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Condensed Matter Seminar

J. Ross, TAMU
Speaker: J. Ross, TAMU Title: Presentation to the students. Abstract: Host:

31 Aug 2018, 4:00PM | MIST M102

Hector Ochoa de Eguileor, UCLA
Speaker: Hector Ochoa de Eguileor, UCLA Title: Spin hydrodynamics in non-collinear magnets Abstract: Understanding out-of-equilibrium phenomena at the interface between metals and magnetic insulators constitutes a step forward in the design of energetically-efficient spintronic devices. Advances in this area also enable the realization and experimental detection of new forms of collective behavior like spin superfluidity: nearly dissipationless, coherent spin transport mediated by topologically stable textures. In this talk, I will discuss this phenomenology in the case of amorphous or polycrystalline magnets. The key observation is that structural disorder in this class of materials can average out the effect of anisotropies inhibiting the onset of spin superfluidity. I will present a hydrodynamical description of the flow of angular momentum injected by spin accumulation in adjacent heavy metals. The theory is valid for a broad class of magnets with frustrated interactions, where spin currents are defined in terms of coherent rotations of a non-collinear texture. Their stability, ascribed to the topology of the emergent SO(3) order parameter, resembles the phenomenology of chiral superfluids like 3He-A or spinor condensates with ferromagnetic order. Host: Ar. Abanov

7 Sep 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Anton Burkov, University of Waterloo
Speaker: Anton Burkov, University of Waterloo Title: Magnetotransport in topological metals. Abstract: Topological metals continue to attract attention as novel gapless states of matter. While there by now exists an exhaustive classification of possible topologically nontrivial metallic states, their observable properties, that follow from the electronic structure topology, are less well understood. In this talk I will present my recent work on magnetotransport phenomena in topological metals, which may be related to the chiral anomaly. I will demonstrate that the chiral anomaly leads to strong anisotropic magnetoresistance in such materials, which manifests in very unusual negative longitudinal magnetoresistance and planar Hall effect. I will also argue that a smoking-gun feature of the chiral anomaly in topological metals is the existence of propagating chiral density modes even in the regime of weak magnetic fields. Finally, I will show that the optical conductivity of such metals exhibits an extra peak, which exists on top of the standard metallic Drude peak. The spectral weight of this peak is transferred from high frequencies and its width is proportional to the chiral charge relaxation rate. Host: A. Belyanin

14 Sep 2018, 4:00PM | MIST M102
Hosted By: A. Belyanin

Pengcheng Dai, Rice
Speaker: Pengcheng Dai, Rice Title: Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe1−xNixAs Abstract: Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe1−xNixAs, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature Ts in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie–Weiss temperature dependence and become suppressed below the superconducting transition temperature Tc, suggesting that they result from the large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry breaking above Ts, and attest to the presence of significant nematic fluctuations near optimal superconductivity. Host: Ar. Abanov

21 Sep 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Evert van Nieuwenburg, caltech
Speaker: Evert van Nieuwenburg, caltech Title: How confused is my neural network? Abstract: There is information to be gained by studying how well a neural network is able to learn physics data. I will discuss the 'learning by confusion' method and its extension to 'discriminative cooperative networks'. Both of these methods rely on the ability to order physics data along one or multiple tuning parameters, which is a feat that is often impossible in non-physics data-sets. In the second part, I will describe how reinforcement learning methods may help in quantum error decoding/correction. Host: Helmut Katzgraber

28 Sep 2018, 4:00PM | MIST M102
Hosted By: Helmut Katzgraber

Sergey Frolov, University of Pittsburgh
Speaker: Sergey Frolov, University of Pittsburgh Title: Status of the Search for Majorana Fermions in Semiconductor Nanowires Abstract: Majorana fermions are non-trivial quantum excitations that have remarkable topological properties and can be used to protect quantum information against decoherence. Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the topological phase transition. A potent alternative explanation for many of the recent experimental Majorana reports is that a non-topological Andreev state localizes near the end of a nanowire. We compare Andreev and Majorana modes and investigate ways to clearly distinguish the two phenomena. We are also exploring how Andreev states can be chained together along the nanowire to realize the one-dimensional Kitaev model, a discrete way of generating Majorana modes. Host: Ar. Abanov

5 Oct 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

CANCELED. Aashish Clerk, the University of Chicago
Speaker: CANCELED. Aashish Clerk, the University of Chicago Title: Photonic analogues of topological superconductors Abstract: Interest continues to grow in photonic and phononic analogues of topological electronic phases. These systems are typically non-interacting, and have the same band structure and edge state dispersion as their fermionic counterparts. In this talk I’ll discuss recent theory work in my group on a class of bosonic systems where this correspondence fails. They involve using parametric “two-photon” driving, and have Hamiltonians that superficially resemble those of topological superconductors. Among the surprising effects that emerge are the presence of topologically-protected instabilities that can be harnessed for non-reciprocal quantum amplification, and effective non-Hermitian dynamics in a bosonic analogue of the Kitaev-Majorana chain. I’ll discuss how these ideas could be realized in a variety of different experimental platforms, including superconducting quantum circuits and coupled arrays of optical resonators. Host: Ar. Abanov

12 Oct 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Sara Haravifard, Duke University
Speaker: Sara Haravifard, Duke University Title: To order or not to order – Solving the Ground State of Geometrically Frustrated Magnets Abstract: The interactions that define how spins arrange themselves in a material play a fundamental role in a wide variety of physical phenomena. Frustrated quantum magnets are systems for which the exchange interactions governing the interacting spins cannot be simultaneously satisfied, leading to a highly degenerate ground state and new states of matter such as quantum spin liquid. In this talk I will discuss how chemical doping, external pressure and application of magnetic field regulates the underlying electronic and magnetic interactions in these systems, ultimately driving the ground state across the phase diagram and leading to emergent quantum critical phenomena. For examples, I will present our recent magnetometry, thermal transport, xray and neutron scattering studies performed under extreme sample environments on 2D AFM triangular and Shastry-Sutherland systems, as well as on 3D breathing pyrochlore system. Host: Ar. Abanov

19 Oct 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Lena Kourkoutis, Cornell
Speaker: Lena Kourkoutis, Cornell Title: Visualizing Atomic Lattice Behavior in Charge Ordered Materials Abstract: Charge-ordered phases permeate the phase diagrams of strongly correlated systems such as layered transition-metal dichalcogenides, colossal magnetoresistive manganites and cuprate high-temperature superconductors. In such charge ordered states, modulations of the electron density break lattice translational symmetry and induce periodic lattice displacements via electron–lattice coupling. Bulk measurements have shown complex interactions between charge order and electronic phases, including direct competition with superconductivity or mediation of colossal magnetoresistance, however, the microscopic structure and origin of charge-ordered states remains actively debated. In this talk, I will discuss room temperature and cryogenic scanning transmission electron microscopy techniques that have enabled direct mapping of the nature and evolution of charge order in manganites. We measure picometer-scale displacive modulations of the cations, distinct from existing manganite charge-order models, and reveal temperature-dependent phase inhomogeneities in the modulations, such as shear deformations and topological defects. At temperatures well below Tc phase coherence emerges. Our observations underscore the importance of lattice locking in manganites and provide a visualization of the real space structure of incommensurate order. More broadly, cryogenic STEM can now be used to investigate and quantify the role of the lattice in a variety of low temperature electronic phases. Host: Ar. Abanov

26 Oct 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Sebastian Deffner, University of Maryland Baltimore County
Speaker: Sebastian Deffner, University of Maryland Baltimore County Title: Fluctuation theorems go Quantum Computers Abstract: Near term quantum hardware promises unprecedented computational advantage. Crucial in its development is the characterization and minimization of computational errors. We will review recent developments in informational quantum fluctuation theorems and propose the use of them to characterize the performance of quantum annealers. We will see that this versatile tool provides simple means to determine whether the quantum dynamics are unital, unitary, and adiabatic, or whether the system is prone to thermal noise. Our proposal was experimentally tested on two generations of the DWave machine, which illustrates the sensitivity of the fluctuation theorem to the smallest aberrations from ideal annealing. Host: Ar. Abanov

2 Nov 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Wayne Saslow, TAMU
Speaker: Wayne Saslow, TAMU Title: Theory of near-equilibrium response of Ferromagnets Abstract: The area of spintronics — so-called because it utilizes the electrical and spin (magnetic) properties of electrons — has its origins in the 1987-1988 discovery of Giant MagnetoResistance. GMR refers to the dependence of the electrical resistance on the relative orientation of the magnetization in two nearby ferromagnets. It is commonly used in one form or another in magnetic read devices, and yielded the 2007 Nobel Prize for Fert and Grunberg. In 1996, as a method to cause magnetic writes, Slonczewski proposed sending a large enough spin polarized electric current from ferromagnet 1 to ferromagnet 2, thereby flipping magnetization M2. This is known as spin transfer. With Berger he received the 2013 Buckley Prize from the APS for this idea. Despite the observational success of Slonczewski’s idea it comes with an unnecessary approximation — that the transfer of spin takes place almost immediately when the spin current enters the ferromagnet. This well known approximation has been accepted as a model for conductors, where the adjustment indeed takes place over a nm scale or smaller. However, for ultrathin conducting magnets the adjustment will not be complete, and for insulating magnets the adjustment takes place over a large enough scale that it cannot necessarily be neglected. Using the methods of irreversible thermodynamics, for small amplitude disturbances Chen Sun and I find that near equilibrium the usual magnetization M tips and the transverse magnetization of the spin excitations, called the spin accumulation m, couple, yielding two sets of coupled transverse modes. By varying the thickness of experimental samples the associated lengths should be observable. Extensions of this work will be made to other types of magnetic system, such as antiferromagnets and spin glasses, which have shown promising behavior in spintronics studies. Host: W. Saslow

9 Nov 2018, 4:00PM | MIST M102
Hosted By: W. Saslow

Eugene Mishchenko, University of Utah
Speaker: Eugene Mishchenko, University of Utah Title: The phenomenon of minimal conductivity of graphene Abstract: Graphene is a two-dimensional crystal of carbon atoms arranged in a honeycomb lattice. Graphene happens to be a semi-metal that can, in some instances, display insulating properties yet reveal metallic behavior in others. For example, conductivity of graphene is predicted by the band theory to be metallic. In contrast, screening of Coulomb interaction between electrons is expected to be rather weak, very much like in typical insulators. Accordingly, one would expect interactions to be strong and result in significant corrections to the conductivity. Surprisingly, experiments show little such corrections, if any. I will discuss theoretical efforts expended over the last decade to understand this phenomenon. Host: Ar. Abanov

16 Nov 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Speaker: THANKSGIVING Title: Abstract: Host:

23 Nov 2018, 4:00PM | MIST M102

Ivan Borzenets, City U. of Hong Kong
Speaker: Ivan Borzenets, City U. of Hong Kong Title: Graphene-Superconductor Hybrids: towards graphene-based topological computing devices. Abstract: The current push towards topological computing specifically and scalable quantum computing in general has put great pressure towards the study of low dimensional nano-materials coupled with superconductors. Such hybrid systems are predicted to support novel topological quantum states such as: Majorana and Parafermions. While the phase-space of low dimensional nanomaterials is vast, graphene (a single layer of crystalline carbon) remains as the flagship material to this day. Here, we discuss the physics of graphene coupled to a superconducting material in the ballistic (ultra-clean) regime as well as the diffusive (dirty) regime. We discuss the governing parameters of such devices and comment on the apparent discrepancies with current theories. Moving beyond the conventional Josephson junction format, we look at more complicated supercurrent flow. Namely, the coexistence of normal and super-currents in the same physical space, and the observation of supercurrent in the Quantum Hall regime. Finally, future potential work is briefly discussed. Host: Ar. Abanov

30 Nov 2018, 4:00PM | MIST M102
Hosted By: Ar. Abanov

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