Abstracts for Condensed Matter Seminars
Xi Dai, Friday, Aug. 30, 2013, Abstract:
In this talk, I will propose that the mix valence phenomena in some of the rare earth compounds
will naturally lead to non-trivial topology in band structure.One of the typical example is SmB6,
where the intermediate valence of Sm generates band inversion at the X point and the non-trivial
Z2 index. Other than SmB6, YbB6 and YbB12 are both mix valence compounds. By applying
LDA+Gutzwiller to these materials, we find that YbB6 has non-trivial Z2 index, indicating that YbB6 is another three dimensional topological insulator with strong correlation effects. Our calculation also finds that YbB12 is a trivial insulator in the sense of Z2 but it can be classified as topological crystalline
insulator with non-zero mirror Chern number. The electronic structure at finite temperature
has also been studied using LDA+DMFT, indicating YbB6 is still in the mix valence region while
YbB12 is quite close to the Kondo limit.
Hong Ding, Sept. 9, 2013, Abstract:
Angle-resolved photoemission spectroscopy (ARPES) has been used extensively in studying electronic structure and superconducting gap of the iron-based superconductors (IBSCs). In this talk, I will present our ARPES results on the IBSCs, mainly focus on high-resolution measurements of the superconducting gap of many different IBSCs. Our results strongly suggest that the pairing mechanism of IBSCs is likely to be driven by local antiferromagnetic exchange interactions and collaborative Fermi surface topology, in a fashion similar to the case of cuprate superconductors.
Yigal Meir, Sept. 30, 2013, Abstract:
Quantum point contacts (QPCs) are the basic building blocks of any mesoscopic structure, and display quantized conductance, reflecting the quantization of the number of transparent channels. An additional feature, coined the "0.7 anomaly", has been observed in almost all QPCs, and has been a subject of intensive debate in the last couple of decades. I will demonstrate that this feature can be attributed to the emergence of a quasi-localized state at the QPC, which explains all the phenomenology of the effect. I will describe the theory behind a new experiment which measured the conductance through length-tunable QPC. The experimental findings support the picture of the localized state(s). Interestingly, with increasing QPC length, it was found that both the 0.7 anomaly and the zero bias peak in the differential conductance oscillate and periodically split with channel length, supporting the idea that the number of the localized states increases with length, leading to an alternating Kondo effect.
Leonid Levitov, Nov. 4, 2013, Abstract
Since the discovery that electrons in graphene behave as massless Dirac fermions, the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics, such as Klein tunneling and the fractional quantum Hall effect. Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. The atomic collapse was predicted to manifest itself in quasistationary states which have complex-valued energies and which decay rapidly. However, the atoms created artificially in laboratory have nuclear charge only up to Z = 118, which falls short of the predicted threshold for collapse, Interest in this problem has been revived with the advent of graphene, where because of a large fine structure constant the collapse is expected for Z of order unity. I this talk I will discuss the symmetry aspects of atomic collapse, in particular the anomalous breaking of scale invariance. I will also describe recent experiments that use scanning tunneling microscopy (STM) to probe atomic collapse near STM-controlled artificial compound nuclei.
Hsin Lin, Nov. 5, 2013, Abstract:
The recently discovered topological crystalline insulators harbor Dirac surface states protected by a discrete set of crystalline space group symmetries and show immense promise for novel quantum applications. In this talk, I will present a first principles investigation as well as a model Hamiltonian of the nontrivial surface states and their spin and orbital texture in the topological crystalline insulator SnTe and related compounds. The (001) surface states exhibit two distinct energy regions of the Fermi surface topology separated by a Van-Hove singularity at the Lifshitz transition point. The surface state band structure around X(pi,0)-point consists of two “parent” Dirac cones centered at X and vertically offset in energy. When they intersect, the hybridization between the electron-branch of the lower parent Dirac cone and the hole-branch of the upper parent Dirac cone opens a gap at all points except along the mirror line, leading to the formation of a pair of lower-energy “child” Dirac points shifted away in momentum space from the time-reversal-invariant point X. Interestingly, the two parent Dirac cones must have different orbital character since they were found to be associated with orbitals with opposite sign of mirror eigenvalues in order to deliver the correct spin texture and band dispersion. I will also discuss the breaking of crystal symmetry and mass acquisition of Dirac fermion.
James Murray, Nov. 22, 2013, Abstract
Using a controlled weak-coupling renormalization group approach, we establish the mechanism of unconventional superconductivity in the vicinity of spin or charge ordered excitonic states for the case of electrons on the Bernal stacked bilayer honeycomb lattice. With one electron per site this system exhibits nearly parabolically touching conduction and valence bands. Such a state is unstable towards a spontaneous symmetry breaking, and repulsive interactions favor excitonic order, such as a charge nematic and/or a layer antiferromagnet. We find that upon adding charge carriers to the
system, the excitonic order is suppressed, and unconventional superconductivity appears in its place, before it is replaced by a Fermi liquid. We focus on firmly establishing this phenomenon using
the RG formalism within an idealized model with parabolic touching.
Victor Galitski, Nov. 25, 2013: Abstract
In this talk I will review recent theoretical work on a new class of topological material systems - topological Kondo insulators, which appear as a result of interplay between strong correlations and spin-orbit interactions. I will start with introducing the by now standard theory of topological band insulators and explain the Fu-Kane method to calculate the Z2 topological index for time-reversal-invariant band structures in three dimensions. The method will be used to show that hybridization between the conduction electrons and localized f-electrons in certain heavy fermion compounds gives rise to interaction-induced topological insulating behavior. A mean field theory of these Kondo topological insulators will be derived. I will also discuss recent experimental results, which have conclusively confirmed our predictions in the Samarium hexaboride compound, where the long-standing puzzle of the residual low-temperature conductivity has been shown to originate from topological surface states. This material system represents the first true topological insulator observed experimentally with low-temperature transport dominated by the surface and essentially no conduction in the bulk. In conclusion, I will mention our ongoing theory work, which focuses on very unusual non-linear transport properties of Samarium hexaboride devices, which mimic neuron-like behavior in biological systems.
Waseem Bakr, Dec. 9, 2013: Abstract
Recent advances in preparing, probing and manipulating ultracold atomic gases enable studying condensed matter physics in a very controlled setting. In the first part of my talk, I will describe quantum gas microscopy, a powerful tool for imaging and manipulating strongly interacting quantum gases containing thousands of atoms at the single atom level. I will describe its application to studying quantum phase transitions of Mott insulators and quantum magnets in bosonic systems of atoms.
In the second part of the talk, I will shift focus to topological physics in fermionic systems. I will explain how spin-orbit coupling, a crucial ingredient of time-reversal invariant topological insulators, can be engineered in a Fermi gas by dressing it with laser light. In addition, I will present results on strongly interacting fermions in two dimensions. These two ingredients can be combined to create topological superfluids analogous to topological superconductors that have been possibly realized in the solid state. Finally, I conclude with a brief outlook on experiments starting at Princeton with the goal of studying Chern insulators in optical lattices.