## 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.

### Dale Van Harlingen, Feb. 10, 2014: Abstract

We are studying the transport properties of hybrid S-TI-S devices fabricated by depositing superconductor electrodes onto topological insulators. In top-gated Nb-Bi2Se3-Nb junctions, we have measured the Josephson supercurrent and conductance as a function of geometry, temperature, and gate voltage in order to determine the nature of the charge transport. The supercurrent exhibits a sharp drop as a function of gate doping that may be explained by the relocation of the topological surface state from above to below the trivial conducting surface states formed by band-banding near the surface. We find that the magnetic field modulation of the supercurrent in Josephson junctions and dc SQUIDs exhibits anomalous features that are consistent with a sin(ϕ/2) component in the current-phase relation that we attribute to the presence of low energy Andreev-bound states and possibly Majorana modes in the junction. We present a model for the current-phase relation that describes many of the observed features and predicts the nucleation and motion of these states with applied field.

We have also measured transport in TI-S-TI structures that allows us to search for crossed Andreev reflection and electron co-tunneling processes allowed by Cooper pair coherence. We find an anomalous asymmetry in the nonlocal transport that we attribute to a chiral p-wave superconducting order parameter component induced in the topological insulator by the superconducting contact.

### Leonid Glazman, Feb. 17, 2014: Abstract

Electron puddles created by doping of a 2D topological insulator may violate the ideal helical edge conductance. Because of a long electron dwelling time, even a single puddle may lead to a significant inelastic backscattering. We find the resulting correction to the perfect edge conductance. Generalizing to multiple puddles, we assess the dependence of the helical edge resistance on temperature and on the doping level. Puddles with odd electron number carry a spin and are effective in backscattering in a broad temperature range. That may explain the experimentally found weak temperature dependence of the excess resistance of helical edges.

### Enam Chowdhury, Feb. 18, 2014: Abstract

As femtosecond laser interacts with matter in fundamentally different ways than how continuous wave (CW) or nano-second laser interacts, it gives rise to many interesting phenomena which may open doors for exciting future applications, like plasmon coupled high harmonic generation [Park 2011] nano-electronics [Chimmalgi 2005], etc. CW and nano-second laser pulses can be focused down to a small spot to limit the spread of laser energy to surroundings to create fine cutting, or drilling; however, laser material coupling in this way causes melting and ablation of heated up materials not only at the intended focal region, but also at significantly larger surrounding volume by heat diffusion. In the case of femtosecond lasers, electrons in interacting materials gain energy non-thermally by being accelerated in strong laser fields before the ions in the material has a chance to respond.

Although much has been understood about mechanism, many fundamental aspects of laser to electron and electron to ion coupling and how it affects material modification, damage and ablation is not well understood [Bonse 2012]. We approach this problem both experimentally and using simulation focusing on i) mechanism for electron transition from valence to conduction band (dielectric) ii) interaction of light with free electrons in conduction band/vacuum (ionized dielectrics/metals) iii) how electrons couple energy to ions to melt/ablate/restructure surfaces, and how surface structure affect the interaction. Laser wavelength is a great tuning parameter in understanding these effects, because it affects all three steps described above. At the AFOSR funded Femtosecond Solid Dynamics Laboratory, we are building capabilities to probe light matter interaction with wavelengths ranging from 200 – 10,000 nm, and pulse widths from 5 – 500,000 fs. I’ll also be discussing some of our recent results on a possible new mechanism of femtosecond laser ablation with few cycle mid IR lasers, and laser ablation from femtosecond to nanosecond scale using hybrid-PIC (particle in cell) simulations.

### Dan Desssau, February 25, 2014: Abstract

Using high resolution ARPES we analyze the electronic scattering rates of cuprate superconductors as a function of temperature, energy, and doping. In the normal state we show the smooth and continual evolution from the heavily overdoped Fermi liquid limit to the optimally doped Marginal Fermi liquid limit and beyond, with these results showing a new type of universality for these non-Fermi liquid interactions. These results are also very relevant for discussions of the pseudogap as well as possible quantum-criticality under the dome. We also consider how these self-energy terms affect the superconducting state, focusing not just on the pairing energy scale (the gap ) but also scattering terms that cause pair-breaking. In contrast to conventional superconductors in which the superconducting transition temperature Tc is set by the pairing energy alone, we show that Tc in the cuprates is set by a crossover between the pairing and pair-breaking energy scales, each of which is strongly temperature-dependent.

### Christian Schonenberger, March 10, 2014: Abstract

An elegant idea for the creation of entangled electrons in a solid-state device is to split Cooper pairs, which are in a spin singlet state, by coupling a superconductor to two parallel quantum dots (QDs) in a Y-junction geometry [1]. Cooper pair splitting (CPS) was investigated recently in devices based on InAs nanowires [2,3] and carbon nanotubes (CNTs) [4,5] and identified by a positive correlation between the currents through the QDs. I will first discuss recent experiments that demonstrate high splitting efficiencies > 90%. A high CPS efficiency is a prerequisite for Bell state measurements, a clear way of proving that Cooper pairs can be extracted coherently and lead to spatially separated entangled electron pairs. Further requirements on entanglement measurements will be addressed in the talk as well. I will then continue to discuss new results in semiconducting nanowires with Nb contacts that display a great variety of correlations. Using also Nb as the injector another distinct experiment with CNT devices will be discussed. In the regime of a strong tunnel coupling between the QDs and superconducting contact, the CPS efficiency is expected to be small [5]. However, the superconducting proximity effect can support so-called Andreev bound states (ABS) on a QD, which can be detected by conventional transport spectroscopy [6]. Here we use a Niobium contacted CNT Cooper pair splitter and investigate the response of the ABS formed on one QD to CPS. We find an appreciable non-local conductance when the bias is large enough to excite charge fluctuations in the ABS. These non-local signals change sign with opposite bias and, more intriguingly, when the ABS ground state changes from a spin singlet to a doublet. Our experiments can be understood qualitatively in an intuitive picture for ABS and CPS and show that CPS can be used as a tool to investigate complex hybrid nanoelectronic structures.

This is a collaborative effort with the groups of Szabolcs Csonka, Budapst University of Technology and Economy, Jesper Nygard, Nano-Science Center, Niels Bohr Institute of the University of Copenhagen, and Jan Martinek- IFM-PAN, Poznan, Polen. I acknowledge funding from the Swiss NFS, SNI , NCCR-QSIT, FP7-SE2ND and ERC-QUEST.

### Eun-Ah Kim, April 7, 2014: Abstract

Much interest in the superconducting proximity effect in 3D topological insulators (TIs) has been driven by the potential to induce exotic Majorana bound states. Most candidate materials for 3D TI, however, are bulk metals, with bulk states at the Fermi level coexisting with well-defined surface states exhibiting spin-momentum locking. In such topological metals, the proximity effect can differ qualitatively from that in TIs. In this talk, I will first discuss how topological metal as we define it interpolates between topological insulator and trivial metal. Then I will discuss how a topological metal, which is not an ideal topological insulator, is better than ideal for superconducting proximity effect.

### Gil Refael, April 14, 2014: Abstract

In my talk I will present fresh findings on the possibility of producing a topological polariton (nicknamed topolariton) in trivial quantum-wells coupled to extended optical cavities. The idea of a topolariton emerges naturally from the concept of Floquet topological insulators. First, I will review how an external periodic drive can lead to a Floquet topological spectrum in a trivial 2d quantum well. Next, I will focus on exciton-polaritons, and present some toy models of quantum-wells coupled to optical cavities that exhibit 'topolaritons' - i.e., chiral edge modes that are superpositions of photons and excitons.