USC Physics Seminars

                                   Quantum Information – Condensed Matter – Biophysics

These seminars are scheduled on Fridays at 2:00pm, and the location is SSL 150, unless otherwise noted. Some of the seminars will be held jointly with UCLA and CALTECH.
For more information contact Lorenzo Campos Venuti (condensed matter) or Ben Reichardt (quantum information). Biophysics seminars are held the first Friday of each month and are indicated in blue.
UPC seminar location: Ahmanson Center for Biological Research, ACB 238. Location for remote viewing of HSC seminars: ACB 238
HSC seminar location: Herklotz seminar room, Zilkha Neurogenetic Institute HSC.  Location for remote viewing of UPC seminars: Herklotz

Friday October 26, 2pm SSL 150

Ron Naaman (Weizmann Institute of Science)
Chiral Molecules and the Electron’s Spin: From Spintronics to Enantio-Separation

Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. However, we found that chiral organic molecules act as spin filters for photoelectrons transmission, in electron transfer, and in electron transport. 
The new effect, termed Chiral Induced Spin Selectivity (CISS),, was found, among others, in bio-molecules and in bio-systems. It has interesting implications for the production of new types of spintronics devices, and on electron transfer in biological systems. Recently we found that charge polarization in chiral molecules is accompanied by spin polarization. This finding shed new light on enantio-specific interactions and it opens the possibility to construct novel methods for enantio-separation.

Friday November 9, 2pm SSL 150

Angelo Lucia (Caltech)
Size-Driven Quantum Phase Transitions

The standard approach to mathematically describe phase transitions in classical and quantum many-body models is to consider sequences of increasing size and then study their limit, which is usually called the "thermodynamic limit". Nonetheless numerical simulations and real world experiments can only consist of a finite number of particles, from which is sometimes possible to extrapolate information about what happens in the limit.  While this approach has been generally very successful, it has been shown recently that some properties of the thermodynamic limit are undecidable, i.e. there cannot exist an algorithm which predicts them. This implies that very exotic behaviour can appear: as an example, I will show how to construct 2D quantum spin models which appear to be classical at finite sizes but will reveal their nature of topological models at system sizes inaccessible for all practical purposes.

Friday November 16, 2pm SSL 150
Claudia Ojeda-Aristizabal (California State University, Long Beach)
Thin film C60: a new available block for the building of van der Waals heterostructures

C60, the organic molecule made of sixty carbon atoms arranged in a truncated icosahedron, has brought a lot of excitement to condensed matter physicists, chemists and material scientists since it appearance in 1985. The combination of the buckyball structure with important electron-electron and electron-phonon interactions, bring unique properties not seen in ordinary non-molecular crystalline materials, such as superconductivity at relatively high temperatures when doped with alkaline metals. In its bulk form, C60 arranges itself in a face-centered cubic (fcc) lattice (one C60 molecule centered at each lattice site), while in its thin film form it is deposited in layers corresponding to the (111) direction of the bulk, forming a triangular lattice. As an initial approximation one would think that C60’s electronic structure is dominated by the electronic interactions within a single molecule. Instead, we have found that long range interactions between the molecules have a profound effect shaping the electronic structure of the material. In this talk, I will show angle resolved photoemission spectroscopy experiments and density functional calculations that support this claim. I will also discuss an electronic device made of a van der Waals heterostructure composed of a thin film crystalline C60, graphene and hexagonal boron nitride.
C. Ojeda-Aristizabal*, E. J. G. Santos* et. al.  ACS Nano 11, 4686 (2017).
D. W. Latzke*, C. Ojeda-Aristizabal* et. al. Submitted (2018)
D. W. Latzke*, C. Ojeda-Aristizabal* et. al. In preparation (2018)

Friday November 30, 2pm SSL 150

Andreas Bill
(California State University, Long Beach)
Intertwined states in Superconducting-Magnetic Hybrid Nanostructures

Modern developments in spintronics and quantum computing rely on nanoscale devices where the role of interfaces between materials with different physical properties is dominant. Typically, none of the competing ground states is established and the resulting electronic state is intertwined with new features. We discuss multiple facets of intertwined states in superconducting-magnetic hybrid nanostructures with localized or itinerant magnetism. Superconducting pair correlations, magnetization and variations of the charge density are determined self-consistently. Josephson currents through various inhomogeneous magnetic multilayers are calculated. We provide an overview of possible states, how the symmetry of pair correlations is strongly modified by tunable magnetic inhomogeneities and discuss their measurable signature in the Josephson current. We also show how electronic phase separation and the competition of magnetic and superconducting orders occur in itinerant ferromagnets.

Support from the National Science Foundation (DMR-1309341) is gratefully acknowledged.

Friday December 7, 2pm SSL 150
Takahiro Sagawa (University of Tokyo)
Second law and eigenstate thermalization in isolated quantum many-body systems

In recent years, the fundamental mechanism of thermalization of isolated many-body quantum systems has attracted renewed attentions, in light of quantum statistical mechanics, quantum information theory, and quantum technologies. In particular, it has been recognized that the eigenstate thermalization hypothesis (ETH) plays a crucial role in understanding the mechanism of thermalization, which states that even a single energy eigenstate is thermal if the system is quantum chaotic.
In this talk, I will discuss our recent results on the second law of thermodynamics for pure quantum states [1].   In our setup, the entire system obeys unitary dynamics, where the initial state of the heat bath is not the Gibbs ensemble but a single energy eigenstate.   Our proof is mathematically rigorous, and the Lieb-Robinson bound plays a crucial role.   In addition, I will talk about our numerical result on large deviation analysis of the ETH [2], which directly evaluates the number of athermal energy eigenstates and validates the ETH.    Our results would reveal a general scenario that thermodynamics emerges purely from quantum mechanics.
[1] E. Iyoda, K. Kaneko, and T. Sagawa, Phys. Rev. Lett. 119, 100601 (2017).  
[2] T. Yoshizawa, E. Iyoda, and T. Sagawa, Phys. Rev. Lett. 120, 200604 (2018).  

Friday February 1, 2pm SSL 150
Masoud Mohseni (Google)
Approximate optimization and sampling with tensor networks

We introduce a deterministic physics-inspired classical algorithm to efficiently reveal the structure of low-energy spectrum for certain low-dimensional spin-glass systems that encode optimization problems. We employ tensor networks to represent Gibbs distribution of all possible configurations. We then develop techniques to extract the relevant information from the networks for quasi-two-dimensional Ising Hamiltonians. In particular, for the hardest known problems devised on Chimera graph known as Deceptive Cluster Loops, for up to 2048 spins, we we find 10^{8} of the high quality solutions in a single run of our algorithm. To this end, we apply a branch and bound strategy over marginal probability distributions by approximately evaluating tensor contractions. Our approach identifies configurations with the largest Boltzmann weights corresponding to low energy states. Moreover, by exploiting local nature of the problems,  we discover spin-glass droplets geometries. This naturally encompasses sampling from high quality solutions within a given approximation ratio which is #P hard. It is thus established that tensor networks techniques can provide profound insight into the structure of disordered spin complexes, with ramifications both for machine learning and noisy intermediate-scale quantum devices. At the same time, limitations of our approach highlight alternative directions to establish quantum speed-up and possible quantum supremacy experiments.

Friday February 15, 2pm SSL 150
Matthew Gilbert (Stanford)
The Non-Hermitian Chern Insulator

During the past decade, the landscape of condensed matter physics has been dominated by a desire to understand the topological nature of materials and the observable consequences that are associated with the presence of topology. To this end, topological band theory has provided a foundation that has allowed for the definition of topological invariants, or conserved quantities that do not change based on adiabatic changes to the system parameters. Nonetheless, each of the systems that has been considered to this point, have relied on the fact that the system under consideration is both closed and Hermitian. Recent work has extended topological band theory to open, non-Hermitian Hamiltonians but little is understood about how non-Hermicity alters the topological quantization of associated observables. In this talk, we will begin to address these problems by examining the non-Hermitian Chern insulator where we focus on changes in observables and its relation to our current understanding about non-Hermitian topological band theory.

Friday March 22, 2pm SSL 150
Philip Stamp (University of British Columbia)


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