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 SSC 319
David Hsieh (Caltech)
Hidden Odd-Parity Orders in Spin-Orbit Coupled Correlated Electron Systems

The 5d transition metal oxides are predicted to host a variety of exotic electronic phases emerging from the interplay of strong spin-orbit coupling and electron-electron correlations. In this talk, I will describe the application of our recently developed nonlinear optical anisotropy and microscopy techniques to uncover novel parity-breaking phases in such materials. In particular, I will present evidence of a magnetic multipolar ordered state in the doped spin-orbit assisted Mott insulator Sr2IrO4 and draw comparisons to the ordered state underlying the pseudogap region of high-Tc superconducting cuprates. I will also show evidence of a 3D electronic nematic phase in the correlated spin-orbit coupled metal Cd2Re2O7 and discuss its implications for the superconductivity state that emerges at lower temperature. 

Friday October 20, 2pm SSL 150
Guifre Vidal (Perimeter Institute)
Tensor networks, conformal field theory, and discrete geometry

At a quantum critical point, the universal properties of a quantum spin chain are captured by an emergent conformal field theory (CFT). First we will explain how to numerically characterize the emergent CFT starting only from a generic microscopic lattice Hamiltonian, using the Koo-Saleur formula augmented with the matrix product state formalism. Then we will explain in which sense a tensor network representation of critical systems can be interpreted as describing a discretized geometry -- as it is often suggested in the context of the AdS/CFT correspondence of high energy physics.

Friday October 27, 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.

November 3, 2pm HSC
Ralf Langen, Ph.D. (USC)

Friday November 10, 2pm SSL 150
Eliot Kapit (Tulane)

Improved quantum annealer performance from oscillating transverse fields

Quantum annealing is a promising application of quantum hardware for solving hard classical optimization problems. The runtime of the quantum annealing algorithm, in absence of noise or other effects such as the constructive interference of multiple diabatic crossings, and at constant adiabatic evolution rate, is proportional to the inverse minimum gap squared. In this article, we show that for a large class of problem Hamiltonians, one can improve in the runtime of a quantum annealer (relative to minimum gap squared scaling) by adding local oscillating fields, which are not amenable to efficient classical simulation. For many hard $N$-qubit problems these fields can act to reduce the difficulty exponent of the problem, providing a polynomial runtime improvement. We argue that the resulting speedup should be robust against qubit energy fluctuations ($1/f$ noise), in contrast to variable-rate annealing, which is not.  We demonstrate quantum speedups for two classes of hard first order transition (the Grover problem and $N$-spin transitions between polarized semiclassical states), and provide analytical arguments and numerical evidence to support our claims. The oscillating fields themselves can be added through current flux-qubit based hardware by simply incorporating oscillating electric and magnetic lines, and could thus be implemented immediately.

Friday November 17, 2pm SSL 150
Michael Fink (Harvard)
Water and Protein Surfaces: An Unfavorable Coexistence

Water is much smaller than most other biological molecules, and still, its peculiar property to form networks of hydrogen bonds amplifies the effects of aqueous solvation: they are large enough to strongly influence, or even dominate many interactions of apolar compounds in water. We argue that binding of hydrophobic molecules in water is largely determined by a mechanism that depends on the energetically unfavorable structure of water networks close to hydrophobic surfaces, and the rearrangement of these water molecules upon association of the surfaces. We test our hypotheses using a robust physical-organic model, the enzyme human carbonic anhydrase, rationalize thermodynamic and crystallographic results using molecular dynamics, and present experimental evidence for both enthalpic and entropic hydrophobic effects, and surprising instances of enthalpy-entropy compensation. These results help to improve computational methods in drug design, and to understand hydrophobic phenomena in physiological events.

Friday January 26, 2pm SSL 150
Jason Alicea (Caltech)


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Januar 2015 - July 2016

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