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.
April – September 2019
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.
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
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, CuIr2S4 (CIS) spinel, where the orbital degrees of freedom play crucial role.
CuIr2S4 displays temperature driven metal to insulator transition (MIT), where the low temperature insulating state consists of long range ordered Ir3+ (5d6) and Ir4+ (5d5) ions, with a four-fold periodicity, an example of tetrameric charge ordering . Concurrently, spin dimerization of Ir4+ 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 . Notwithstanding the complexities of the insulating state, including formation of remarkable three-dimensional Ir3+8S24 and Ir4+8S24 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 . 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 CuIr2S4 . 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.
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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
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 . The spin Hall effect for polaritons (SHEP) in a TMDC monolayer embedded in a microcavity is predicted . 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 . 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 Aand Bpolariton 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 . The components of polariton conductivity tensor were obtained for polaritons without Bose-Einstein condensation (BEC) and in the presence of BEC and superfluidity . 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 .
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 D. W. Snoke and J. Keeling, Physics Today 70, 54 (2017).
Unconventional thermal transport
Department of Physics
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, kL, 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 kL that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering . Our ab initio calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited kL comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor . In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting kL [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5]. It gives rise to anomalous non-monotonic pressure dependence of kL . I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high kL. 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 kL that is nearly temperature independent, contrary to the typical behavior.
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 T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).
 F. Tian et al., Science 361, 582 (2018).
 J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).
 S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).
 N. K. Ravichandran and D. Broido, Nature Communications (2019).
 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
Room B-131 Physics, Stony Brook University. Tuesday June 25, 1:30 pm. host: PBA
Physical properties of (Mn 0.85 Fe 0.15)Si along the critical trajectory
A.E. Petrova and S.M. Stishov
Institute for High Pressure Physics of RAS, Troitsk, Moscow, Russia
Institut für Physik der Kondensierten Materie, Technische Universität Braunschweig, D-38106 Braunschweig, Germany
The magnetic phase transition temperature in the helical magnet MnSi decreases with pressure and practically reaches zero value at ~15 kbar. However a nature of this transition at zero temperature and high pressure is still a subject of controversial interpretations. Early it was claimed an existence of tricritical point on the phase transition line that might result in a first order phase transition in MnSi at low temperatures, preventing observation a quantum critical point in MnSi. On the other hand some experimental works and the recent Monte-Carlo calculations may indicate a strong influence of inhomogeneous stress arising at high pressures and low temperatures on characteristics of phase transitions that could make any experimental data not entirely conclusive. In this situation it would be appealing to use a different approach to discover a quantum criticality in MnSi, for instance, making use doping as a control parameter. The results of studying the magnetization, specific heat and thermal expansion of a single crystal with nominal composition Mn 0.85 Fe 0.15 Si show that the trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e. approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.
Title: Structure Inference from X-ray Absorption Spectroscopy: Pilot Projects for Operando Experimentation
host: Phil Allen
abstract: X-ray Absorption Near Edge Structure (XANES) is well-adapted for in situ and operando experiments. It is both atomically specific and it encodes local structure of the surrounding atoms. Due to multiple scattering effects, inferring that structure from the spectra can be complex. In the Center for Functional Nanomaterials, we are pursuing several prototype projects with collaborating groups to explore approaches to solve this inverse problem and pursue nanomaterials research enabled by these methods. We seek to go beyond the empirical fingerprint method, particularly to broaden applicability to structural motifs that emerge in studies of new or nanostructured materials. Our over-arching approach is two-fold: exploit theory for direct computation of XANES spectra for a pertinent database of material structures to support and train data analysis techniques; develop and validate these techniques through comparison to experiments. Following a brief introduction of operando experimental techniques and the role of X-ray absorption spectroscopy, I will describe a series of pilot projects [1-3] that illustrate different aspects of structure inference, including the training of artificial neural network models.
Work performed in part at the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.
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 M. R. Carbone, et al., Phys. Rev. Mater. 3, 033604 (2019).
 D. Yan, et al., Nano Lett. 19, 3457, (2019).
Brief Biography: Mark S. Hybertsen holds a BA in Physics from Reed College in Portland, OR (1980) and a PhD in Physics from The University of California, Berkeley (1986) where his thesis research was directed to many-body perturbation theory and the GW approach. Dr. Hybertsen joined Bell Laboratories in 1986, pursuing a variety of research projects in the theory of the electronic properties of materials. He supervised the Device and Materials Physics Group in the Semiconductor Photonics Research Department for four years. From 2003 to 2006, Dr. Hybertsen was a Senior Research Scientist in the Department of Applied Physics and Applied Mathematics at Columbia University in New York, where he has also been an Adjunct Professor in the Department of Electrical Engineering. In 2006, Dr. Hybertsen joined the new Center for Functional Nanomaterials at Brookhaven National Laboratory. He is a Senior Scientist, leading the Theory and Computation Group. He has also had adjunct research appointments at Columbia University. Dr. Hybertsen is a fellow of the American Physical Society and a member of the IEEE and the American Chemical Society.