Department of Physics & Astronomy

2011 – 2012

*Where:
Brockman Hall 300When: Mondays at 4:00 p.m.*

Prof. Jan van Ruitenbeek Leiden University Abstract |

Prof. Milton Torikachvili San Diego State University The iron-arsenide based compounds with general composition AFe2As2 (A = Ca, Sr, and Ba) display quite remarkable properties. A structural transformation from tetragonal (T) at high temperature (T) to orthorhombic (O) at low temperature takes place in the temperature range from 100 to 200 K, and it is accompanied by the onset of long-range magnetic order. The orthorhombic/ordered phase is not superconducting. However, suppression of the T/O transformation by pressure and/or partial substitution leads to the emergence of superconductivity. Pressure studies in the parent and substituted compounds will be reviewed. |

Dr. Liang Fu Harvard University Topological superconductors are unconventional superconductors which have topologically protected gapless surface Andreev bound states hosting itinerant Majorana fermions. There is currently intensive search for topological superconductors. In this talk, I will propose a recently discovered superconductor Cu-doped Bi2Se3 may have an unconventional odd-parity pairing symmetry due to its strongly spin-orbital coupled band structure. I will show on general grounds that odd-parity pairing leads to topological superconductivity. The resulting surface Andreev bound state spectrum will be discussed and related to a recent point-contact spectroscopy experiment. |

Prof. Mark van Schilfgaarde Arizona State University A new type of self-consistent scheme within the GW approximation is pre-" "sented, which we call the quasiparticle self-consistent GW (QSGW ) approxi- mation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is used to minimize the difference between the many-body and single-particle hamiltonians. QSGW describes optical properties in a wide range of materials rather well, including cases where the local-density and LDA-based GW approximations fail qualitatively. Self-consistency dra- matically improves agreement with experiment, and is sometimes essential. QSGW avoids some formal and practical problems encountered in conven- tional self-consistent GW , which will be discussed. It handles both itinerant and correlated electrons on an equal footing, without any ambiguity about how a localized state is defined, or how double-counting terms should be sub- tracted. Weakly correlated materials such as Na and sp semiconductors are described with uniformly high accuracy. Discrepancies with experiment are small and systematic, and can be explained in terms of the approximations made. Its consistently high accuracy make QSGW a versatile method that can reliably predict critical energy band properties of graphene, CuInSe2 , CaFe2 As2 and NiO in a unified framework. Many other properties attendant to the elec- tronic structure can be calculated, such as magnetic excitations, the Auger recombination process, the transmission through a metal-semiconductor con- tact. In principle it can serve both as a framework to construct effective hamiltonians for many-body physics, and as an engine to build models for device design from first principles, with unprecedented reliability. How to do this in practice is a major challenge today. I will briefly present some discussion of each. |

Prof. Shiwei Zhang College of William and Mary The Hubbard model is realized in both optical lattices and many dilute Fermi gas systems. In the context of condensed matter, understanding the properties of the repulsive Hubbard model and its connection to high-Tc superconductivity has long been a challenge. Recent advances in quantum Monte Carlo simulation methods have greatly improved our capabilities to do accurate many-body calculations in such model systems, as well as in realistic strongly correlated materials. I will discuss two applications in fundamantal models, one to study itinerant ferromagnetism in a dilute gas of cold atoms, and the other to determine the nature of antiferromagnetism in doped Hubbard model with repulsive interaction. In the latter, a spin-density wave state with long wavelength modulation is found, which is related to FFLO states in optical lattices of spin-imbalanced atoms with attractive interaction. |

Prof. Malcolm Kennett Simon Frazer University, Canada By considering a generalization of a proposal by Sorensen et al. [1] for generating an artificial magnetic field in an optical lattice of cold atoms, we are led to a tight-binding model with an unusual dispersion when there is an average of half a flux quantum per plaquette. The dispersion contains Dirac points and the spectrum of low-energy excitations about these points can be described by massless Dirac fermions in which the usually doubly degenerate Dirac cones split into cones with different "speeds of light" which can be tuned to give a single Dirac cone and a flat band. These gapless birefringent Dirac fermions arise because of broken chiral symmetry in the kinetic energy term of the effective low energy Hamiltonian. We characterize the effects of various perturbations to the low-energy spectrum, including staggered potentials and interactions. We also study the zero energy modes that arise in an unusual vortex configuration involving both the kinetic energy and an appropriate mass term in this model. We find the surprising feature that the symmetry between states with a vortex background and and an anti-vortex background is broken, which is reflected in differing length scales for the vortex and anti-vortex zero modes, but that fractionalization of quantum numbers such as charge is unaffected. We discuss this situation from a symmetry point of view and present numerical results for a specific lattice model realization of this scenario. |

Prof. Philipp Gegenwart Georg-August-University, Germany Abstract |

Dr. Gang Chen University of Colorado, Boulder We construct and analyze a theoretical model for the pyrochlore iridates R$_2$Ir$_2$O$_7$ with R (= Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb) magnetic. The electrons on trivalent rare earth ions R$^{3+}$ form local Ising doublets due to the local crystal field. Based on a space group symmetry analysis, we write down the generic Kondo coupling between the Ising spin at R sites and the effective spin at Ir sites. Besides this interaction, we also include direct electron tunneling between Ir sites and indirect electron tunneling via intermediate oxygens for Ir-Ir coupling. This simple minimal model gives a rich phase diagram with broad regions of topological semi-metal and axion insulator phases. Based on these findings, we propose R$_2$Ir$_2$O$_7$ to be one of the most promising candidates to realize the topological semi-metal and axion insulator phases. Implications for existing and future experiments are discussed. |

Dr. SungBin Lee University of California at Santa Barbara Rare earth pyrochlores, with a chemical formula A2B2O7, exhibit many interesting features in A site spin system. Depending on A site rare earth elements, spin ice and magnetically ordered phases are shown in several experiments. Moreover, they have been also focused as possible candidates of U(1) spin liquid. In order to explore such versatile phases, we study the pseudospin-1/2 model, which is quite generic to describe rare earth pyrochlores with integer spins, in the presence of spin-orbit coupling and crystalline electric field. Using a new "gauge mean field theory", we show the possible ground states, corresponding to several phases listed above. |

Prof. Ilya Vekhte Louisiana State University In many correlated electron materials different ordering phenomena compete and, occasionally, coexist. One well studied example of such cohabitation is between antiferromagnetism and superconductivity: the two phenomena coexist in a region of parameter space for several families of unconventional superconductors. The case of heavy fermion CeCoIn5 has challenged our understanding since this is the only situation where antiferromagnetism appears under an applied magnetic field in the superconducting state, but vanishes as soon as superconductivity is suppressed. After reviewing various scenarios for this observation and comparing them with the experimental data, I will present a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn5. I will show that in Pauli-limited superconductors with zeroes (nodes) in the energy gap the nesting of the quasiparticle pockets induced by Zeeman pair breaking leads to incommensurate antiferromagnetic state. I will show that salient features of this theory are in agreement with experiments, and will predict a new double-Q magnetically ordered phase. |

Dr. Yan He University of California, Riverside Recently neutron scattering experiments have discovered a new magnetic order in the pseudogap phase of under-doped cuprates. In the same regime, two branches of weakly dispersive collective modes have also been found by inelastic neutron scattering. We show that these two branches of collective modes are a necessary consequence of the quantum fluctuation of the ordered loop-current state. Using parameters to fit the observed modes, we show that quantum fluctuations change the direction of the effective moments in the ground state to lie at an angle to the c axis as observed in experiments. |

Dr. Wenxin Ding Florida State University The last few years have seen a surge in study of entanglement entropy in different fields of theoretical physics. In condensed matter, entanglement entropy, and various generalizations such as Renyi/Tsallis entropy, mutual information and entanglement spectrum, have been considered a useful tool for characterizing exotic quantum phases and quantum phase transitions that denies a description by a conventional symmetry breaking paradigm of Landau. Yet relatively little attention has been dedicated to that of the conventional long-range ordered phases or critical phases. In this talk, I will talk about a series of work that focuses on the entanglement entropy of such "well-understood" phases, and show that they have non-trivial entanglement properties due to either the long-range order, or the quantum criticality. In the first part, I will focus on entanglement entropy of long-range ordered systems[1]: a ferromagnetic model, an antiferromagnetic model, and a set of free boson models with long-range hopping, and show that in all these cases, the entanglement entropy (or the mutual information at finite temperatures) scales as ~ log L.. For the second part, I shall discuss the entanglement entropy of Fermi liquids in two dimensions[2], which is one of the simplest and well-understood quantum critical phases. In the absence of interaction, the entanglement entropy of free fermions is solved by Gieov and Klich[3], using the Widom's conjecture, and scales as the area-law enhanced by a logarithmic factor, E ~ L^{d-1} log L. We re-derive this result via high dimensional bosonization, generalize the method to take into account the Fermi liquid interaction, and show that the entanglement entropy is not renormalized by the interactions. |

Prof. Yoshihiro Asai AIST, Japan Although it might not be expected easily from the mesoscopic physics text book, electron-phonon (e-ph) coupling effect on electric current is not negligible and idealistic ballistic transport is not realized even in atomic scale nanostructures such like atomic wires and molecules bridging two electrodes. The inelastic scattering effect puts some finger prints on I-V curve (I is current and V is voltage) as such we obtain specific line shapes of dI/dV and/or (d^2 I)/(dV^2 ) which enables us to make inelastic tunneling spectroscopy (IETS). The local heating effects were measured experimentally in Raman spectroscopy or junction life time. These features are well explained theoretically using Keldysh Green’s function theory treating inelastic heat generation and heat dissipation self-consistently on the same footing. After making review of theoretical efforts1-3 to describe these phenomena, I will put my second focus on the anomalous inverse temperature dependence of zero bias conductance observed at low temperature. The zero bias anomalies of the IETS will be discussed in this context. |

Prof. Frank Steglich Max-Planck-Institute for Chemical Physics of Solids, Dresden, Germany In this talk, I discuss combined heat- and charge-transport measurements on YbRh2Si2 across its Kondo-destroying quantum critical point (QCP) [1]. The latter had been inferred previously from disparate temperature dependences of thermodynamic and electrical-transport properties [2], an abrupt Fermi-surface reconstruction [3, 4] and the vanishing of a quantum-critical energy scale [5]. The new low-T measurements of the electrical and thermal resistivity reveal a violation of the Wiedemann-Franz law at the QCP of YbRh2Si2, where the residual thermal resistivity is estimated to exceed the residual electrical resistivity by about 10 %. This finding sheds new light on the break-up of the composite Landau quasiparticles, due to inelastic electronic scatterings that persist in the T = 0 limit. 1. H. Pfau et al., to be published 2. J. Custers et al., Nature 424, 524 (2003). 3. S. Paschen et al., Nature 432, 881 (2004). 4. S. Friedemann et al., Proc. Natl. Acad. Sci. USA 107, 14547 (2010). 5. P. Gegenwart et al., Science 315, 969 (2007). |

Prof. Jorge Dukelsky CSIC, Spain The exact solution of the BCS Hamiltonian with non-degenerate single particle orbits was introduced by Richardson in the early sixties. Although it passed almost unnoticed, it was recovered in the last decade in an effort to describe the disappearance of superconductivity in ultrasmall superconducting grains. Since then it has been extended to several families of integrable pairing models, the Richardson-Gaudin models. However, only the rational family has been widely applied to mesoscopic systems where finite size effects play an important role. Even in the thermodynamic limit, the exact many-body wavefunction provides a unique view to the Cooper pair structure in the BCS-BEC crossover. We have recently found two complementary implementations of the hyperbolic Richardson-Gaudin family in condensed matter and nuclear physics. The first implementation gives rise to a p-wave pairing describing a gas of spinless fermions in a 2D lattice with $p_x + i p_y$ pairing symmetry. Using this new tool we study the quantum phase diagram which, unlike the case of s-wave pairing, displays a third order quantum phase transition. We make use of the exact solution to characterize the quantum phase transition and the properties of the weak and strong paring phases. The exact wavefunction of the p-wave pairing Hamiltonian gives a beautiful insight into the nature of the quantum phase transition. Moreover, it suggests the existence of an experimentally accessible characteristic length scale, associated with the size of the Cooper pairs, that diverges at the transition point. The second implementation leads to a separable pairing Hamiltonian with two free parameters that can be adjusted to give an excellent reproduction of the superfluid properties of heavy nuclei as described by the Gogny force in the Hartree-Fock-Bogoliubov (HFB) approximation. As such, this new exactly solvable model could be used as a benchmark model to test many-body approximations beyond HFB, or it can be implemented into a self-consistent Hartree-Fock plus exact pairing method to describe heavy nuclei and other mesoscopic systems. |

Prof. David Huse Princeton University A fundamental question in quantum statistical mechanics is: What quantum systems of many degrees of freedom are able to function as a heat bath and thermally equilibrate themselves? That certain infinite and strongly-interacting disordered systems are localized and fail to equilibrate themselves at nonzero temperature was originally suggested by Anderson (1958). The Anderson localization transition with interactions and at nonzero temperature is a quantum phase transition between this localized phase and the "ergodic" phase that does succeed in thermally equilbrating itself and thus apparently obeys the "eigenstate thermalization hypothesis". The many-body localized phase is an interesting potential quantum memory: it has local two-level systems that constitute "permanent" q-bits with infinite coherence time, even though they are interacting strongly with many other degrees of freedom. Approximations to this physics might be realized with spins in solids (as Anderson originally considered) or with cold atoms in a random optical lattice. |

Dr. Max Metlitski University of California, Santa Barbara States of matter with a sharp Fermi-surface but no well-defined Landau quasiparticles are expected to arise in a number of physical systems. Examples include i) quantum critical points associated with the onset of order in metals, ii) the spinon Fermi-surface (U(1) spin-liquid) state of a Mott insulator and iii) the Halperin-Lee-Read composite fermion charge liquid state of a half-filled Landau level. In this talk, I will use renormalization group techniques to investigate possible instabilities of such non-Fermi-liquids to pairing. I will show that for a large class of phase transitions in metals, the attractive interaction mediated by order parameter fluctuations always leads to a superconducting instability, which preempts the non-Fermi-liquid effects. On the other hand, the spinon Fermi-surface and the Halperin-Lee-Read states are stable against pairing for a sufficiently weak attractive short-range interaction. However, once the strength of attraction exceeds a critical value, pairing sets in. I will describe the ensuing quantum phase transition between i) the U(1) and the Z_2 spin-liquid states, and ii) the Halperin-Lee-Read and Moore-Read states. |

Dr. Tamar Mentzel MIT In this talk, I will present a novel method for probing charge transport in nanostructured films. We perform a time-resolved measurement of charge in nanostructured films using an integrated nanoscale charge sensor. The sensor measures slow electron dynamics in materials where the electrical current is immeasurably small. This technique is insensitive to contact effects, enabling the measurement of electrical conductance even in the presence of blocking contacts. The integrated charge sensor is also capable of detecting single-electron hopping events in nanostructured films. In the second portion of the talk, I will present the first electrical measurements of nano-patterned arrays of semiconductor nanocrystals. The electrical conductance increases and additional noise arises in the current in the nanopatterned arrays compared to disordered, microscopic arrays. Random telegraph noise in the current may be indicative of conductance fluctuations from a single-electron switching event or correlated electronic behavior. I will conclude with an outlook on our work on integrating charge sensors with nano-patterned nanocrystals to study single-electron hopping events in tunable, ordered arrays of nanocrystals. |

Prof. Laura Greene University of Illinois at Urbana-Champaign Quasiparticle scattering spectroscopy (QPS), also called point contact spectroscopy (PCS), has proven to be a powerful probe of the superconducting order parameter in conventional and unconventional superconductors. This technique is also effective in detecting strong electron correlations in the normal state. In the heavy fermion URu2Si2 we detect the Fano resonance and hybridization gap as a distinct asymmetric double-peaked structure [1].In a variety of the Fe pnictides and chalcogenides, an enhanced conductance onsets well above the magnetic and structural transition temperatures, and in Ba(Fe1-xCox)2As2 we identify a new region in the underdoped side of the phase diagram which may be explained by orbital fluctuations [2,3]. Finally, the need for some kind of a microscopic theory that explains how QPS detects strong electron correlations will be discussed. |

Prof. Andre-Marie Tremblay University of Sherbrooke TBA |

Dr. Wei-Cheng Lee University of Illinois at Urbana-Champaign The discovery of new classes of high-temperature superconductors, iron pnictides in 2008, launched an international wave of research in the past few years. Whether or not these materials are strongly correlated is one of the central issues under hot debate. In this talk, I will present a theoretical study pointing out that a non-Fermi liquid behavior could be present in various iron based superconductors, and I will discuss several new experiments supporting this proposal. Using a five orbital tight binding model with generalized Hubbard on-site interactions, we find that within a one-loop treatment, a branch of overdamped collective modes develops at low frequency in scattering channels associated with quasi-1D $d_{xz}$ and $d_{yz}$ bands. When the critical point for the $C_4$ symmetry broken phase (structural phase transition) is approached, these overdamped collective modes soften, and acquire increased spectral weight, leading to a non-Fermi liquid behavior at the Fermi surface. We argue that this non-Fermi liquid behavior is responsible for the recently observed zero-bias enhancement in the tunneling signal in point contact spectroscopy. Our result suggests that quantum criticality plays an important role in understanding the normal state properties of iron-pnictide superconductors. |

Prof. Cristian Batista Los Alamos National Laboratory TBA |

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