*Joe Ross*

**Speaker:***Presentation for new graduate students*

**Title:***The presentation is aimed at the incoming students, at the students who are choosing their field, and at those who are just curious of what we do.*

**Abstract:***Joe Ross*

**Host:**1 Sep 2017, 4:00PM | MIST M102

Hosted By: Joe Ross

*Lucia Steinke, TAMU*

**Speaker:***Chiral transport and electronic correlations in surface states of HfNiSn single crystals*

**Title:***The large family of half-Heusler compounds hosts a number of topological insulator materials and potential topological superconductors, making these compounds interesting candidates to study physical phenomena on the verge of a topological phase transition. Here we present first magnetotransport measurements on high-quality single crystals of HfNiSn, which according to density functional theory calculations is a nonmagnetic, topologically trivial semiconductor without a bulk band inversion. Our samples show unconventional transport properties already at moderately low temperatures T < 200 K. Instead of the thermal carrier freeze-out expected for a bulk semiconductor, electrical conduction in HfNiSn is increasingly dominated by metallic surface states, with a saturation of the longitudinal resistance and highly nonlocal transport. X-ray diffraction shows no structural transitions that could potentially lead to anisotropic conduction in this temperature regime. Magnetoresistance measurements are consistent with weak anti-localization, a signature of low-dimensional transport in a system with strong spin-orbit coupling. An anomalous transverse resistance at zero magnetic field possibly indicates chiral transport. This suspicion is confirmed by experimental evidence for time reversal symmetry (TRS) breaking: four-point measurements related by Onsager-Casimir symmetry show a measurable difference below ~ 30 K. This result is unexpected in the light of magnetic susceptibility and muon spin rotation measurements showing that HfNiSn is a diamagnetic compound with no magnetic moments and no evident magnetic order. The chiral component of the conductance depends very weakly on applied magnetic fields up to 9 T, however it is strongly temperature dependent and vanishes above ~ 30 K. The transverse conductance shows a nonlinear dc bias dependence where a moderate dc current of 1 mA leads to an 85 % suppression of the transverse voltage. The nonlinearities in I(V) curves are suppressed above 30 K, the same energy scale established for the broken TRS. Despite being topologically trivial, the surface of HfNiSn appears to host chiral states with broken TRS, where the observed nonlinear transport could indicate collective phenomena.*

**Abstract:***Ar. Abanov*

**Host:**8 Sep 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Helmut Katzgraber, TAMU*

**Speaker:***Quantum vs classical optimization: A status update on the arms race*

**Title:***Can quantum computers meet the tantalizing promise of solving complex calculations - such as optimization problems or database queries - faster than classical computers based on transistor technologies? Although IBM recently opened up their five-qubit programmable quantum computer to the public to tinker with, the holy grail of a useful large-scale programmable universal quantum computer is decades away. While working mid-scale programmable special-purpose quantum optimization machines exist, a conclusive detection of quantum speedup remains controversial despite the recent promising results. In this talk a head-to-head comparison between quantum and classical optimization approaches is given. Current quantum annealing technologies must outperform classical devices to claim the crown in the race for quantum speedup.*

**Abstract:***Helmut Katzgraber*

**Host:**15 Sep 2017, 4:00PM | MIST M102

Hosted By: Helmut Katzgraber

*Morten Eskildsen, University of Notre Dame*

**Speaker:***Vortex Structure and Broken Time Reversal Symmetry in the B phase of UPt3.*

**Title:***With three different superconducting mixed (vortex) phases the heavy-fermion material UPt3 can be considered a paradigm for unconventional superconductivity. Despite more than three decades of study, a definitive understanding of the superconducting state in this material has remained elusive. The order parameter structure that is consistent with a number of experiments is an odd-parity, f-wave orbital state of E2u symmetry. Here the order parameter is chiral and breaks time reversal symmetry in the low-temperature superconducting B-phase. We have performed small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in UPt3 in the B- and C-phases with H // c. This led to the discovery of a previously unknown field-induced VL rotation in the B-phase. Furthermore, the magnitude of the VL rotation show a subtle magnetic field history dependence; VLs prepared with the field parallel or anti-parallel with respect to initial direction with which one enters the B-phase are rotated by different amounts. This demonstrates an internal degree of freedom associated with the vortex cores, and provides direct evidence for broken time reversal symmetry in the B-phase of UPt3 in non-zero fields by a bulk measurement. We propose that this correspond to an order parameter chirality that is respectively in the same or opposite sense as the VL supercurrent circulation. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Awards DE-FG02-10ER46783 (University of Notre Dame; neutron scattering) and DE-FG02-05ER46248 (Northwestern University; crystal growth and characterization).*

**Abstract:***Ar. Abanov*

**Host:**22 Sep 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Guillaume Gervais, McGill University*

**Speaker:***Quantum Matter On-a-Chip!*

**Title:***I will describe my quantum (hopefully not random) walk in the area of low temperature nanoscience in an effort to elucidate "bizarre" quantum phenomena. I will discuss several experiments that are underway in my laboratory. These include: - our effort to detect bizarre particles that are neither bosons or fermions; - the ‘Coulomb drag’ interaction of electrons in two closed-pack quantum wires, separated by only ~10 nm; - a nano-engineered quantum faucet (for real fluids). It's a lot of (quantum) fun!*

**Abstract:***Ar. Abanov*

**Host:**29 Sep 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Shinsei Ryu, University of Chicago*

**Speaker:***Many-body topological invariants for topological insulators and superconductors*

**Title:***Topological insulators and superconductors are fermionic symmetry-protected topological phases. Namely, they are topologically distinct from trivial states of matter in the presence of some symmetries. For example, topological insulators are topological only in the presence of time-reversal symmetry. Topological properties of such fermionic symmetry-protected topologically phases are commonly discussed by using single-particle Bloch wave functions. While such single-particle formulations have been tremendously successful, there still remains a question to address –– the effect of interactions. In this talk, I will describe a fully many-body formulation of topological invariants for various symmetry-protected topological phases of matter, which does not refer to single particle wave functions at all.*

**Abstract:***Ar. Abanov*

**Host:**6 Oct 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Leonid Rokhinson, Purdue*

**Speaker:***Experimental search for non-Abelian excitations*

**Title:***I will start with a brief introduction into the physics of Majorana fermions in semiconductor/superconductor hybrids and description of our experiments where the fractional ac Josephson effect, a hallmark of topological matter, has been observed. I will continue with the discussion of challenges facing the field, namely that in currently pursued hybrid topological superconductors topological states are well localized which makes manipulation and demonstration of non-Abelian statistics extremely challenging. In the second part I will introduce a new concept based on the ferromagnetic transition in a quantum Hall regime where manipulation of topological defects may be possible via reconfigurable network of domain walls. We pursue two systems, one based on magnetic semiconductor CdTe:Mn, and one based on GaAs high mobility heterostructures, where we developed gate control of quantum Hall ferromagnetic transitions. Such control allows formation of isolated domain walls which consist of counter-propagating edge states of opposite polarization. Apart from interesting spintronics applications, these re-configurable domain walls, coupled to superconducting contacts, can form a new platform where Majorana fermions, parafermions, and, possibly, Fibonacci fermions can be created, braided, manipulated and fused.*

**Abstract:***Ar. Abanov*

**Host:**13 Oct 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Sarang Gopalakrishnan, CUNY, Staten Island,*

**Speaker:***Dynamics of monopoles in quantum spin ice.*

**Title:***"Spin ice" materials are described by a lattice version of electrodynamics, with elementary excitations that resemble magnetic monopoles. At low temperatures, the response of quantum spin ice is governed by the coherent quantum dynamics of monopoles. Monopoles move by flipping physical spins; the matrix element for a spin flip depends strongly on the configuration of the other spins in the vicinity of a monopole. Thus, monopole motion is "kinetically constrained." Using numerical simulations and an approximate mapping to quantum percolation, we explore the properties of monopole wavefunctions, which we find to be at or near criticality, and discuss the implications for experimental measurements of spin response.*

**Abstract:***Ar. Abanov*

**Host:**20 Oct 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Debanjan Chowdhury, MIT*

**Speaker:***Scrambling, chaos and transport in quantum many-body systems.*

**Title:***The growth of commutators of initially commuting local operators diagnoses the onset of chaos in quantum many-body systems. I will discuss the onset of scrambling in a few different examples of many-body quantum systems. I will argue that in all of these examples, chaos spreads in a ballistic fashion with a butterfly velocity that acts as a speed limit for the propagation of quantum information even though transport is diffusive. I will also comment on various interesting and universal aspects of the growth (Lyapunov) exponent and the butterfly velocity for these systems.*

**Abstract:***Ar. Abanov*

**Host:**27 Oct 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Ania Bleszynski Jayich, UCSB*

**Speaker:***TBA*

**Title:**

**Abstract:***Ar. Abanov*

**Host:**3 Nov 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Brian DeMarco, University of Illinois Urbana-Champaign*

**Speaker:***TBA*

**Title:**

**Abstract:***Ar. Abanov*

**Host:**10 Nov 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Romain Vasseur, Berkeley*

**Speaker:***TBA*

**Title:**

**Abstract:***Ar. Abanov*

**Host:**17 Nov 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*THANKSGIVING*

**Speaker:**

**Title:**

**Abstract:**

**Host:**24 Nov 2017, 4:00PM | MIST M102

*Erez Berg, University of Chicago*

**Speaker:***TBA*

**Title:**

**Abstract:***Ar. Abanov*

**Host:**1 Dec 2017, 4:00PM | MIST M102

Hosted By: Ar. Abanov

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