The extremal-grain onset of superconductivity has not previously been observed experimentally, and explains how superconductivity can emerge in granular or inhomogeneous superconductors.

Faculty Host: Vlad Pribiag

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