CENTER FOR QUANTUM MATERIALS AND CONDENSED MATTER PHYSICS SEMINARS

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.

The lectures in this series will attract a broad audience of physicists from SBU and BNL,
and SBU graduate students.

November 2018 – April 2019

Nov
30
Fri
Peter D. Johnson: Topology meets High Tc Superconductivity in the FeTe1-xSex family
Nov 30 @ 1:30 pm – 2:30 pm

Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, New York 11973

Abstract

 

Low energy, laser-based ARPES with variable light polarization, including both linear and circularly polarized, is used to examine the Fe-based superconductor family, FeTe1-xSex.  At the center of the Brillouin zone we observe the presence of a Dirac cones with helical spin structure as expected for a topological surface state and as previously reported in the related FeTe0.55Se0.45.  These experimental studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system.  Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with the experimental observations.  In the bulk superconducting state for FeTe0.7Se0.3 the system appears to reflect the presence of some level of orbital selectivity in the pairing even though the system is in the tetragonal phase above and below the transition temperature Tc.  At the same time the topological state appears to acquire mass at the superconducting transition, possibly indicative of time reversal symmetry breaking.  These observations are discussed in detail.

Feb
22
Fri
Benjamin Jungfleisch (U. of Delaware)
Feb 22 @ 1:30 pm – 2:30 pm

NEW TWISTS FOR MAGNONS
Matthias Benjamin Jungfleisch
Department of Physics and Astronomy
University of Delaware

mbj@udel.edu

In recent years, the exploration of magnons, the quanta of spin waves, as carriers of spin-angular momentum has flourished in spintronics. Magnon spintronics aims at developing novel functional devices that combine magnonic and electronic spin transport phenomena.
In particular, magnetic metamaterials such as artificial spin ice and magnonic crystals offer unique possibilities in magnon spintronics. Here, we present results on high-frequency dynamics in metallic artificial spin-ice lattices by employing broadband ferromagnetic resonance spectroscopy [1]. Furthermore, we explore the possibility to drive and to detect spin dynamics in those systems by dc electrical means using the spin Hall effect [2,3].
Besides magnetic metamaterials, magnetic insulators such as yttrium iron garnet (YIG) are ideal materials for magnonic and spintronic research since they feature long magnon propagation distances and coherence times. Here, we demonstrate the propagation of spin waves in nanometer-thick YIG waveguides [4] and the electric excitation and detection of spin dynamics via pure spin currents by the spin Hall effect in YIG/Pt micro- and nanostructures [5,6].

This work was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.

REFERENCES:
[1] M. B. Jungfleisch et al., Phys. Rev. B 93, 100401(R) (2016).
[2] M. B. Jungfleisch et al., Appl. Phys. Lett. 108, 052403 (2016).
[3] M. B. Jungfleisch et al., Phys. Rev. Applied 8, 064026 (2017).
[4] M. B. Jungfleisch et al., J. Appl. Phys 117, 17D128 (2015).
[5] M. B. Jungfleisch et al., Phys. Rev. Lett. 116, 057601 (2016).
[6] M. B. Jungfleisch et al., Nano Lett. 17, 8 (2017).

 

Mar
1
Fri
Layla Hormozi (BNL)
Mar 1 @ 1:30 pm – 2:30 pm
Quantum computing with topological qubits 
Abstract: A topological quantum computer is a hypothetical device in which intrinsic fault-tolerance is embedded in the hardware of the quantum computer. It is envisioned that in these devices quantum information will be stored in certain topologically-ordered states of matter and quantum computation will be carried out by braiding the world-lines of quasiparticle excitations that obey non-Abelian statistics, around one another, in specific patterns. I will review some of the properties of these states, and describe a general method for finding braiding patterns that correspond to a universal set of quantum gates on encoded topological qubits, based on quasiparticles that can be realized as excitations of certain fractional quantum Hall states.
Mar
8
Fri
Evan J. Philip (Stony Brook University)
Mar 8 @ 1:30 pm – 2:30 pm

Chiral Photocurrents and Terahertz Emission in Dirac and Weyl Materials

Abstract:
Recently, chiral photocurrents have been observed in Weyl materials. I will discuss a new mechanism we proposed for photocurrents in Dirac and Weyl materials in the presence of magnetic fields that, unlike previously proposed effects, does not depend on any asymmetry of the crystal. This Chiral Magnetic Photocurrent would be an independent probe of the chiral anomaly. I will also discuss an observation of terahertz emission in the Weyl material TaAs with tunable ellipticity due to chiral photocurrents induced by an ultrafast near infrared laser.
References:
arXiv:1810.02399 [cond-mat.mes-hall]
arXiv:1901.00986 [cond-mat.mtrl-sci]
Mar
11
Mon
Pablo Ordejon (ICN2, CSIC), Barcelona: Charge Density Waves in the Blue Bronzes and some 2D Transition Metal Dichalcogenides
Mar 11 @ 1:30 pm – 2:30 pm

NOTE THIS IS A MONDAY (SPECIAL SEMINAR)

I will present some of our recent work on the understanding of Charge Density Wave (CDW) instabilities of several materials, by means of Density Functional Theory (DFT) calculations. The presentation will focus on the correlation between the crystal structure and the electronic properties, with special emphasis on the structural instabilities which have an electronic origin. I will present results for the blue bronze, K0.3MoO3, a tradicional system in which the CDW is originated by a Peierls instability. For this material, the Lindhard response function computed from DFT is able to account quantitatively for the Peierls scenario. I will also show results in connection with recent experimental studies that have been able to demonstrate the presence of charge density waves in several 2D single-layer materials like NbSe2, TiSe2 and TiTe2. For NbSe2, we have focused on the nature and atomic displacements associated with the CDW. The evolution of the CDW with external electrostatic doping, which has been achieved experimentally using field effect transistor setups, will be analysed for the case of TiSe2. For the case of TiTe2, we focus on the recently observed CDW in the single layer, which is not present in the bulk material.

Mar
15
Fri
Binghai Yan (Weizmann Institute): Anomalous electric and thermal properties of magnetic Weyl semimetals
Mar 15 @ 1:30 pm – 2:30 pm

Topological Weyl semimetals provide a new stage to examine exotic transport phenomena such as the chiral anomaly and the anomalous Hall effect. In the ordinary longitudinal transport, the Wiedemann-Franz law links the ratio of electronic charge and heat conductivity to fundamental constants. It has been tested in numerous solids, but the extent of its relevance to the anomalous transverse transport, which represents the topological nature of the wave function, remains an open question. In this talk, I will first introduce recently-discovered Weyl materials Mn3Sn and Mn3Ge. Their noncollinear chiral spin structure induces huge anomalous Hall effect. Then I will talk about our recent work on the thermal Hall effect. In collaboration with the experiment, we reveal a finite temperature violation of the Wiedemann-Franz correlation. This violation is caused by the Berry curvature, rather than the inelastic scattering as observed in ordinary metals.

Mar
18
Mon
Denis Golosov (Bar-Ilan University) @ B-131
Mar 18 @ 1:30 pm – 2:30 pm

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.

Mar
29
Fri
Nicola Marzari (EPFL, Lausanne)
Mar 29 @ 1:30 pm – 2:30 pm

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.

Apr
5
Fri
Jennifer Cano (Stony Brook University)
Apr 5 @ 1:30 pm – 2:30 pm

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

Apr
11
Thu
Ingrid Mertig (Martin Luther University, Halle-Wittenberg, Germany) @ Laufer Center Conference Room 107
Apr 11 @ 9:30 am – 10:30 am

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.