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Condensed Matter Seminar

Andrey Rogachev, University of Utah
Speaker: Andrey Rogachev, University of Utah Title: Superconductor-insulator transition in nanowires Abstract: Superconductivity in one-dimensional wires can be affected by several physical processes with different characteristic length scales. To identify a process dominant in the regime of complete superconductivity suppression we studied transport properties of very narrow (9–20 nm) MoGe wires fabricated by advanced electron-beam lithography in a wide range of lengths, 1–25 micrometers. We observed that the wires undergo a superconductor-insulator transition (SIT) that is controlled by cross sectional area of a wire and possibly also by the width-to-thickness ratio. The results show strong quantitative disagreement with current theories. We also found that the SIT in nanowires can be driven by magnetic field. The scaling analysis of this transition and alternative interpretation based on quantum corrections will be presented. Host: Ar. Abanov

15 Jan 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Open
Speaker: Open Title: Abstract: Host:

22 Jan 2016, 4:00PM | MIST M102


Fuxiang Li, LANL
Speaker: Fuxiang Li, LANL Title: Single spin dynamics in an optically active quantum dot and the measurement of third order spin correlators. Abstract: Understanding the spin dynamics in quantum dot, especially its detailed decoherence and relaxation is not only of theoretical interests, but also a crucial problem towards the application of quantum dot as a solid-state quantum qubit. From the phenomenological models of decoherence developed more than a decade ago, it has been now fairly accepted that the spin dynamics undergoes two stages, first a fast ensemble dephasing due to the coherent precession of spin qubit around nearly static but randomly distributed hyperfine fields (? 2 ns) and then a much slower relaxation process (> 1 ?s) due to dynamics of the nuclear spin bath induced by complex many-body interaction effects. However, this characteristics has never been verified in the experiment, until the recent experiment breakthrough I'm going to talk about. What’s more interesting is that, the experiment unambiguously shows a more complex picture, in which two dips rather than one, develops, which can be attributed to the effect of the comparatively strong quadruple field. We also show that the ability to precisely control and measure the single spin allows us to measure third order correlator which contains a purely quantum effect and can be shown to violate the Leggett-Garg inequality. Host: Ar. Abanov

29 Jan 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Open
Speaker: Open Title: Abstract: Host:

5 Feb 2016, 4:00PM | MIST M102


Open
Speaker: Open Title: Abstract: Host:

5 Feb 2016, 4:00PM | MIST M102


CM Group meeting
Speaker: CM Group meeting Title: Abstract: Host:

12 Feb 2016, 4:00PM | MIST M102


Vladimir Manucharyan, University of Maryland
Speaker: Vladimir Manucharyan, University of Maryland Title: Superconducting Fluxonium qubit for quantum information and beyond. Abstract: Fluxonium can be modeled using only three circuit elements: a Josephson junction, a capacitor, and an inductor all connected in parallel. Despite such circuit simplicity, when the three circuit parameters are chosen appropriately, fluxonium's spectrum can be quite rich, involving very different transition frequencies and efficiencies, a useful feature for quantum information applications. Most importantly, one needs an unusual combination of a large inductance L and a small capacitance C, such that impedance (L/C)^1/2 is on the order of resistance quantum (26.5 kOhm); most conventional circuit impedances are limited by the vacuum impedance (377 Ohm). In fluxonium, such a large and compact inductance (often referred to as "superinductance") is successfully achieved using a chain of Josephson junctions with parameters chosen to minimize quantum phase slip and standing wave resonances.
In this talk, I will introduce circuit quantum electrodynamics (QED) with fluxonium qubits and present our first experiments at UMD. I will also discuss the potential of high-impedance circuits for creating ultra-compact resonators for quantum technology development, exploring utlrastrong coupling QED, and simulating quantum impurity physics. Host: Ar. Abanov

19 Feb 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Alexei Tsvelik, Brookhaven National Laboratory
Speaker: Alexei Tsvelik, Brookhaven National Laboratory Title: Quasi-Ballistic Transport in a Kondo Chain Abstract: Host: A. Finkelstein

26 Feb 2016, 4:00PM | MIST M102
Hosted By: A. Finkelstein


Ben Hunt,Carnegie Mellon
Speaker: Ben Hunt,Carnegie Mellon Title: Quantum metal and Ising superconductivity in two-dimensional NbSe2 Abstract: Atomically-thin transition metal dichalcogenides have recently become a very popular subject. NbSe2, a classic layered superconductor, is the first of these that exhibits superconductivity down to one or two atomic layers. I will discuss our recent experiments on bilayer and few-layer NbSe2 and our observation of an anomalous metallic phase in the zero-temperature limit induced by a small perpendicular magnetic field. This quantum metal phase has only been observed in highly-disordered thin film superconductors and its observation in a crystalline superconductor, along with a distinct magnetic field scaling, forces us to reexamine the diagram of possible electronic phases in two dimensions at zero temperature.
The atomic-scale thickness of the NbSe2 crystals also implies that we can explore the regime where quenching of superconductivity is entirely due to paramagnetic effects. We study this regime, where superconductivity can survive up to 30 T in-plane, and find a strong enhancement of the upper critical field relative to the Pauli limit. I discuss the implications of this for spin-orbit coupling effects and possible exotic phases of superconductivity. Host: Ar. Abanov

4 Mar 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Johannes Pollanen, Caltech
Speaker: Johannes Pollanen, Caltech Title: Superfluids and low-­?dimensional electrons: On the road to hybrid quantum systems Abstract: Creating and controlling novel quantum states of matter is at the forefront of modern condensed matter physics. I will discuss two examples of this “create and control” paradigm from my experiences. First, I will describe how we have utilized anisotropic disorder, in the form of high porosity aerogel, to create new chiral superfluid states of 3 He. The understanding of these states has broad implications regarding the general stability of chiral superconductivity. In the second part of my talk I will discuss our experiments on controlling the orientation of two-­?dimensional (2d) liquid crystalline states existing in ultra-­?clean semiconductor heterostructures. These fascinating states, known as quantum Hall nematics, exhibit a mysterious broken rotational symmetry in the 2d plane. We have found that this symmetry can be experimentally controlled by engineering the device structure; thereby demonstrating a unique technique for controlling the orientation of these exotic quantum states. Finally, I will remark on how the lessons learned from studying superfluid 3 He and 2d electrons can be harnessed to create hybrid quantum systems composed of free electrons floating on the surface of liquid helium coupled to nanoscale structures or topological states of matter. These systems provide a unique platform for studying the fundamental physics of low dimensional quantum systems and their potential quantum computing applications. Host: Ar. Abanov

11 Mar 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Spring Break
Speaker: Spring Break Title: Abstract: Host:

18 Mar 2016, 4:00PM | MIST M102


Open
Speaker: Open Title: Abstract: Host:

25 Mar 2016, 4:00PM | MIST M102


CM Faculty Meeting
Speaker: CM Faculty Meeting Title: Abstract: Host:

1 Apr 2016, 4:00PM | MIST M102


Aaron Finck, UIUC
Speaker: Aaron Finck, UIUC Title: Phase coherent transport in hybrid superconductor-topological insulator devices Abstract: ?Heterostructures of superconductors and topological insulators are predicted to host unusual zero energy bound states known as Majorana fermions, which can robustly store and process quantum information. Here, I will discuss our studies of such heterostructures through phase-coherent transport, which can act as a unique probe of Majorana fermions. We have extensively explored topological insulator Josephson junctions through SQUID and single-junction diffraction patterns, whose unusual behavior give evidence for low-energy Andreev bound states. In topological insulator devices with closely spaced normal and superconducting leads, we observe prominent Fabry-Perot oscillations, signifying gate-tunable, quasi-ballistic transport that can elegantly interact with Andreev reflection. Superconducting disks deposited on the surface of a topological insulator generate Aharonov-Bohm-like oscillations, giving evidence for unusual states lying near the interface between the superconductor and topological insulator surface. Our results point the way towards sophisticated interferometers that can detect and read out the state of Majorana fermions in topological systems. Host: Ar. Abanov

8 Apr 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Vlad Pribiag, University of Minnesota
Speaker: Vlad Pribiag, University of Minnesota Title: Towards topological superconductivity in InAs/GaSb quantum wells Abstract: Topological insulators coupled to conventional superconductors are predicted to host topological superconductivity and to support localized Majorana zero-modes. Majorana modes are predicted to obey non-Abelian exchange statistics, which makes them interesting building blocks for topological quantum computing. In this talk, I will focus on our recent experiments which show gate-tunable superconductivity in Type-II InAs/GaSb quantum wells, a two-dimensional topological insulator (2D TI). Using electrostatic gating, we induce superconductivity in all three transport regimes of our devices: bulk electron transport, edge mode transport and bulk hole transport. We extract the spatial distribution of the supercurrent in all three regimes by using superconducting quantum interference measurements. A clear transition to edge-dominated supercurrent is observed under conditions of high bulk resistivity. I will discuss possible experiments that could be used to detect Majorana modes and topological superconductivity in such 2D-TI-superconductor devices, and highlight some of the experimental challenges and opportunities. Host: Ar. Abanov

15 Apr 2016, 4:00PM | MIST M102
Hosted By: Ar. Abanov


Open
Speaker: Open Title: Abstract: Host:

22 Apr 2016, 4:00PM | MIST M102


Zhifeng Ren, University of Houston
Speaker: Zhifeng Ren, University of Houston Title: High Performance Thermoelectric Materials Abstract: The past decade has witnessed significant advances in the field of thermoelectric materials thanks to extensive research along new pathways to enhance properties, in particular nanostructuring bulk materials to largely reduce the thermal conductivity which has resulted in peak thermoelectric figure of merit (ZT). Achieving high peak ZT has been the goal of the thermoelectric community. Is high peak ZT good enough? Does a high peak ZT necessary translated into a high efficiency? The answer is "NO". I will demonstrate how a high peak ZT cannot warrantee a high efficiency, but engineering (ZT)eng is the right answer. I will also demonstrate how a high conversion efficiency does not warrantee high output power density, but the high engineering power factor (PF)eng does, and this is what matters most for thermoelectric power generators. Host: J. Ross

29 Apr 2016, 4:00PM | MIST M102
Hosted By: J. Ross


Vladimir Zyuzin, University of Nebraska
Speaker: Vladimir Zyuzin, University of Nebraska Title: Dynamic spin susceptibility of interacting fermion systems Abstract: The past decade has witnessed significant advances in the field of thermoelectric materials thanks to extensive research along new pathways to enhance properties, in particular nanostructuring bulk materials to largely reduce the thermal conductivity which has resulted in peak thermoelectric figure of merit (ZT).
Achieving high peak ZT has been the goal of the thermoelectric community. Is high peak ZT good enough? Does a high peak ZT necessary translated into a high efficiency? The answer is "NO". I will demonstrate how a high peak ZT cannot warrantee a high efficiency, but engineering (ZT)eng is the right answer. I will also demonstrate how a high conversion efficiency does not warrantee high output power density, but the high engineering power factor (PF)eng does, and this is what matters most for thermoelectric power generators. Host: A. Finkelstein

6 May 2016, 4:00PM | MIST M102
Hosted By: A. Finkelstein


Open
Speaker: Open Title: Abstract: Host:

13 May 2016, 4:00PM | MIST M102


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