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 September 8, 2pm SSL 150
Tahir Yusufaly (USC)

Friday September 15, 2pm SSL 150
Peter Young (UC Santa Cruz)
Critical Phenomena and Griffiths-McCoy Singularities in Quantum Spin Glasses

Quantum phase transitions (i.e. phase transitions at zero temperature) in disordered systems can display unusual features, such as "infinite-randomness" critical phenomena, and Griffiths-McCoy (GM) power-law singularities even in the paramagnetic phase. In this lecture I will review these concepts in the context of quantum spin glasses, discuss the results of some (mainly old) Quantum Monte Carlo simulations, and conclude by describing some recent calculations with Rajiv Singh using series expansions to explore quantum spin glass critical behavior and GM singularities in a wide range of dimensions.

Friday September 22, 2pm SSL 150
Salvatore Mandrà (NASA)
Numerical and experimental results on problem optimization using classical and quantum heuristics

Quantum technologies have finally reached the astonishing level that commercialized quantum devices can compete with classical devices. Among the possible quantum paradigms, quantum annealing has the potential to be a disruptive technology and overcome classical heuristics in the optimization of energy landscapes. Unfortunately, excluding few theoretical and numerical results, suitable problems that show a quantum speed-up are still missing. In my talk, I will review some of my last results ([1-3]), including experimental and numerical results for a class of promising problems for which the D-Wave quantum device has the potential to show an advantage with respect to classical heuristics.

[1] S. Mandrà, Z. Zhu, W. Wang, A. Perdomo-Ortiz, H.G. Katzgraber, "Strengths and weaknesses of weak-strong cluster problems: A detailed overview of state-of-the-art classical heuristics versus quantum approaches", Physical Review A 94 (2), 022337
[2] S. Mandrà, Z. Zhu, H.G. Katzgraber, "Exponentially-Biased Ground-State Sampling of Quantum Annealing Machines with Transverse-Field Driving Hamiltonians", Physical Review Letters 118 (070502)
[3] S. Mandrà, H.G. Katzgraber, C. Thomas, "The pitfalls of planar spin-glass benchmarks: Raising the bar for quantum annealers (again)", Quantum Science and Technology 2 (3)

Friday October 13, 2pm SSL 150
David Hsieh (Caltech)

Friday October 20, 2pm SSL 150
Israel Felner (Hebrew University of Jerusalem)
Search for new high Tc superconductors and unusual irreversible magnetic behavior in three unrelated  substances

Following the phase diagram of the well-known 122 material Ba-Fe-As in which superconductivity (SC) emerges from magnetic states, we synthesized and measured dozens of 122 materials of the Y(Lu)Fe2-xMx(Si,Ge)2 type (M=3d element).  In all samples pronounced magnetic peaks appear at various temperatures.  Their nature will be discussed.  Unfortunately, no SC traces have been observed.  On the other hand, traces of two SC phases (with TC=32 K and 66 K) have been observed in inhomogeneous commercial and fabricated amorphous carbon doped with sulfur (a-CS).
The non-superconducting a-CS samples  exhibit pronounced peaks in their virgin zero-field-cooled (ZFC) curves at TP ~50-80 K.  Around these peaks the field-cooled (FC) curves cross the ZFC plots, thus in a certain temperature range ZFC>FC.  This complex behavior disappears in the second ZFC run. 

The same peculiar observation (ZFC>FC) was observed in two other unrelated systems:
(i) In a chiral-based magnetic memory device where the main components are α-helix L-polyalanine adsorbed on gold, Al2O3, and Co or Ni layers.  The ZFC>FC phenomenon is observed only in the hard direction of the layers.
(ii) In pathological liver tissues taken from a patient with hematological malignancies.

The unusual ZFC>FC phenomenon cannot be ascribed to extra magnetic phases (oxygen or magnetite), and is believed to be an intrinsic property of the three unrelated systems.  We may assume that in the ground state the intrinsic local magnetic moments in each system are randomly distributed.  In the first ZFC runs, low dc magnetic field aligns these moments to flip along its direction in a FM manner up to TP.  Above TP, an antiparallel exchange (AFM) coupling is more favored and in the next ZFC and FC processes the net magnetic moments are lower and cross the ZFC branches.  Alternatively, we may speculate that all systems are in the so-called two-state system separated by a certain energy barrier.

October 27, 2pm HSC
Ralf Langen, Ph.D. (USC)

Friday November 17, 2pm SSL 150
Michael Fink (Harvard)



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