Brockman Hall 300
When: Mondays at 4:00 p.m.
August 22, 2014
Fermion space charge in narrow-band gap semiconductors, Weyl semimetals and around highly charged nuclei
Prof. Eugene B. Kolomeisky
University of Virginia
|August 28, 2014|
Prof. Predrag Nikolic
George Mason University
|September 2, 2014|
Quantum Engineering Robust 2D Topological Insulator in InAs/GaSb Bilayers
Prof. Rui-Rui Du
|September 8, 2014|
Rare fluctuation effects in disordered Dirac fermion systems
Dr. Rahul M. Nandkishore
Princeton Center for Theoretical Science
Abstract: The traditional theory of disordered systems is based on concepts such as non-linear sigma models and Born approximations. However, the usual implementations of these ideas ignore the contributions of rare regions (`quantum Griffiths effects’), which can dominate the behavior of disordered systems of Dirac fermions. I illustrate this physics with a discussion of two sets of phenomena that are dominated by rare regions: superconductivity on the surface of a topological insulator, and transport near charge neutrality in a three dimensional Dirac semimetal. In both cases, I explain how traditional analyses yield qualitatively incorrect answers, and how the physics is dominated by rare region effects. I also explain how sigma model based analyses may be modified to incorporate the effect of rare regions.
References: Phys. Rev. B 87, 174511 (2013), Phys. Rev. B 89, 245110 (2014), arXiv: 1407.4830
|Novermber 17, 2014|
NON-ABELIAN EXCITATIONS IN A 2 DIMENSIONAL ELECTRON SYSTEM
Dr. Robert Willett
Abstract: Correlation of charges in two-dimensional electron systems can induce numerous novel physical properties, including fractional charge, fractional statistics, and transitions between statistical classes. Perhaps the most unique of the properties within this system is the predicted demonstration of non-Abelian statistics by a specific set of excitations. In this talk we will review these concepts from an experimental perspective, and present charge interference measurements that demonstrate distinct non-Abelian effects.
|Novermber 18, 2014|
SPIN-ORBIT TUNED GROUND STATES IN SINGLE-CRYSTAL IRIDATES
Prof. Gang Cao
University of Kentucky
Abstract: The iridates have become a fertile ground for studies of new physics driven by spinorbit coupling (SOC) that is comparable to the on-site Coulomb and crystalline electric field interactions. This unique circumstance creates a delicate balance between interactions that drives complex magnetic and dielectric behaviors and exotic states seldom or never seen in other materials. A profound manifestation of this competition is the novel “Jeff = 1/2 Mott state” that was observed in the layered iridates with tetravalent Ir4+(5d5) ions. On the other hand, very little attention has been drawn to iridates having pentavalent Ir5+(5d4) ions, primarily because the strong SOC limit is expected to impose a nonmagnetic singlet ground state (Jeff = 0). In this talk, we review the underlying physical properties of the iridates including perovskites, honeycomb lattices and double perovskites with pentavalent Ir5+ ions, and report results of our recent studies that emphasize spin-orbit-tuned ground states stabilized by chemical doping, application of pressure and magnetic field. In addition, we address the urgent question that the Jeff states may not survive in the presence of strong noncubic crystal fields and/or exchange interactions.
|November 19, 2014|
Valley Zeeman effect in a two-dimensional semiconductor
Dr. Ajit Srivastava
Abstract: A monolayer of transition metal dichalcogenide (TMD) such as WSe2 is an atomically thin direct band-gap semiconductor with a honeycomb lattice which breaks inversion symmetry. The existence of two inequivalent minima or valleys in the Brillouin zone represents a pseudo-spin degree of freedom which can be optically addressed using circularly polarized light. Furthermore, due to time-reversal symmetry but broken inversion symmetry, there is equal but opposite Berry curvature and a closely related quantity called intercellular orbital magnetic moment, in the two valleys. This unique contribution to the total magnetic moment can be thought to arise from the self-rotation of a wavepacket constructed from the Bloch states of a band and points out-of-plane for a two-dimensional material. In this talk, I will present our recent results providing evidence for the observation of such an orbital magnetic moment in monolayer WSe2. We perform magneto-optical spectroscopy and observe a splitting of the exciton peak corresponding to a magnetic moment of ~ 4.3 Bohr magneton only in a magnetic field perpendicular to the sample. The intercellular orbital magnetic moment calculated from a particle-hole asymmetric tight-binding Hamiltonian agrees well with the experimental value when strong excitonic interactions are taken into account. On the other hand, the magnetic moment of the trion is found to be anomalously large in comparison with exciton which we attribute to the large exchange-induced Berry curvature of trions. As an outlook, I will discuss the possibility of using strong-light matter interactions in TMDs to tune the trion Berry curvature by forming cavity trion-polaritons.
|Strain-induced partially flat band, helical snake states and interface superconductivity in topological crystalline insulators|
Dr. Evelyn Tang
|Correlation effects on double-Weyl semimetals|
Dr. Hsin-Hua Lai
|Surface and vortex states in topological nodal superconductors|
Dr. Po-Yao Chang
U. Illinois Urbana-Champaign
|Unconventional quantum phase transition in a quantum dimer model on the kagome lattice|
Dr. Zhihao Hao
Dr. Baratunde Cola
|March 9, 2015|
Quantum spin ice
Prof. Nic Shannon
Okinawa Institute of Science and Technology
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 , and recent experiments which suggest that the spin ice Dy2Ti2O7 may undergo a phase transition at very low temperature .
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 , 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 .
 K. Ross et al., Phys. Rev. X 1, 021002 (2012).  L.-J. Chang et al., Nature Commun. 3, 992 (2012)  K. Kimura et al., Nature Commun. 4, 1934 (2013)  D. Pomaranski et al., Nature Phys. 9, 353 (2013).  M. Hermele et al., Phys. Rev. B 69, 064404 (2004).  A. Banerjee et al., Phys. Rev. Lett. 100, 047208 (2008)  N. Shannon et al., Phys. Rev. Lett. 108, 067204 (2012).  O. Benton et al., Phys. Rev. B 86, 075154 (2012).  Y. Kato et al., arXiv:1411.1918  P. McClarty et al., arXiv:1410.0451
|March 16, 2015|
ENHANCEMENT OF SUPERCONDUCTIVITY NEAR A NEMATIC QUANTUM CRITICAL POINT
Prof. Steven Kivelson
Abstract: 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.
|March 17, 2015|
SUPERCONDUCTOR-INSULATOR TRANSITION IN (Li,Fe)OHFeSe AND ENHANCED SUPERCONDUCTIVITY BETWEEN BaFe2As2 and CsFe2As2
Prof. Xianhui Chen
University of Science and Technology of China
Abstract: In this talk, we present a first-order transition from superconductor to insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe, and a novel phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out. The most intriguing feature is the appearance of an insulating phase immediately adjacent to the optimal superconductivity, which is reported for the first time in iron-based superconductors. Similar phase diagrams have been observed in the two-dimension organic superconductors and Cs3C60. This phase diagram of FeSe-derived superconductors also bears resemblance with that of high-Tccuprate superconductors. The similarity of these phase diagrams transcends the diversity of various unconventional superconducting materials, suggesting that all of them share a universal mechanism in superconductivity. Moreover, our work suggests that the gate-controlled strong charge doping is a very powerful practice for the exploration of novel states of matter that cannot be realized using traditional methods.
Interface superconductivity offers an alternative avenue, different from bulk doping or external pressure, to convert a non-superconducting material into a superconductor and also to explore the physics of high-Tc superconductivity. The reduced dimensionality at the interface can amplify quantum fluctuations and induce or enhance underlying interactions, which may result in unexpected physical properties, though the exact nature could be elusive. Here, we report on a superconductivity with transition temperature (Tc) up to 30 K in BaFe2As2/CsFe2As2 intergrowth crystals, where BaFe2As2 is an antiferromagnetic metal and CsFe2As2 possesses a very low Tc of 1.8 K. Combining structural, transport and magnetic measurements, we found that the unexpected high transition temperature arises from the interface between BaFe2As2 and CsFe2As2. Our finding provides a new platform to understand the mechanism of interface superconductivity, and also indicate that the interface superconductivity could be an important way to achieve high-Tc superconductivity in FeAs-based superconductor.
|March 24, 2015|
Quantum Criticality and Unconventional Properties of Heavy Fermion Superconductor Ce1-xYbxCoIn5
Dr. Yogesh Singh
Kent State University
Abstract: Ce1-xYbxCoIn5 is a unique member of the heavy fermion family Ce1-xRxCoIn5 where ‘R’ is a rare earth. Many of the properties observed with Yb substitution on Ce site are highly unusual. Due the intermediate valence nature of Yb, many of the properties of this system appear to be influenced by Yb. In this presentation, I will talk about some of these properties and also will discuss my experimental findings for this material. A relatively small Yb substitution drastically decreases quantum critical point (HQCP) of CeCoIn5 and brings it to 0 T for a very small impurity concentration. On the other hand superconducting transition temperature (Tc) and Kondo coherence temperature (Tcoh) are robust and survive over the whole Yb doping range. These results imply that superconductivity and quantum criticality are decoupled in this system, i.e., unconventional superconductivity is not triggered by spin fluctuations. We also find a scaling of the normalized Tcoh and normalized Tc which suggests that the onset of many-body coherence and emergence of superconductivity have same physical origin: hybridization between conduction and localized f-electron states. The robustness of Tcoh and Tc with respect to disorder for Yb compared with the other rare-earth substituents is consistent with Yb atoms forming a cooperative mixed-valence state that significantly reduces the pair-breaking effects.
|April 27, 2015|
Anomalous Hall effect in graphene
Prof. Bruno Uchoa
University of Oklahoma
When graphene is supported on hexagonal boron nitride, the local potentials of boron and nitrogen atoms locally break sublattice symmetry, giving rise to a local mass term in the free Dirac Hamiltonian of graphene. Since the two crystal lattices have a small mismatch, the mass term modulates in real space and forms a Moire pattern with a unit cell as large as a hundred lattice sites. The zero mass lines where the mass term changes sign form topological domain walls containing low energy modes, similarly to edge states in the quantum Hall effect. In this talk, I will examine the role of Coulomb interactions in the emergence of macroscopically ordered states in this system. In the presence of these modes, I will show that Coulomb interactions lead to spontaneous formation of chiral loop currents in bulk and to macroscopic spin-valley order at zero temperature. This exotic order describes an anomalous Hall state, which can be detected with interferometry and polar Kerr measurements.
|July 13, 2015|
Nematic quantum criticality in a 2+1-dimensional metal
Dr. Samuel Lederer
Evidence continues to mount that quantum critical points lie beneath the superconducting “dome” of both the iron-based and cuprate high temperature superconductors. Further, these putative critical points appear intimately related to anomalous properties of both the superconducting and normal (metallic) states of these materials. We consider a model for a quantum critical point in a 2+1-dimensional metal with an Ising nematic order parameter, which captures the spontaneous breaking of fourfold rotational symmetry known to occur in the above materials. We simulate this model using numerically exact, sign-problem-free Quantum Monte Carlo techniques, and find evidence of a new strongly coupled fixed point with nontrivial scaling exponents. At this fixed point, the fermions are strongly, perhaps singularly renormalized, consistent with the expected breakdown of Fermi liquid theory.
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