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.

October 2018 – March 2019

Oct
19
Fri
Renata Wentzcovitch
Oct 19 @ 1:30 pm – 2:30 pm

Spin crossover in iron in lower mantle minerals

Department of Applied Physics and Applied Mathematics and

Department of Earth and Environmental Science,

Lamont Doherty Earth Observatory, Columbia University

Pressure and temperature-induced spin state change in iron in lower mantle minerals is an unusual phenomenon with previously unknown consequences.  High pressure and high temperature experiments have offered a wealth of new information about this class of materials problems, which includes the insulator to metal transition in Mott systems. I will discuss key experimental data, contrast them with ab initio results and thermodynamic models, show the implications for fundamental phenomena taking place at the atomic scale and their macroscopic manifestations, and discuss potential geophysical consequences of this phenomenon.

              

Oct
26
Fri
Michael Martin (Advanced Light Source, LBL) “SINS: Synchrotron Infrared Nano Spectroscopy extending into the far-IR”
Oct 26 @ 1:30 pm – 2:30 pm
Abstract: Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful imaging and spectroscopic tool for investigating nanoscale heterogeneities in biology, quantum matter, and electronic and photonic devices. However, many materials are defined by a wide range of fundamental molecular and quantum states at far-infrared (FIR) resonant frequencies currently not accessible by s-SNOM. Here we show ultrabroadband FIR s-SNOM nano-imaging and spectroscopy by combining synchrotron infrared radiation with a novel fast and low-noise copper-doped germanium (Ge:Cu) photoconductive detector [1]. This approach of FIR synchrotron infrared nanospectroscopy (SINS) extends the wavelength range of s-SNOM to 33µm (330 cm −1 , 10 THz), exceeding conventional limits [2] by an octave toward lower energies. We demonstrate this new nano-spectroscopic window by measuring elementary excitations of exemplary functional materials, including surface phonon-polariton waves and optical phonons in oxides and layered ultrathin van der Waals materials, skeletal and conformational vibrations in molecular systems, and the highly tunable plasmonic response of graphene.
Continued detector development in collaboration with NSLS-II will further extend the range of FIR SINS to ultimately bridge the energy gap with available THz s-SNOM sources, yet in a single nano-spectroscopy instrument. This work highlights the continued advantage of synchrotron radiation as an ultra-broadband coherent light source for near-field nano-spectroscopy, especially in the long wavelength regime where alternative low-noise, broadband, quasi-cw laser sources are not readily available.
References
[1] Khatib, Bechtel, Martin, Raschke, Carr,  ACS Photonics, 5(7), 2773–2779(2018).
[2] Bechtel, Muller, Olmon, Martin, Raschke, PNAS 111(20), 7191–7196 (2014).
Nov
2
Fri
Tyler M. Cocker (Michigan State University)
Nov 2 @ 1:30 pm – 2:30 pm

Ultrafast terahertz microscopy: from near fields to single atoms

 

A new experimental frontier has recently emerged with the potential to significantly impact
physics, chemistry, materials science, and biology: the regime of ultrafast time resolution and ultrasmall spatial resolution. This is the domain in which single atoms, molecules, and electronic orbitals move. It also corresponds, on larger length scales, to the territory of low-energy elementary excitations such as plasmons, phonons, and interlevel transitions in excitons. These processes are of particular importance for nanomaterial functionality and typically survive for only femtoseconds to picoseconds after photoexcitation. In this talk, I will show how these diverse dynamics can be studied with new techniques that combine terahertz technology with scanning probe microscopy. First, I will describe how ultrafast near-field microscopy has been employed to perform sub-cycle spectroscopy of single
nanoparticles [1], reveal hidden structure in correlated electron systems [2], and resolve transient interface polaritons in van der Waals heterostructures [3]. Then I will discuss the development of a related technique: lightwave-driven terahertz scanning tunneling microscopy [4,5]. In this novel approach, the oscillating electric field of a phase-stable, few-cycle light pulse at an atomically sharp tip can be used to remove a single electron from a single molecular orbital within a time window faster than an oscillation cycle of the terahertz wave. I will show how this technique has been used to take ultrafast snapshot images of the electron density in single molecular orbitals (e.g. Figure 1) and watch the motion of a single molecule for the first time [5].

Figure 1: Ultrafast snapshot of the electron density in the highest molecular orbital of a single pentacene molecule resolved with lightwave-driven terahertz scanning tunneling microscopy [5].

References:
[1] M. Eisele et al., Nature Photon. 8. 841 (2014).
[2] M. A. Huber et al., Nano Lett. 16, 1421 (2016).
[3] M. A. Huber et al., Nature Nanotech. 12, 207 (2017).
[4] T. L. Cocker et al., Nature Photon. 7, 620 (2013).
[5] T. L. Cocker et al., Nature 539, 263 (2016).

Nov
9
Fri
Ignace Jarrige (BNL)
Nov 9 @ 1:30 pm

Bright Lights, Big Opportunities for Quantum Materials Research at NSLS-II

 

Abstract: The realization of quantum materials for energy science and quantum information applications requires an understanding of competing electronic phases spanning multiple scales of energy, time and length. Operating since 2014, the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory has become a nexus for X-ray based studies of the electronic properties to advance knowledge of these key issues. Following an overview of NSLS-II and its beamlines tailored for quantum materials research, this talk will review the capabilities and current status of the soft resonant inelastic X-ray scattering (RIXS) beamline, called SIX. Recent research examples involving transition-metal oxides and 4f Kondo systems will be showcased.

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.