Unfortunately, seminars are cancelled for the remainder of the semester until further notice, due to COVID-19.
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.
October 2018 – March 2019
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.
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 , reveal hidden structure in correlated electron systems , and resolve transient interface polaritons in van der Waals heterostructures . 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 .
 M. Eisele et al., Nature Photon. 8. 841 (2014).
 M. A. Huber et al., Nano Lett. 16, 1421 (2016).
 M. A. Huber et al., Nature Nanotech. 12, 207 (2017).
 T. L. Cocker et al., Nature Photon. 7, 620 (2013).
 T. L. Cocker et al., Nature 539, 263 (2016).
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.
Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, New York 11973
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.
NEW TWISTS FOR MAGNONS
Matthias Benjamin Jungfleisch
Department of Physics and Astronomy
University of Delaware
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 . 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  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.
 M. B. Jungfleisch et al., Phys. Rev. B 93, 100401(R) (2016).
 M. B. Jungfleisch et al., Appl. Phys. Lett. 108, 052403 (2016).
 M. B. Jungfleisch et al., Phys. Rev. Applied 8, 064026 (2017).
 M. B. Jungfleisch et al., J. Appl. Phys 117, 17D128 (2015).
 M. B. Jungfleisch et al., Phys. Rev. Lett. 116, 057601 (2016).
 M. B. Jungfleisch et al., Nano Lett. 17, 8 (2017).
Chiral Photocurrents and Terahertz Emission in Dirac and Weyl Materials
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.