Rice University
Department of Physics & Astronomy

Condensed Matter Seminars
2008 – 2009

Where: HZ 116
When: Mondays at 4:00 p.m.

September 8, 2008
Thermodynamic Study of the Supersolid Phase Transition
Dr.  Xi Lin
Penn State University
 Liquid 4He enters the superfluid state and flows without friction below 2.176 K. Thin liquid films adsorbed on solid substrates undergo the same transformation, although at a lower temperature. When the substrate is subjected to oscillatory motion a portion of the film, known as the superfluid fraction, decouples from the oscillation. A similar phenomenon has been observed in solid 4He, in which a fraction of the solid seems to decouple from the motion of the surrounding lattice. This observation has been replicated in various laboratories. In addition, a heat capacity peak, on top of the phonon contribution, that coincides with the onset of mass decoupling has been found. The excess specific heat peaks with a magnitude that decreases with increasing sample quality, in a way similar to the NCRI sample quality effect.
September 29, 2008
One-dimensional fermions beyond the Luttinger liquid paradigm
Prof.  Michael Pustilnik
Georgia Tech
In this talk I will review recent works on dynamic correlation functions in one-dimensional systems of interacting fermions. Due to the energy and momentum conservation, the correlation functions exhibit a characteristic threshold behavior. Surprisingly, the conventional harmonic fluid (a.k.a. Luttinger liquid) framework is not sufficient for the description of the threshold singularities in the correlation functions.

October 10, 2008
Nonequilibrium Effects Near Itinierant Electron Quantum Critical Points
Prof. Aditi Mitra
New York University
An important set of questions in condensed matter relate to the effect of a nonequilibrium drive on a system near a quantumcritical point. In this talk results will be presented for the effect of current flow on two systems: one near a ferromagnetic-paramagnetic quantum critical point, and the other near a superconductor-metal quantum critical point. For a ferromagnetic system it will be shown that current flow has two important effects. One is to produce decoherence that affects the system in ways rather similar to temperature. Second, it causes an inversion symmetry breaking which qualitatively has the effect of producing a drift. For Ising magnets decoherence is the dominant effect, whereas for Heisenberg magnets the inversion symmetry breaking due to current flow can lead to dynamical instabilities of the ordered phase. For a superconducting order-parameter nonequilibrium effects are even more pronounced as the order parameter is charged and couples directly to the electric field. For these systems it will be shown that it is the direct coupling which has the dominant effect on scaling near a critical point, with the effect of current induced drift and decoherence being subdominant.
October 20, 2008
Multi-channel Kondo Models in non-Abelian Quantum Hall Droplets
Prof. Gregory Fiete
UT Austin
The study of topological states of matter has generated much interest recently on both the theoretical and experimental fronts.  The most well established topological states are the quantum Hall states.  In this talk we study the coupling between a quantum dot and the edge of a non-Abelian fractional quantum Hall state which is spatially separated from it by an integer quantum Hall state.  Near a resonance, the physics at energy scales below the level spacing of the edge states of the dot is governed by a $k$-channel Kondo model when the quantum Hall state is a Read-Rezayi state at filling fraction $\nu=2+k/(k+2)$ or its particle-hole conjugate at $\nu=2+2/(k+2)$. The $k$-channel Kondo model is channel isotropic even without fine tuning in the former state; in the latter, it is generically channel anisotropic. In the special case of $k=2$, our results provide a new venue, realized in a mesoscopic context, to distinguish between the Pfaffian and anti-Pfaffian states at filling fraction $\nu=5/2$.
October 27, 2008
Magnetotransport in Microwave-irradiated Quantum Hall Systems
Prof. Michael Zudov
University of Minnesota

When a high quality two-dimensional electron system is irradiated by microwaves, its magnetoresistance exhibits microwave-induced resistance oscillations (MIRO) and zero-resistance states. MIRO were discussed in terms of the “displacement” model, which is based on microwave-assisted impurity scattering, and the “inelastic” model, stepping from the oscillatory electron distribution function. It is believed that the “inelastic” contribution greatly exceeds the “displacement” contribution and can also account for MIRO temperature dependence. This talk will discuss our recent experiments focusing on non-linear response and temperature dependence of MIRO. First, we show that the ‘’displacement’’ mechanism cannot be ignored and might even dominate the response under typical experimental conditions. Further, we find that the MIRO temperature dependence originates primarily from the temperature-dependent quantum lifetime entering the Dingle factor. We suggest that the main source of the modification of the quantum scattering rate is the contribution from electron-electron scattering, a quantity hardly available from other experiments.
November 10, 2008
Fe-As based superconductors: A new class of high-Tc superconducting compounds
Prof. Bernd Lorenz
University of Houston

The discovery of superconductivity in rare earth (R) oxypnictides, ROFeAs, by Hosono et al. has revived the field of high-temperature superconductivity. With transition temperatures of up to 55 K the new class of superconducting compounds has given hope to reach even higher Tc’s exceeding those of the copper oxide superconductors. At the same time, questions have been raised concerning possible similarities of the two high-Tc systems with the perspective that studying the FeAs superconductors might also help to better understand the cuprates.

I will present a brief overview of some recent results and discuss examples of FeAs-based superconductors crystallizing in different basic structure types: (i) The PbFCl-type structure (LiFeAs) and (ii) the ThCr2Si2 -type structure (KF e2As2, CsFe2As2, and the solid solution (K/Sr)Fe2As2). The ternary compounds are all self-doped superconductors. The (K/Sr)Fe2As2 – system reveals an interesting phase diagram, similar to  the high-Tc cuprates, with a maximum Tc at an optimal composition and a spin density wave (SDW ) state at the Sr-rich side. In a narrow composition range the SDW state is followed by a superconducting transition at lower temperatures. The extrapolation of the SDW phase boundary suggests the possible existence of a quantum critical point.

November 17, 2008
The heat generation by electric current in nano-devices
Prof. Xincheng Xie
 Oklahoma State University

Over the past two decades, with the development of micro-machining technology, there has been enormous progress in the operation speed and integrating techniques in the semiconductor electronics. With these advancements, one critical issue arises, namely the dissipation of the conducting electrons (i.e., the thermal generation from electric currents). The dissipation in nano-devices strongly hinders further development of the semiconductor electronics. Thus, it is important to uncover the laws of thermal generation induced by electric currents in nano-devices.
In this work, we study the heat generation in a nano-device with an electric current flowing through. A general formula for the heat generation is derived by using the non-equilibrium Keldysh Green's functions. This formula can be applied in the linear and non-linear transport regions, for time-dependent cases, and with multi-terminal systems. The formula is also valid if the nano-device contains various interactions. As an application of the formula, the heat generation of the device with lead-quantum dot-lead is investigated. The dc and the ac biases are studied in detail. We find several interesting behaviors that are unique to nano-devices, revealing significant difference from heat generation in macroscopic systems.
November 21, 2008
Crystal Growth at CEA Grenoble
Prof. Gerard Lapertot
CEA Grenoble

I will review various material synthesis techniques and various aspects of a flux growth experiments at CEA Grenoble. Crystal growth of CeIn3, MnSi, PrOs4Sb12 or YbCu2Si2 will be used as examples. This will be followed by short presentation about mirror furnace crystal growths, their use and recent developments. Finally I will outline components of "ideal/optimized" crystal growth lab, including characterization techniques.
November 24, 2008
Hydrodynamic Transport in Graphene
Dr. Lars Fritz
Harvard Univeristy
We study the thermal and electric transport of a fluid of interacting Dirac fermions  using a quantum Boltzmann approach. We include Coulomb interactions, a dilute density of  charged impurities and the presence of a magnetic field to describe both the static and  the low frequency response as a function of temperature $T$ and chemical potential $\mu$. In the quantum-critical regime $\mu\lesssim T$ we find pronounced deviations from Fermi  liquid behavior, such as a collective cyclotron resonance with an intrinsic,  collision-broadened width, and significant enhancements of the Mott and Wiedemann-Franz  ratio. Some of these results have been anticipated by a relativistic hydrodynamic theory,  whose precise range of validity and failure at large fields and frequencies we determine.  The quantum Boltzmann approach allows us to go beyond the hydrodynamic regime, and to  quantitatively describe the deviations from magnetohydrodynamics, the crossover to  disorder dominated Fermi liquid behavior at large doping and low temperatures, as well as the crossover to the ballistic regime at high fields. Finally, we obtain the full frequency and doping dependence of the single universal  conductivity $\sigma_Q$ which parametrizes the hydrodynamic response.
December 1, 2008
Recent progress in neutron scattering studies of FeAs-based superconductors
Prof. Pengcheng Dai
University of Tennessee

In this seminar, I will summarize our recent neutron scattering work focused on studying structural and magnetic properties of FeAs-based superconductors [1-9]. Forneutron diffraction, we systematically study the structural and magnetic phase transitions in various FeAs-based materials and compare them with recently discovered Fe(Se,Te) family of materials. For inelastic neutron scattering, we focus on low-energy spin wave excitations and discuss magnetic exchange coupling. Crystal field level excitations sensitive to the spin fluctuations in the FeAs layer will also be discussed. Comparison will be made between our neutron scattering work and current theoretical understanding of these materials.
[1] Clarina de la Cruz et al., Nature 453, 899-902 (2008).
[2] Jun Zhao et al., Nature Materials (AOP,
doi:10.1038/nmat2315).
[3] Ying Cheng et al., Phys. Rev. B 78, 064515 (2008).
[4] Q. Huang et al., Phys. Rev. B 78,
054529 (2008).
[5] Jun Zhao, et al, Phys. Rev. B 78, 140504 (R) (2008).
[6] Jun Zhao et al., Phys. Rev. B 78, 132504 (2008). 
[7] Jun Zhao et al., Phys. Rev. Lett. 101, 167203 (2008)
[8] S. Chi et al., Phys. Rev. Lett. (in press).
[9] Shiliang Li et al., arXiv:0811.0195.
January 12, 2009
Local electronic properties of graphene
Prof. Brian Leroy
University of Arizona

Combining scanning probe microscopy with electrical transport measurements is a powerful approach to probe low-dimensional systems.  The local information provided by scanning probe microscopy is invaluable for studying effects such as electron-electron interactions and scattering.  Using this approach, we have probed the local electronic properties of graphene with atomic resolution.  We studied the effect of ripples, charged impurities and defects on the local density of states.  We find that long-range scattering from ripples and impurities shifts the Dirac point leading to electron and hole puddles.  Short-range scattering from lattice defects mixes the two sublattices of graphene and tends to be strongly suppressed away from the Fermi energy.
January 20, 2009
Time-reversal symmetry breaking and spontaneous anomalous Hall effect in Fermi fluid
Prof. Kai Sun
University of Illinois at Urbana Champaign
We study the spontaneous non-magnetic time-reversal symmetry breaking in a 2D Fermi liquid without breaking either the translational symmetry or the U(1) charge symmetry. Using a Berry phase approach, we found that for a large class of models, including all one- and two-band models, the time-reversal symmetry breaking states can be classified into two types, dubbed I and II, depending on the accompanying spatial symmetry breaking patterns. The properties of each class are studied. In particularly, we show that the states breaking both time-reversal and chiral symmetries (type II) are described by spontaneously generated Berry phases and exhibit anomalous Hall effect in the absence of magnetic fields and magnetic impurities. We also show examples of the time-reversal symmetry breaking phases in several different microscopically motivated models and calculate their associated Hall conductance within a mean-field approximation. In particularly, we found a simple lattice structure in which the time-reversal symmetry breaking phases is stabilized by infinitesimal interactions.

January 23, 2009
Charge fractionalization in two-dimensional Dirac fermions
Dr. Chang-Yu Hou
Boston University
Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. We will show that quasiparticle excitations with irrational charge exist in tight-biding systems, such as graphene-like structure, described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge re-rationalizes to the value 1/2.

January 26, 2009
Interaction-Induced Localization in an Inhomogeneous Quantum Wire
Prof. Harold Baranger
Duke University

Localization of electrons induced by electron-electron interactions is a key issue in strongly interacting systems which dates from the dawn of solid state physics when
Wigner introduced the notion of an electron crystal. We have studied such interaction-induced localization in several nanoscale settings: a homogeneous quasi-1D electron gas, a circular quantum dot, and an inhomogeneous 1D system, a wire with two regions, one at low density and the other high. Quantum Monte Carlo techniques are used in order to treat the strong Coulomb interactions in the low density region. In all three systems, the electrons become localized if the density is sufficiently low. In the homogeneous wire, preliminary results for the "zig-zag" transition will be presented. In the inhomogeneous wire, we focus on the separation between the high density and low density regions. If the external potential changes abruptly at the interface, a barrier develops between the two regions, causing Coulomb blockade effects. For a short low density region, as in a quantum point contact, a single localized electron naturally occurs. The picture emerging here is in good agreement with the experimental measurements of tunneling between two wires (the Yacoby group), and has implications for the "0.7 effect" in the conductance of quantum point contacts.

February 2nd, 2009
Multiferroics: New (and old) Materials
Dr. Art Ramirez
Bell Labs
The science of both ferromagnets and ferroelectrics is mature, as are their device manifestations. Materials possessing both types of order parameters, "multiferroics", also exist but are much less.  Of particular interest are "induced multiferroics" which exhibit a magnetoelectric response that becomes large and therefore potentially useful in a critical region.  An additional challenge is to discover systems that possess a large magnetoelectric response at room temperature. I will discuss our recent progress in developing useful multiferroics.

February 6th, 2009
Magnetic Bose-Einstein condensation vs. localization in a
                  site-diluted spin-1 antiferromagnet
Dr. Rong Yu
University of Tennessee
In this talk I will report on the magnetic phase diagram of coupled spin-1 chains with site diltion, modeling the behavior of doped NiCl$_2$-tetrakis thiourea (DTN). In absence of doping, this compound
displays a field-induced Bose-Einstein condensation of magnons, which has been revealed by the mean-field scaling of the critical temperature, $T_c \sim |H-H_c|^{\phi}$ with $\phi=2/3$. At zero temperature, the system experiences a quantum phase transition (QPT) from a gapped phase to a gapless ordered phase. I show that site dilution introduces a novel gapless quantum disordered phase, corresponding to a Bose glass phase of localized magnons. The existence of the Bose glass phase leads to a change in the universality class of the QPT, and in the scaling of T_c. A crossover temperature is found, above which the mean-field scaling of $T_c$ at finite temperature is observed. But a new universal exponent $\phi\approx 1.2$ is obtained below the crossover temperature. Such a crossover is explained via a scenario of thermal percolation of magnons.
February 9th, 2009
A tale of inhomogeneities and length-scale “hierarchy” in some cuprates
Prof. Zahirul Islam
X-Ray Science Division
Advanced Photon Source
Argonne National Laboratory
   Scattering studies have always played a critical role in elucidating microscopic nature of and intrinsic correlations in complex phenomena such as high-temperature superconductivity, spin-charge stripes, colossal magnetoresistance and polaronic effects, orbital ordering, etc., in d-electron systems. This talk focuses on x-ray-scattering studies of length-scale “hierarchy” due to incommensurate modulations that may profoundly influence electronic properties in cuprates.
   As a copper-oxide based Mott insulator is doped with charge carriers (“holes”), the long-range-ordered antiferromagnetic state is suppressed, and a superconducting (SC) ground state emerges above a certain doping level. Numerous experiments, such as electron microscpic studies, indicate that cuprates are electronically “inhomogeneous” even in their SC state. Although the origin of these inhomogeneities may well be electronic instabilities such as spin-charge stripes, a general consensus remains elusive. Systematic x-ray diffuse scattering studies of YBa2Cu3O6+x (YBCO), Bi2Sr2CaCu2O8+d, and (La1-xSrx)2CuO4 (LSCO) have revealed lattice modulations  on a “hierarchy” of length scales. In the case of YBCO, as charge carriers are tuned via O-stoichiometry variations, short-range ordered modulations characterized by a wavevector of the form q=(qx, 0, 0) are observed. These modulations correspond to correlated atomic displacements of Ba, Cu, and O atoms, respectively, with correlations extending several unit cells to form “nano-scopic” patches. They form well above room temperature, persist down to the lowest temperatures studied, and, intriguingly, at high doping level appear to be susceptible to Fermi-surface effects. In addition, “bowtie”-shape Huang scattering indicates characteristic coherent strain, signifying an intrinsically inhomogeneous lattice in both YBCO and LSCO from mesocopic to microscopic length scales. The role of lattice distortions in modifying electron-pair potential and exchange interactions has been highlighted recently. It seems that a close interplay between electronic inhomogeneities and lattice modulations on various length scales is inevitable in cuprates as well as oxides, in general.
    Applications of high magnetic fields to perturb these systems and to suppress the superconducting order parameter can have profound implications on our understanding of their electronic phase diagram. Studies of these modulations in magnetic fields and future directions are briefly summarized.

*Use of the Advanced Photon Source is supported by the U. S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
February 20th, 2009
Some Properties of Iron-based Superconductors-Magnetism, Junctions, Multi-orbital models, etc.
Dr. Daoxin Yao
Purdue University
In this seminar, I will first give a brief review about the Iron-based superconductors. Then we study the magnetism of parent components using a local moment picture. Through the Heisenberg model, we estimate the exchange couplings from experimental data. We notice that the interlayer coupling is rather large, and the system may be very anisotropic along the a- and b-directions in the low temperature orthombic phase.  In the third part, I will talk about the possible superconducting states and how to use Josephson junctions to distinguish them in the iron-based superconductors.  Within the two-orbital exchange coupling model, we find the non-trivial in-gap states in the sign-changed s-wave pairing state (cos(kx) cos(ky)). This can be taken as a sharply distinct feature in contrast to other singlet pairing states. In addition, we propose a novel trilayer Pi-junction as a possible signature of the sign-changed s-wave. I will also discuss the necessity of multi-orbital models.
February 23rd, 2009
TBA
Dr. Tobias Micklitz
Argonne National Laboratory
March 9th, 2009                                                      
Towards Molecular Modualtion of Electronic Devices.
New and renewed vistas for solar cells?
Prof. David Cahen*
Dept. of Materials and Interfaces,
Weizmann Inst. of Science, Rehovot, Israel 76100
david.cahen@weizmann.ac.il
We explore two main ways to use molecules in molecular electronics, electrostatically, as dipolar films and electrodynamically, where current passes through them. For the former we find that incomplete partial dipolar molecular films can control diode behaviour and that this effect extends to poly- and nano-crystalline solar cells. For the latter complete monolayers are needed, as ideal as possible to assure current transport through the molecules. Such systems with alkyl chain monolayers behave like ideal MIS diodes with significant photovoltaic activity, an effects that is much more chemical in nature than conventional wisdom says (which described everything to the insulating behavior of the intervening layer).  Recently we are attempting to combine the two effects by adding dipolar character to the completely covering alkyl monolayers, enabling modulating the electronic quality of the monolayers. The idea is to use molecular interface modifications to learn about device operation and optimizations.

* Work done. with Ayelet Vilan, Omer Yaffe, Ariel Biller, Rotem Har Lavan, as well as D. Gal, H. Haick, G. Hodes, L. Kronik, S. Ruehle, A. Salomon, O. Seitz, H. Shpaisman, F. Thieblemont, I. Visoly-Fisher, and J. Gooding (UNSW), H. Haick (Technion), A. Kahn (Princeton U), C. Sukenik, A. Zaban (Bar Ilan U), R. Tung (CUNY), E. Umbach (Wuerzburg/Karlsruhe), N. Ueno (Chiba), H. Zuilhof (Wageningen)  and coworkers.

March 30th, 2009
Static and Dynamic Properties of a Fermi-Gas of Cooled Atoms Near a Wide Feshbach Resonance
Prof. Artem G. Abanov
  Texas A& M University
I will discuss the static and dynamic properties of the ultracold fermionic gas near the broad Feshbach resonance. I will show that the problem of molecular production, in a single-mode approximation, is
reduced to the linear Landau-Zener problem for operators. The strong interaction leads to significant renormalization of the gap between adiabatic levels. Two main physical results of our theory is the high
sensitivity of molecular production to the initial conditions and generation of a large BCS condensate distributed over a broad range of momenta in a wide range of parameters. 
                                                 
April 9th, 2009
Nonequilibrium dynamics of ultracold fermions in optical lattices
Prof. Eugene Demler
Havard University
April 10th, 2009                                             
TBA
April 13th, 2009
TBA
Prof. Yeong-Ah Soh
Dartmouth College
April 20th, 2009
TBA
Dr. Mumtaz Qazilbashb
UCSD

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