# 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.

## November 2018 – April 2019

*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, FeTe_{1-x}Se_{x}. 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 FeTe_{0.55}Se_{0.45}. 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 FeTe_{0.7}Se_{0.3} 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 T_{c}. 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.

**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).

**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.

**Chiral Photocurrents and Terahertz Emission in Dirac and Weyl Materials**

**Abstract:**

NOTE THIS IS A MONDAY (SPECIAL SEMINAR)

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.

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.