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 2019 – January 2020

Oct
11
Fri
Riley Hanus (Northwestern U and ORNL)
Oct 11 @ 1:30 pm – 2:30 pm

host: Phil

Title: Heat conduction in defective and complex crystals:  phonon scattering and beyond

Riley Hanus*

* Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.

The flow of heat through materials is a topic of scientific interest and technological importance in fields of microelectronics, power generation, heat management, and thermoelectrics. For example, advancement in microelectronic technologies (e.g. microprocessors, and high-power electronics) demands ever more efficient removal of the heat generated in these devices. In contrast, technologies such as thermal barrier coatings and thermoelectric materials are designed to stop the flow of heat. In simple, defect-free crystals the thermal conductivity is generally well understood. However, in materials containing defects and/or in those with very complex crystal structures there is a lack of basic understanding which inhibits technological progress.

In this presentation, I will highlight several experimental and theoretical results which aim to establish a fundamental understanding of heat transport in defective materials. First, I will discuss several studies related to heat conduction across interfaces. Secondly, I will demonstrate in several material systems how defects can soften a materials lattice which reduces the phonon group velocity and thus decreases thermal conductivity. Lastly, the transition from crystalline-like to amorphous-like thermal conduction is investigated by studying the lattice dynamics of crystals with very complex crystal structures both computationally and experimentally. Through this analysis emerges a description of phonon transport which is divided between two channels. One is the standard phonon-gas transport mechanism and the other we term the diffuson-channel since it is mathematically the same mechanism in which ‘diffusons’ were defined.

Oct
18
Fri
Mehdi Jadidi (Columbia)
Oct 18 @ 1:30 pm – 2:30 pm

Title:  Optical Control of Chiral Charge Pumping in a Topological Weyl Semimetal

host: Dima Kharzeev

Abstract: Solids with topologically robust electronic states exhibit unusual electronic and optical transport properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the chiral population relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.
Oct
25
Fri
Laszlo Forro (EPFL, Lausanne)
Oct 25 @ 1:30 pm – 2:30 pm

host: Laszlo Mihaly

title: Why are hybrid halide perovskites exciting materials?
László Forró
Laboratory of Physics of Complex Matter
Ecole Polytechnique Fédérale de Lausanne
CH-1015 Lausanne
Recently, it has been shown that CH3NH3PbI3 is very promising material in photovoltaic devices1 reaching light conversion efficiency (η) up to 22%2. A strong research activity has been focused on the chemistry of the material to establish the most important parameters which could further improve η and to collect photons from a broad energy window. The major trend in this field is in photovoltaic device engineering although the fundamental aspects of the material are not yet understood.
In my lab we have devoted considerable effort to the growth of high quality single crystals at different length scales, ranging from large bulk crystals (up to 100 mm3) through nanowires3,4 down to quantum dots of tens of nanometers of linear dimensions. The structural tunability of the material allows to study a broad range of physical phenomena including electrical and thermal transport, magnetism, optical properties, band structure by photoemission etc. With a selected set of measurements I will demonstrate our enthusiasm for this material both in basic science and in device applications5,6.
Acknowledgement: The work has been performed in collaboration with Endre Horvath, Massimo Spina, Balint Nafradi, Peter Szirmai, Alla Araktcheva, Andrea Pisoni, Jacim Jacimovic, Andrzej Sienkiewicz, Claudio Grimaldi, Hugo Dil, Henrik Ronnow and many others.

References:
1. Lee, M. M. et al.,Science 338, 643-647 (2012).
2. see reports of the Gaetzel and Hagfeldt groups
3. Horvath et al., Nano Letters 14, 6761, (2015)
4.Spina et al., (2016) Scientific Reports, 6, 1
5.Spina et al., (2015) Small, 11, 4823 ; Spina et al., Nanoscale, 2016, 8, 4888
6.Nafradi et al., J. Phys. Chem. C 2015, 119, 25204

Nov
1
Fri
Vidvuds Ozolins (Yale)
Nov 1 @ 1:30 pm – 2:30 pm

host: Phil Allen

title: Carrier lifetime effects on thermoelectric efficiency

abstract

Recent developments in electronic structure algorithms based on the Wannier function interpolation of electronic wave functions have enabled accurate first-principles calculations of electron-phonon interactions and intrinsic carrier lifetimes in the relaxation time approximation. This has supplied the final missing piece of the puzzle for predicting the thermoelectric figure of merit zT=s S2 T/k, where the conductivity s, the Seebeck coefficient S, and the total thermal conductivity k now can all be obtained from the density-functional theory (DFT). This opens up exciting possibilities for theoretically understanding and reliably predicting new materials with high values of zT. We will review several examples from our recent work, including a Li-intercalated analogue of lead telluride (Li2TlBi), an intermetallic compound with unexpectedly high value of S (CoSi), and a theoretically predicted full Heusler compound with ultrahigh zT (Ba2BiAu). General factors for high thermoelectric power factors in these compounds include energy dependence of carrier lifetimes for high S, high degeneracy of carrier pockets at the Fermi level, weak electron-phonon scattering for high mobility, and concomitantly low Lorentz numbers for low electronic thermal conductivity.

Nov
8
Fri
Anderson Janotti (University of Delaware)
Nov 8 @ 1:30 pm – 2:30 pm

Title: Emergent Phenomena at the Interface of Complex Oxides

Host: Cyrus Dreyer

Progress in epitaxial growth of complex oxides have led to heterostructures with exquisite physical phenomena, such as the formation of a high-density two-dimensional electron gas (2DEG) at the interface between two normally insulating materials—e.g. SrTiO3/LaAlO3. Superconductivity and magnetic ordering have been demonstrated in these systems, sparking the interest in novel device applications. The formation of a 2DEG at the interface between SrTiO3 and Mott insulators, such as GdTiO3, has also been demonstrated, with electron densities that are over an order of magnitude higher than those realized with conventional semiconductors. Charge transport in these systems exhibit intriguing behavior, varying drastically from metal to insulator depending on the thickness of the building-block layers. Intensive research efforts in the last decade have raised questions regarding the origin of the excess charge, the mechanisms that determine the density of the 2DEG, and fundamental properties of the 2DEG. In this presentation, we will discuss how computer simulations can provide insights into the origin and nature of the 2DEG. Based on results of first-principles calculations we will discuss electron correlation effects and how the electronic structure of these heterostructures can be drastically altered, turning from metallic into insulating, through charge localization in ultrathin layers. Finally, we will address the interplay between orbital, charge, and spin in the manipulation of the magnetic ordering observed in some of these heterostructures.

Nov
15
Fri
Peter W. Stephens (SBU P&A)
Nov 15 @ 1:30 pm – 2:30 pm

title: Manganese Cyanide Tinkertoys

Nov
22
Fri
tentative special seminar
Nov 22 @ 1:30 pm – 2:30 pm

tentatively reserved by phil

Dec
6
Fri
Justin Wilson (Rutgers)
Dec 6 @ 1:30 pm – 2:30 pm

Host: Jen Cano

Dec
13
Fri
Navaneetha K. Ravichandran (Boston College)
Dec 13 @ 1:30 pm – 2:30 pm

host: Phil

Jan
31
Fri
John Tranquada (BNL)
Jan 31 @ 1:30 pm – 2:30 pm

host: Phil