# NEWS: The **CQM Distinguished Lecture series** has been established in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.

## March – May 2019

Notice special day (Monday).

**An on-site density matrix description of the extended Falicov-Kimball model at finite temperatures**

**Abstract:** In an extended Falicov-Kimball model, an excitonic insulator phase can be stabilised at zero temperature. With increasing temperature, the excitonic order parameter (interaction-induced hybridisation on-site, characterised by the absolute value and phase) eventually becomes disordered, which involves fluctuations of both its phase and (at higher T) its absolute value. In order to build an adequate mean field description, it is important to clarify the nature of degrees of freedom associated with the phase and absolute value of the induced hybridization, and the corresponding phase space volume. We show that a possible description (including the phase space integration measure) is provided by the on-site density matrix parametrization. In principle, this allows to describe both the lower-temperature regime where phase fluctuations destroy the long-range order, and the higher temperature crossover corresponding to a decrease of absolute value of the hybridization relative to the fluctuations level. This picture is also expected to be relevant in other contexts, including the Kondo lattice model.

This work was supported by the Israeli Absorption Ministry.

**How simulations drive the discovery of novel materials, and novel physics**

First-principles simulations are one of the greatest current accelerators in the world of science and technology. To provide some context, one could mention that 30,000 papers on density-functional theory are published every year (this corresponds to an investment of roughly 3 billion US$ PPP); that 12 of these are in the top-100 most-cited papers in the entire history of science, engineering, and medicine; and that initiatives based on open science for codes, data, and simulation services are multiplying worldwide.

I’ll highlight some of our own scientific and technological perspectives on this, starting with the goals and the infrastructure needed to deliver on the promise of materials discovery, and applying it to the case study of ~1800 novel two-dimensional materials (including e.g. the first Kane-Mele quantum spin Hall insulator) and their possible applications in electronics or energy.

I’ll then argue how the need to compute some of the most relevant materials properties – in this case transport – forces us to critically re-evaluate some of the stalwarts of condensed-matter physics: learning that phonons are just a high-temperature approximation for the heat carriers, or discovering that the Boltzmann transport equation can be generalized to describe simultaneously the propagation and interference of phonon wavepackets, thus unifying the description of thermal transport in crystals and glasses.

**Partial lattice defects in higher order topological insulators**

Non-zero weak topological indices are thought to be a necessary condition to bind a single helical mode on lattice dislocations. In this work we show that higher-order topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations (including step edges) in the absence of weak topological indices. This helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. We present two examples of HOTIs, one intrinsic and one extrinsic, that show helical modes at partial dislocations. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can in principle significantly affect the expected conductivity in these materials.

Reference: https://arxiv.org/abs/1809.03518

**Note: special date (Thursday), time (9:30), and place (Laufer Center) just before the IACS research day begins.**

**Transversal transport coefficients and topological properties**

Spintronics is an emerging field in which both charge and spin degrees of freedom of electrons are utilized for transport. Most of the spintronic effects—like giant and tunnel

magnetoresistance—are based on spin- polarized currents which show up in magnetic

materials; these are already widely used in information technology and in data storage

devices. The next generation of spintronic effects is based on spin currents which occur in metals as well as in insulators, in particular in topologically nontrivial materials. Spin currents are a response to an external stimulus—for example electric field or temperature gradient — and they are always related to the spin-orbit interaction. They offer the possibility for future low energy consumption electronics. The talk will present a unified picture, based on topological properties, of a whole zoo of transversal transport coefficients: the trio of Hall, Nernst, and quantum Hall effects, all intheir conventional, anomalous, and spin flavour. The formation of transversal charge andspin currents as response to longitudinal gradients is discussed. Microscopic insight into all phenomena is presented by means of a quantum mechanical analysis based on the Dirac equation in combination with a semi-classical description which can be very elegantly studied within the concept of Berry curvature.

**Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition**

**abstract**

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the d-electron orbitals and the influence of this filling on structural configurations and responses. Curiously, some of the transition metal systems feature a large discrepancy between the long-range ordering temperatures (tens to hundreds of Kelvin) and the energy scales of the underlying electronic phenomena involved (hundreds to thousands of meV). In this presentation I will address this often ignored and largely unexplained disparity through a study of one such compound, CuIr_{2}S_{4} (CIS) spinel, where the orbital degrees of freedom play crucial role.

CuIr_{2}S_{4} displays temperature driven metal to insulator transition (MIT), where the low temperature insulating state consists of long range ordered Ir^{3+} (5d^{6}) and Ir^{4+} (5d^{5}) ions, with a four-fold periodicity, an example of tetrameric charge ordering [1]. Concurrently, spin dimerization of Ir^{4+} pairs occurs within the tetramer, with large associated structural distortions (0.5 Å) as they move towards each other, making this charge-order particularly amenable to detection using structural probes [2]. Notwithstanding the complexities of the insulating state, including formation of remarkable three-dimensional Ir^{3+}_{8}S_{24} and Ir^{4+}_{8}S_{24} molecule-like assemblies embedded in the lattice, its quasi-one-dimensional character was unmasked, and MIT attributed to an orbital-selective Peierls mechanism, postulated from topological considerations [3]. By utilizing a sensitive local structural technique, x-ray atomic pair distribution function analysis, we reveal the presence of fluctuating local-structural distortions deep in the high temperature metallic regime of CuIr_{2}S_{4 }[4]. The distortions are the fingerprints of a precursor high temperature state that enables the rich phenomenology observed at low temperature. Through judicious chemical substitutions, we show that this hitherto overlooked fluctuating symmetry lowering has electronic origin that can be understood as a local, fluctuating, orbital-degeneracy-lifted (ODL) state. This is related to, but qualitatively different from, the dimer-state observed in the insulating phase. Observation of the ODL state provides a natural way to understand the observed energy-scale discrepancy in a range of transition metal systems. Our study also presents a very new view on MIT and related phenomena in the material studied – CIS, and CIS-derived spinel systems – and experimentally verifies that the orbital sector indeed drives the physics in this material class.

While the electronic driving force for the formation is ubiquitous, the mechanisms of achieving the ODL state may be diverse (e.g. Jahn-Teller, local crystal field, covalency, molecular orbital formation, relativistic spin-orbit coupling, etc.). Our study exemplifies that such states exist but are difficult to detect and should be studied in a more systematic manner. The ODL state, characteristic of the high temperature regime, could be a critical ingredient and a missing link enabling more comprehensive understanding of phenomena as widespread as nematicity, pseudogaps, metal insulator transitions, spin glass behavior etc. Time permitting, the presentation will also spotlight a few other ODL systems such as perovskites, pyroxenes, and delafossites.

[1] P. G. Radaelli *et al*., Nature **416**, 155–158 (2002).

[2] E. S. Bozin *et al*., Physical Review Letters **106**, 045501 (2011).

[3] D. I. Khomskii & T. Mizokawa, Physical Review Letters **94**, 156402 (2005).

[4] E. S. Bozin *et al*., submitted, arxiv 1901.10104 (2019).

Increasing reproducibility of first principles methods and availability of computer power made first principles calculations an integral part of solid state physics and materials science. It is now possible to support and reproduce various experimental observations ab initio, and furthermore, it is also common to solve the inverse band structure problem to perform *materials by design*, i.e. to predict yet-to-be-synthesized solid compounds with desired electronic functionalities.

In this talk I am going to discuss some of my group’s recent efforts to perform correlated materials design. Correlated electronic phenomena, such as heavy fermion or Mott insulating physics require methods that go beyond the commonly used workhorse of materials simulations, *the *density functional theory (DFT). After a brief introduction of one such method, namely the Dynamical Mean Field Theory (DMFT), I will explore how DFT+DMFT can be used to come up with novel compounds that can be used as transparent conductors. Specific examples will include vanadates and niobates, and their double perovskites which display interesting Hund’s physics.

**Superfluidity of excitons and polaritons in novel two-dimensional materials**

**Abstract:**

This talk reviews the theoretical studies of the Bose-Einstein condensation (BEC) and superfluidity of indirect excitons and microcavity polaritons in quasi-two-dimensional (quasi-2D) van der Waals nanomaterials such as transition metal dichalcogenide (TMDC) heterostructures and phosphorene. Indirect excitons are the Coulomb-bound pairs of electrons and holes confined to different parallel monolayers of a layered planar nanomaterial structure. The high-T superfluidity of the two-component weakly-interacting Bose gas of the *A*-type and *B*-type indirect excitons in the TMDC heterostructures is proposed [1,2]. The critical temperature and superfluid velocity of the indirect excitons in a bilayer phosphorene nanostructure is shown to be anisotropic, dependent strongly on the particular direction of the exciton propagation [3]. The spin Hall effect for polaritons (SHEP) in a TMDC monolayer embedded in a microcavity is predicted [4]. It is demonstrated that two counterpropagating laser beams incident on a TMDC monolayer can deflect a superfluid polariton flow due to the generation the effective gauge vector and scalar potentials [4]. The polaritons cloud is formed due to the coupling of excitons created in a TMDC layer and microcavity photons. It was demonstrated that the polariton flows in the same valley are splitting: the superfluid components of the *A*and *B*polariton flows propagate in opposite directionsalong the counterpropagating beams, while the normal components of the flows slightly deflect in opposite directionsand propagate almost perpendicularly to the beams [4]. The components of polariton conductivity tensor were obtained for polaritons without Bose-Einstein condensation (BEC) and in the presence of BEC and superfluidity [4]. The possible experimental observation of SHEP is discussed. These results open up new avenues for the experimental realization of the exciton and polariton BEC and superfluidity phenomena as well as their practical applications in optoelectronics [5].

[1] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B **93**, 245410 (2016)

[2] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B **96**, 094502 (2017).

[3] O. L. Berman, G. Gumbs, and R. Ya. Kezerashvili, Phys. Rev. B **96**, 014505 (2017).

[4] O. L. Berman, R. Ya. Kezerashvili, and Yu. E. Lozovik, Phys. Rev. B **99**, 085438 (2019).

[5] D. W. Snoke and J. Keeling, Physics Today **70**, 54 (2017).

**Unconventional thermal transport **

David Broido

Department of Physics

Boston College

According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity, *k*_{L}, and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. In this talk, I will discuss aspects of a new paradigm for achieving high *k*_{L} that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering [1]. * *Our* ab initio* calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited *k*_{L} comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor [1]. In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting *k*_{L} [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5]. It gives rise to anomalous non-monotonic pressure dependence of *k*_{L} [6]. I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high *k*_{L}. An interesting case is that of group V transition metal carbides, NbC, TaC and VC. These metals have ideal vibrational properties for the desired weak phonon-phonon scattering. But, their nested Fermi surfaces give rise to strong scattering between phonons and electrons, which results in an orders-of-magnitude lower *k*_{L} that is nearly temperature independent, contrary to the typical behavior.

[1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013).

[2] T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).

[3] F. Tian et al., Science 361, 582 (2018).

[4] J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).

[5] S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).

[6] N. K. Ravichandran and D. Broido, Nature Communications (2019).

[7] C. Li, N. K. Ravichandran, L. Lindsay, and D. Broido,Phys. Rev. Lett. 121, 175901 (2018).

The advent of two-dimensional materials with hexagonal crystal symmetry offers a new electronic degree of freedom, namely valley, the manipulation and detection of which could potentially be exploited to form new many-body ground states as well as new paradigms of electronic applications. In this talk, I will describe our work in creating valley-momentum locked quantum wires, namely quantum valley Hall kink states, along artificial domain walls created by gating in Bernal stacked bilayer graphene. The (quantum valley Hall) kink states can carry current ballistically with a mean free path of several um’s. I will also demonstrate the operations of a topological valley valve and a tunable electron beam splitter, which exploit unique characteristics of the kink states. Because it uses topology, the operation of the valley valve does not require valley-polarized current. The high quality and versatile controls of the system open the door to many exciting possibilities in valleytronics and in pursuing fundamental physics of helical 1D systems.

J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. F. Ren, K. Watanabe, T. Taniguchi, Z. H. Qiao, J. Zhu,** **“Gate-controlled topological conducting channels in bilayer graphene”, *Nature Nanotechnology*, **11**, 1060 (2016)

Jing Li, Rui-Xing Zhang, Zhenxi Yin, Jianxiao Zhang, Kenji Watanabe, Takashi Taniguchi, Chaoxing Liu, Jun Zhu, “A valley valve and electron beam splitter”, *Science ***362**, 1149 (2018)

Jun Zhu - Penn State