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Condensed Matter Seminar

Joe Ross, TAMU
Speaker: Joe Ross, TAMU Title: NMR studies of Thermoelectric Materials Abstract: In this talk I will review some of our recent and current work, with a focus on thermoelectric materials, and in particular the understanding of these materials that can be obtained from NMR studies. In recent years there has been a concerted effort to develop new thermoelectric materials systems in an effort toward improved energy recovery and efficiency, as well as for microelectronic and related applications. Among the requirements are semiconducting solids with low lattice thermal conductivity, and high electron mobility. In this process there have been a number of recent developments in understanding such materials including phonon interactions and anharmonic vibrational properties. In my lab we use several experimental techniques to analyze such materials, with NMR as a primary tool providing a local probe of electronic and vibrational properties. I will discuss how we use these techniques to analyze such behavior, and focus on some recent results in Ba8Ga16Sn30, a clathrate semiconductor, and in Cu2-xSe and related materials, a superionic conductor system. Host: J. Ross

23 Jan 2015, 4:00PM | MIST M102
Hosted By: J. Ross

Vladimir L Safonov
Speaker: Vladimir L Safonov Title: Nonequilibrium Magnons and Energy Transformations in Nano-Magnetic Systems. Abstract: shall discuss non-equilibrium magnetic excitations (magnons) in magneto-ordered systems (particles and films). A universal language of bose-gas in a nonlinear system will be used to help finding possible analogies in other nonlinear systems. Two mechanisms of microwave energy transformation of the noisy pumping to the coherent magnetic signal at room temperature will be discussed. One is the Bose-Einstein condensation of quasi-equilibrium magnons (the phenomenon was experimentally observed about 7 years ago), and another is the nonlinear noise squeezing by the microwave resonator (the phenomenon was discovered about 20 years ago). We shall also briefly consider a possibility and advantages of developing parallel processing of digital operations using the long-lived excitations in nano-systems. Host: W. Saslow

30 Jan 2015, 4:00PM | MIST M102
Hosted By: W. Saslow

Winfried Teizer, TAMU
Speaker: Winfried Teizer, TAMU Title: Molecular Motors - A Different Kind of Transport Abstract: Nature has generated sophisticated and complex molecular motors, employed for nanoscale transport at the intracellular level. As a complementary tool to nanofluidics, these motors have been envisioned for nanotechnological devices. In order to pave the way for such applications, a thorough understanding of the mechanisms governing these motors is needed. Because of the complexity of their in-vivo functions, this understanding is best acquired in-vitro, where functional parameters can independently be controlled. I will report on work in my group that studies and harnesses the transport properties of molecular motors on functionalized structures of reduced dimensionality, such as carbon nanotubes, 1 lithographically designed electrodes 2 and microwires. 31 Molecular Motor-Powered Shuttles along Multi-walled Carbon Nanotube Tracks. A. Sikora et al. Nano Lett. 14, 876-881 (2014).2 Surface Manipulation of Microtubules Using Self-Assembled Monolayers and Electrophoresis. J. A. Noel et al., ACS Nano 3, 1938 (2009).3 Microtubule shuttles on kinesin-coated glass micro-wire tracks. K. Kim et al. Biomed. Microdev., 16, 501-508 (2014). Functional Localization of a Kinesin/Microtubule-Based Motility System along Metallic Glass Microwires. K. Kim et al. Appl. Phys. Lett. 105, 143701 (2014). Host: W. Teizer

6 Feb 2015, 4:00PM | MIST M102
Hosted By: W. Teizer

Konstantin Tikhonov, TAMU
Speaker: Konstantin Tikhonov, TAMU Title: Spectral non-uniform temperature, non-local heat transfer, and the spin Seebeck effect Abstract: Recently discovered spin-dependent thermoelectric effects have merged spin, charge, and thermal physics, known as spin caloritronics, of which the spin Seebeck effect is its most puzzling. Here we present a theory of this effect driven by subthermal non-local phonon heat transfer and spectral non-uniform temperature. The theory explains its non-local behavior from the fact that phonons that store the energy (thermal) and the phonons that transfer it (subthermal) are located in different parts of the spectrum and have different kinetics. This gives rise to a spectral phonon distribution that deviates from local equilibrium along the substrate and is sensitive to boundary conditions. The theory also predicts a non-magnon origin of the effect in ferromagnetic metals in agreement with observations in recent experiments. Equilibration of the heat flow from the substrate to the Pt probe and backwards leads to a vertical spin current produced by the spin-polarized electrons dragged by the thermal phonons Host: A. Finkelstein

13 Feb 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

Keji Lai, UT Austin
Speaker: Keji Lai, UT Austin Title: Nanoscale Electrical Imaging of Novel Quantum Materials Abstract: The research of complex quantum materials, in which a dazzling number of emergent phenomena take place in the nanoscale, is a major theme in modern condensed matter physics. For real-space imaging of complex systems, electrical impedance microscopy fills an important void that is not well represented by the existing local probes. Using shielded cantilever probes and sensitive microwave electronics, we can now perform non-invasive electrical imaging with sub-100nm resolution and sub-aF sensitivity. In this talk, I will focus on the visualization of metal-insulator transition of functional materials in field-effect-transistor configurations, which is enabled by the sub-surface imaging capability of impedance microscopy. Local probing on the anomalous conduction in multiferroic domain walls will also be discussed. Host: Ar. Abanov

20 Feb 2015, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Georg Schwiete, Johannes Gutenberg Universitat Mainz
Speaker: Georg Schwiete, Johannes Gutenberg Universitat Mainz Title: Transport of heat in the disordered Fermi and electron liquids Abstract: The main subject of this talk is thermal transport in the disordered Fermi and electron liquids at low temperatures. I plan to start with a brief introduction to the physics of quantum corrections to conductivity in disordered systems and the phenomenology of the metal-insulator transition in two-dimensional electron systems. Then, I will contrast approaches to the calculation of electric and thermal transport. A principle difficulty for the description of thermal transport is that temperature is an internal parameter, and a temperature gradient does not correspond to an external "mechanical" force like a the one originating from an electric potential. This problem becomes particularly severe when the description of low temperature phenomena requires a renormalization group (RG) analysis, which is hard to implement in a kinetic equation approach. We therefore use Luttinger's gravitational potentials as sources for finding the heat density and its correlation function. For a comprehensive study, we extend the RG analysis developed for electric transport by including the gravitational potentials into the RG scheme. The analysis reveals that for the disordered Fermi liquid the Wiedemann-Franz law remains valid even in the presence of quantum corrections caused by the interplay of diffusion modes and the electron-electron interaction. In the present scheme this fundamental relation is closely connected with a fixed point in the multi-parametric RG flow of the gravitational potentials. For the disordered electron liquid we additionally analyze inelastic processes induced by the Coulomb interaction at sub-temperature energies. While the general form of the correlation function has to be compatible with energy conservation, these inelastic processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. The interplay of various terms in the heat density-heat density correlation function therefore differs from that for densities of other conserved quantities, such as the total number of particles or spin. References: G. Schwiete and A. M. Finkel'stein, Renormalization group analysis of thermal transport in the disordered Fermi liquid, Phys. Rev. B 90, 155441 (2014) G. Schwiete and A. M. Finkel'stein, Thermal transport and the Wiedemann Franz law in the disordered Fermi liquid, Phys. Rev. B 90, 060201(R) (2014) G. Schwiete and A. M. Finkel'stein, Keldysh approach to the renormalization group analysis of the disordered electron liquid, Phys. Rev. B 89, 075437 (2014) Host: A. Finkelstein

27 Feb 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

APS March Meeting,
Speaker: APS March Meeting, Title: Abstract: Host:

6 Mar 2015, 4:00PM | San Antonio, Tx

Nic Shennon, Okinawa Institute of Science and Technology
Speaker: Nic Shennon, Okinawa Institute of Science and Technology Title: Quantum Spin Ice Abstract: Spin ice, with its magnetic monopole excitations, is perhaps the outstanding example a classical, topological spin liquid. Nonetheless, the role of quantum effects in spin-ice materials remains poorly understood. This question gain fresh urgency from studies of "quantum spin-ice" materials such as Yb2Ti2O7 [1,2] and Pr2Zr2O7 [3], and recent experiments which suggest that the spin ice Dy2Ti2O7 may undergo a phase transition at very low temperature [4].

In this talk, we explore some of the new phenomena which can arise as a result of quantum fluctuations in a spin-ice material. We show how quantum tunnelling between different spin-ice configurations can convert spin-ice into a quantum spin liquid with photon-like excitations [5], review the numerical evidence that such a state exists [6-9], and discuss how it might be identified in experiment [8,9].

We also consider the nature of the quantum ground state in a realistic model of spin ice, directly motivated by Dy2Ti2O7. We identify the principles which govern magnetic order in the presence of long-range dipolar interactions, and use quantum Monte Carlo simulation to show that only a very small amount of quantum tunnelling is needed to convert these ordered states into a quantum spin liquid [10].

[1] K. Ross et al., Phys. Rev. X 1, 021002 (2012).[2] L.-J. Chang et al., Nature Commun. 3, 992 (2012)
[3] K. Kimura et al., Nature Commun. 4, 1934 (2013)
[4] D. Pomaranski et al., Nature Phys. 9, 353 (2013).
[5] M. Hermele et al., Phys. Rev. B 69, 064404 (2004).
[6] A. Banerjee et al., Phys. Rev. Lett. 100, 047208 (2008)
[7] N. Shannon et al., Phys. Rev. Lett. 108, 067204 (2012).
[8] O. Benton et al., Phys. Rev. B 86, 075154 (2012).
[9] Y. Kato et al., arXiv:1411.1918
[10] P. McClarty et al., arXiv:1410.0451 Host: host

13 Mar 2015, 4:00PM | MIST M102
Hosted By: host

Daniel Packwood, Advanced Institute of Materials Research, Tohoku University, Japan
Speaker: Daniel Packwood, Advanced Institute of Materials Research, Tohoku University, Japan Title: Magnetic stability and relaxation in single-molecule magnets: Insights from new stochastic theories Abstract: Single molecule magnets (SMMs) are molecules possessing large magnetic moments due to coupling between the electron spins of transition metal ions. When SMMs are deposited onto a metal surface they tend to undergo uncontrolled structural deformations, which weakens the interactions between transition metal ions and reduces the magnetic moment. This is one reason why SMMs are yet to be used in real applications. We have been exploring the effect of structural deformations on the magnetic properties of SMMs using stochastic models. In the first part of this presentation, a Heisenberg-type model with randomly fluctuating exchange couplings between transition metal ions will be discussed. Analysis of this model predicts that SMMs containing 20 - 50 transition metal ions arranged in a small number of ring-type configurations possess -weak topological invariant- magnetic moments. This means that the magnetic moments only depend upon how the transition metals in the molecule are arranged, and are very insensitive to deformations in the shape of the molecule (Proc. Roy. Soc. A. 469, 2013, 20130373). The second part of this presentation will introduce the stochastic Boltzmann equation approach to studying magnetic relaxation in SMMs. This equation gives the time-evolution of the probability density of the density matrix for the spin state of the molecule, and assumes that the vibrational modes undergo a stochastic frequency modulation due to coupling with the environment. A trajectory simulation method for solving the stochastic Boltzmann equation under a new -relaxation time approximation- will be outlined, and a calculation for the important case of the Mn12O12(O2CCH3)16 molecule will be presented. It will be shown that the stochastic Boltzmann equation is an efficient method for studying the competition between thermal relaxation and spin tunneling in the magnetic relaxation of SMMs under a variety of parameter regimes. This is joint work with Kelley T. Reaves (Lynntech, formerly Texas A&M University and Advanced Institute for Materials Research, Tohoku University, Japan), Filippo F. Federici (Helsinki University), Ikutaro Hamada (National Institute for Materials Research, Japan), Helmut G. Katzgraber (Texas A&M University and Santa Fe Institute), and Winfried Teizer (Texas A&M University and Advanced Institute for Materials Research, Tohoku University, Japan) Host: W. Teizer

27 Mar 2015, 4:00PM | MIST M102
Hosted By: W. Teizer

Rebecca Flint, Iowa State University
Speaker: Rebecca Flint, Iowa State University Title: Hidden (hastatic) order in URu2Si2: hybridization with a twist Abstract: 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. Host: Ar. Abanov

3 Apr 2015, 4:00PM | MIST M102
Hosted By: Ar. Abanov

W. Saslow, TAMU
Speaker: W. Saslow, TAMU Title: A Tale of Two Theories (for Non-Experts) -- The Spin Hall Effect and its Inverse Abstract: We discuss both the Dyakonov and Perel theory (1971) and the Hirsch hand-waving physical reasoning (1979) that leads to the spin Hall effect (SHE) and to the inverse spin Hall effect (ISHE).
(1) In the SHE an electric current along a conducting wire or strip produces a transverse "spin accumulation" (excess or deficit of spin, relative to equilibrium). Sometimes this is called "spin voltage". (2) In the ISHE a spin current along a wire or strip produces a transverse electrical voltage. Both theory and experiment have been active in this area for the past fifteen years.
We will show both how the anomalous Hall effect coefficient (AHE) follows from the DP theory, and that DP theory (based on a kinetic theory hidden from the reader) is largely consistent with the more stringent requirements of the Onsager relations. Time-permitting we will discuss a new effect second order in the spin-orbit parameter (spin-Hall angle). Host: W. Saslow

10 Apr 2015, 4:00PM | MIST M102
Hosted By: W. Saslow

Alexander Chaplik, Institute of Semiconductor Physics, Novosibirsk, Russia
Speaker: Alexander Chaplik, Institute of Semiconductor Physics, Novosibirsk, Russia Title: ELECTROSTATIC SCREENING IN NANOSTRUCTURES Abstract: Part 1. Screening by charged fermions ( electrons). Nanotubes, double quantum wells, multilayer superlattices. Matrix dielectric functions.
Part 2. Screening by neutral bosons (spatially indirect dipolar excitons). It will be shown that screening of a Coulomb impurity center by bosons depends much stronger on the concentration than in the Fermi case. The screening changes drastically when the dipolar excitons form the Bose-Einstein condencate. This results from the Gross-Pitaevsky equation. Host: V. Pokrovsky

17 Apr 2015, 4:00PM | MIST M102
Hosted By: V. Pokrovsky

Ilya Vekhter, LSU
Speaker: Ilya Vekhter, LSU Title: Orbital order and Hunds rule frustration in Kondo lattices Abstract: Origin of exotic orders in correlated systems have been a long-standing focus of research. It is generally believed that Kondo screening that gives rise to the heavy fermion state competes with both magnetic and orbital order. In this talk I will give an example of an opposite trend. I will analyze microscopic origin of the Kondo effect-assisted orbital order in heavy-fermion materials. I will show that, for periodic two-orbital Anderson model with two local electrons, frustration of local Hunds rule coupling due to Kondo screening leads to an incommensurate spiral orbital and magnetic order, which exists only inside the Kondo (heavy-electron) phase. This spiral state can in principle be observed in measurements in U and Pr-based heavy-fermion compounds, and realized in cold atomic gases, e.g. fermionic 173Yb. I will discuss the implications of this result for the phase diagram of such systems. Host: Ar. Abanov

24 Apr 2015, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Andrea Young, MIT
Speaker: Andrea Young, MIT Title: Layer-resolved thermodynamic probe of charge order in bilayer graphene Abstract: Bilayer graphene is a highly tunable electronic system in which electric fields can be used to control both the carrier density as well as the electronic structure. Like its monolayer cousin, the bilayer graphene Landau levels are characterized by approximate spin and valley degeneracy; unlike monolayer, however, the three dimensional structure of the bilayer allows control of the sublattice splitting with a perpendicular electric field. This feature has been used extensively to probe the phase diagram of interacting electrons, particularly within the zero energy Landau level, revealing a number of interacting states characterized by spin and/or valley order. Typically, however, the spin or valley order is inferred indirectly by varying conjugate fields and inferring the order from the resulting changes in conductivity. Here I will describe a technique capable of resolving layer-polarization directly through high sensitivity capacitance measurements. The measurements confirm the known features of the bilayer graphene phase diagram, while revealing several new phases and a series of sharp features associated with phase transitions between states of different layer polarization. These features suggest a new mechanism for inversion symmetry breaking in Bernal stacked bilayer graphene. Host: Ar. Abanov

1 May 2015, 4:00PM | MIST M102
Hosted By: Ar. Abanov

CANCELED: Maria Iavarone, Temple University
Speaker: CANCELED: Maria Iavarone, Temple University Title: Emergence of Superconductivity and Vortex Confinement in Superconductor/Ferromagnet Hybrid Systems Abstract: Magnetically coupled superconductor-ferromagnet systems have been studied by low temperature Scanning Tunneling Microscopy and Spectroscopy. The stray field of the ferromagnet induces a non-uniform superconducting state characterized by a local superconducting critical temperature Tc and a non monotonic behavior of Tc vs H close to the critical temperature [1,2]. We studied Pb/[Co/Pd] systems and we visualized the emergence of superconductivity in regions above the separation between adjacent magnetic domains, as well as reverse domain wall superconductivity. Moreover, deep in the superconducting state vortices of opposite polarity are induced by the stray field of the ferromagnet in zero applied external field and they are strongly confined on the stripe of the same polarity. The nucleation of spontaneous vortex-antivortex strongly depends on the domain width [3]. Our results demonstrate that such S/F structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nuclei.

[1] Yang Z., Lange M., Volodin A., Szymczak R., and Moshchalkov V.V., Nature Mater. 3, 793 (2004)
[2] Aladyshkin A. Yu., Buzdin A. I., Fraerman A.A., Melnikov A.S., Ryzhov D.A., and Sokolov A. V., Phys. Rev. B 68, 184508 (2003)
[3] M. Iavarone, S.A. Moore, J. Fedor, S. T. Ciocys, G. Karapetrov, J. Pearson, V. Novosad, and S. D. Bader, under Review Nature Communications 5, 4766 (2014) Host: D. Naugle

8 May 2015, 4:00PM | MIST M102
Hosted By: D. Naugle

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