« fall 2014 - spring 2015 - summer 2015 »

This week | Next week | This semester | All future | Print view

This week | Next week | This semester | All future | Print view

Thursday, January 23rd 2014

1:25 pm:

A film of singlet s-wave superconducting material placed in direct contact with a ferromagnet develops a superconducting condensate with unusual properties. Most notably, the condensate can acquire a triplet component immune to pair breaking by exchange field and thereby can penetrate deep into the ferromagnet. In this talk, I will show how the amplitude of this triplet component can be measured and efficiently tuned by controlling the magnetic state of a pair of ferromagnetic layers (a spin valve) proximate to the superconducting film. I will also discuss applications of the magnetically tunable triplet condensate in superconducting spintronic devices such as nonvolatile memory for energy-efficient cryogenic computing.

Thursday, January 30th 2014

1:25 pm:

ABSTRACT: In strongly correlated electron materials (CEMs), the delicate interplay between spin, charge, and lattice degrees of freedom often leads to extremely rich phase diagrams exhibiting intrinsic phase inhomogeneities. The key to studying and disentangling such complexities usually lies in characterization and control of these materials at their fundamental energy, time and length scales. Using the prototypical correlated insulator vanadium dioxide (VO2) as a case study, I will show in this talk that ultrafast and ultrasmall optical spectroscopy offers unique insights into this electronic/structural interplay with unprecedented spatial and temporal resolutions. Specifically, with scanning near-field infrared microscopy we resolved the long-lasting enigma of electronic anisotropy in VO2 and revealed three distinct stages of the insulator to metal transition (IMT) at nanoscopic length scales. Using ultrafast terahertz pump terahertz probe spectroscopy we have also unambiguously demonstrated that the IMT occurs at picosecond time scales via electric field-induced electron liberation. These results set the stage for future spectroscopic investigations to access the fundamental time and length scales of CEMs.

Monday, February 3rd 2014

3:30 pm:

Recently, there has been intense interest in realizing excitations of Majorana fermions(MFs) in solid-state systems. Circuits of Josephson junctions (JJs) made of closely spaced conventional superconductors on 3D topological insulators have been proposed to host low energy Andreev bound states (ABSs) which can include MFs. In this talk, I will present signatures of an anomalous supercurrent carried by such topologically non-trivial ABSs in various superconducting quantum interference devices (SQUIDs) made of Nb/Bi2Se3/Nb junctions. An electrostatic top gate placed on the JJs allows strong modulation of the Josephson current and the realization of a topological phase transition in which the spatial location of the supercurrent-carrying states in the junction is changed. This transition is accompanied by qualitative changes in the SQUID oscillations, single-junction diraction patterns, and temperature dependence of the critical current.

Thursday, February 6th 2014

1:25 pm:

Conventional semiconductors with strong spin-orbit coupling and topological insulators have emerged as promising platforms for spintronics and quantum information processing. In the first part of this talk, I will present experiments relying on the strong spin-orbit interaction in InSb nanowires to demonstrate all-electrical control of individual spins in quantum dots. These experiments highlight the potential of electric dipole spin resonance (EDSR) as both a means for controlling spin-based qubits and as a powerful spectroscopic tool for electrons and holes. Holes, in particular, are very attractive for quantum information processing due to the possibility of longer spin coherence and stronger spin-orbit coupling than electrons, both consequences of the p-orbital symmetry of the Bloch wavefunction. Our work demonstrates that hole-spin states can be manipulated and probed in a transport experiment, providing a path for the implementation of hole-spin based spintronic and opto-electronic devices. In the second part of the talk, I will focus on our current efforts to develop 2D topological insulators (2D TIs) into an experimental platform for Majorana zero modes. These exotic fermionic excitations are predicted to have non-Abelian statistics, making them ideal candidates for fault-tolerant quantum computation. I will discuss superconducting junctions based on InAs/GaSb double quantum wells as a first step towards Majorana experiments based on this gate-tunable 2D TI.

Monday, February 10th 2014

3:30 pm:

Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as nanowires. Recently, a new method has emerged with the recognition that gapped symmetry-protected topological (SPT) phases can host robust surface states that remain gapless as long as the relevant global symmetry remains unbroken. The nature of the charge carriers in SPT surface states is intimately tied to thesymmetry of the bulk, resulting in one-and two-dimensional electronic systems with novel properties such as the locking of spin and momentum. I will describe our recent experimental realization of such helical states on the edge of a graphene flake subjected to very large magnetic fields. In contrast to its time-reversal-symmetric cousin, thegraphene quantum spin Hall state is protected by a symmetry of planar spin rotations that emerges as electron spins in a half-filled Landau level are polarized by the applied field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting cantedantiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with a tunable bandgap and an associated spin texture.

Thursday, February 13th 2014

1:25 pm:

The mechanism of high-temperature superconductivity is unresolved, both in terms of how the phases evolve with doping, and in terms of the actual Cooper pairing process. In my talk, I will introduce spectroscopic-imaging STM as a tool to explore these questions, by direct imaging of the relevant quantum mechanical waves on the atomic scale. I will discuss our discovery of strong electronic nematicity in the parent state of iron-based superconductors, and the recent realization of how dopant-atom induced unidirectional impurity states and anisotropic scattering can explain the mysterious anisotropic transport characteristics in these materials. Next, I will describe our exploration of the superconducting energy gaps and electron-boson interaction in the canonical Fe-based superconductor LiFeAs. The interactions generating unconventional Cooper pairing are often conjectured to be of electronic nature. We introduced Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of both the superconducting gaps in momentum space and the electron-boson coupling self-energy. We identified anisotropic gaps and strong effects in the interband direction only, pointing towards a spin-fluctuation mechanism for Cooper pairing. Finally, I will outline the new techniques for atomic-scale imaging that I propose in order to address the most pressing open questions in quantum materials.

Thursday, February 20th 2014

1:25 pm:

Quantum materials are those in which the interactions between constituent particles are too strong to be treated semiclassically, resulting in exotic emergent properties such as high Tc superconductivity. They represent the front line in the quest to understand the organizing principles that lead to complex collective behavior, and to eventually apply these principles to do something useful.

In this talk I will discuss the application of a wide range of optical and electron spectroscopies to quantum materials. In the cuprate superconductors, for example, we have used femtosecond pulses of laser-light to excite collective modes of the charge density wave order recently discovered in these materials. In these experiments we are able to directly observe the coupling between Cooper pairs and charge density waves, which may hold the key to high Tc superconductivity. In similar experiments, we have studied the collective modes of exotic spin textures like the Skyrmion lattice in MnSi. These experiments are complimented by time-resolved x-ray scattering to directly observe the dynamics of spin, charge and orbital order. I will also discuss the use of transient four-wave mixing to manipulate spins in a spin-orbit coupled 2D electron gas. And, finally, I will discuss some of our very recent work on Iridium oxides, which possess both strong electron-electron interactions and strong spin-orbit coupling, providing an ideal playground for demonstration of exotic new ground states.

Thursday, February 27th 2014

1:25 pm:

Thursday, March 6th 2014

1:25 pm:

Monday, March 10th 2014

12:20 pm:

Tuesday, March 11th 2014

3:00 pm:

Thursday, March 13th 2014

1:25 pm:

See image below for abstract

Thursday, March 27th 2014

1:25 pm:

The helical spin texture of surface electrons in topological insulator has attracted a great deal of interest in the past few years. Although this texture was predicted with the discovery of topological insulators and experimentally confirmed in in few points in the momentum space, its full experimental verification has been non trivial because of the low efficiency of spin resolved experiments.

In this talk I will present new compelling experimental findings where we discovered that photoelectrons emitted from the surface states of Bi2Se3, a typical 3D topological insulator, are nearly fully polarized, and moreover their spin orientation can be manipulated in three dimensions through selection of the light polarization. When the polarization is changed from linear to circular, photoelectrons can be tuned to be completely opposite to their original in-plane spin orientation, and even flipped to be out-of-plane, suggesting the possibility of full control by light.

These results challenge previous works in the field, which due to the lack of a comprehensive momentum dependent study of the spin texture because of the low efficiency of spin-resolved photoemission technique, made conclusions based on the incorrect assumption that electronic spin is always conserved through photoconversion. Finally, the manipulation of the highly polarized photoelectrons may mark an important step towards the longer-term goal of utilizing topological insulators for spintronics by using light control.

Thursday, April 3rd 2014

1:25 pm:

Electrokinetic phenomena in liquid crystals (LCs) show a number of distinct features not met in the isotropic media, thanks to the long-range orientational order and ensuing anisotropy of physical properties such as ionic mobility, elasticity and viscosity [1]. The electric field causes hydrodynamic flows and transport of particles by various mechanisms: dielectric torque-triggered backflow [2], LC-enabled dielectrophoresis [2], LC-enabled electrophoresis (LCEP) [3] and LC-enabled electro-osmosis (LCEO) [4]. LCEP and LCEO are rooted in broken symmetry of the medium. Namely, a spherical particle placed in a uniform nematic, causes director distortions of the dipolar type. Because of anisotropy of ionic mobility, the director distortions break the symmetry of the field-induced ionic flows. As a result, free colloidal particles move with a velocity that is proportional to the square of the applied electric field, Fig.1. A similar effect of liquid crystal – enabled electro-osmosis occurs when the nematic flows past a spherical obstacle. Micro particle image velocimetry establishes the angular and radial dependencies of electro-osmotic flows around particles with different symmetry of director distortions. The LCEP and LC-enabled electro-osmosis are nonlinear phenomena that are complementary to the induced charge electrokinetics. The important difference is that the symmetry breaking that enables the transport is caused by the medium rather than by a particle itself. The difference allows one to transport particles that are symmetric (for example, spherical droplets of water shaped by surface tension) and particles that are only weakly polarizable (glass, polymers, etc.). By controlling the properties of the LC (such as director orientation and sign of dielectric anisotropy) and the particle, one can design various 3D trajectories and move the particles from one location to another along a curvilinear path.

The work is supported by NSF DMR 1104850 grant.

References

1. O. D. Lavrentovich, Soft Matter 2014, 10, 1264.

2. O. P. Pishnyak, S. V. Shiyanovskii, O. D. Lavrentovich, Phys. Rev. Lett. 2011, 106, 047801.

3. O. D. Lavrentovich, I. Lazo and O. P. Pishnyak, Nature 2010, 467, 947; I. Lazo, O. D. Lavrentovich, Phil. Trans. Royal Society A 2013, 371, 20120255.

4. I. Lazo, S. Shiyanovskii, O. D. Lavrentovich, Submitted, 2014.

5. T.M. Squires, M.Z. Bazant, J. Fluid Mech. 2004, 509, 217.

Thursday, April 10th 2014

1:25 pm:

Quantum criticality is often implicated for both the breakdown of Fermi liquid theory and the superconductivity seen in many correlated metals. In this talk I will describe recent progress in studying these issues theoretically in two dimensional metals pushed close to the onset of nematic order which breaks crystal rotation but not translation symmetries. Controlled calculations show that superconductivity is strongly enhanced near such quantum critical points. If time permits I will describe some other contrasting examples where superconductivity is suppressed.

Thursday, April 17th 2014

1:25 pm:

Thursday, April 24th 2014

1:25 pm:

Thursday, May 1st 2014

1:25 pm:

Thursday, September 11th 2014

1:25 pm:

Electrons have three quantized properties – charge, spin, and Fermi

statistics – that are directly responsible for a vast array of

phenomena. Here we show how these properties can be coherently and

dynamically stripped from the electron as it enters certain exotic

states of matter known as quantum spin liquids (QSL). In a QSL,

electron spins collectively form a highly entangled quantum state that

gives rise to emergent gauge forces and fractionalization of spin,

charge, and statistics. We show that certain QSLs host distinct,

topologically robust boundary types, some of which allow the electron

to coherently enter the QSL as a fractionalized quasiparticle, leaving

its spin, charge, or statistics “at the door.” We use these ideas to

propose a number of universal, “smoking-gun” experimental signatures

that would establish fractionalization in QSLs.

Thursday, September 18th 2014

1:25 pm:

In this talk, I will revisit a few old issues in the quantum Hall effect in high quality two-dimensional electron systems (2DES). I will first present our experimental results to quantitatively examine the theoretical model of spin splitting collapse in the quantum Hall regime [by Fogler and Shklovskii, Phys. Rev. B 52, 17366 (1995)] at fixed magnetic fields as a function of electron density in a high quality heterojunction insulated-gate field effect transistor. In the second part, I will talk about the quantum criticality of quantum Hall (QH) plateau to plateau transitions in alloy disordered two-dimensional electron systems. In one short-range random alloy disorder dominated sample, a perfect temperature (T) scaling, (dRxy/dB)|Bc T-, was observed through two full decades of T from 1.2K down to 12 mK. This manifests unequivocally that in an Anderson disordered 2DES the scaling behavior indeed prevails. Moreover, our temperature and size dependent data allow direct determination of both the localization critical exponent, = 2.38, and the dynamical exponent, z = 1. If time permits, I will also talk about the resistivity law observed ~ 30 years ago. Our recent work in ultra-high mobility 2DES has brought new insight on this problem.

Thursday, September 25th 2014

1:25 pm:

Charged colloidal particles present a controllable system for study a host of condensed matter/many body problems such as crystallization. 2D crystals are invariably hexagonal. Hexagons perfectly tile a flat plane but a soccer ball requires exactly 12 pentagons dispersed among the hexagons on its curved surface. Pentagons and hexagons are positive and negative topological charges, disclinations, sources for positive and negative curvature. But we have discovered that “Pleats”, grain boundaries which vanish on the surface (and play a similar role to fabric pleats) can provide a finer control of curvature.

We experimentally investigate the generation of topological charge as flat surfaces are curved. For positive curvature, domes and barrels, there is one pentagon added for every 1/12 of a sphere. Negative curvature is different! For capillary bridges forming catenoids, pleats relieve the stress before heptagons appear on the surface. Pleats are important for controlling curvature from crystals on surfaces, to the shape of the spiked crown of the Chrysler building.

Adding a particle to a flat surface produces an interstitial - usually an innocuous point defect. On a curved surface interstitials are remarkable, forming pairs or triplets of dislocations which can fission dividing the added particles into fractions which migrate to disclinations.

We have also used “Topological Tweezers” to create grains and study the dynmaics of dislocations in gran boundaries.

Friday, September 26th 2014

1:30 pm:

Non-equilibrium systems have been of tremendous interest both in physics

and engineering. Recent experimental breakthroughs in atomic, condensed

matter physics and optics have given birth to new paradigms for studying

out-of-equilibrium quantum systems. Understanding such phenomena require

an inter-disciplinary approach uniting ideas from these diverse fields.

For an isolated Bose gas, I will present a deep connection between the

Gross-Pitaevskii equation and Kardar-Parisi-Zhang universality class of

stochastic dynamics [1]. In a different scenario, an ensemble of atoms

placed in a leaky optical cavity and pumped with a laser constitutes an

exotic light-matter open quantum system. I will describe the

non-equilibrium aspects close to an interesting phase transition in such

driven-dissipative systems [2]. I will also propose protocols to prepare

light-mediated entangled states [3] of matter by optimally designing the

laser and cavity in other hybrid light-matter systems. Our work shows

that far-from-equilibrium many-body physics is a rich field where theory

and experiments across several disciplines thrive together.

[1] M. KULKARNI, A. Lamacraft, Phys. Rev. A 88, 021603, Rapid

Communication, M. KULKARNI, D. A. Huse, H. Spohn (2014) , M. KULKARNI,

J. Stat. Phys (invited article, 2014)

[2] M.KULKARNI, B. Oztop and H. E. Tureci, Phys. Rev. Lett, 111,

220408, M.KULKARNI, K. G. Makris and H. E. Tureci (2014)

[3] C. D. Aron, M. Kulkarni, H. E. Tureci, arXiv:1403.6474 (2014)

Thursday, October 2nd 2014

1:25 pm:

I'll talk about quantum criticality in the quasi one-dimensional topological Anderson insulators and superconductors. It turns out that the systems may be fully described in terms of two parameters representing localization and topological properties, respectively. Surfaces of half-integer valued topological parameter define phase boundaries between distinct topological sectors. Upon increasing system size, the two parameters exhibit flow similar to the celebrated two parameter flow describing the quantum Hall transitions. However, unlike the quantum Hall system, an exact analytical description of the entire phase diagram can be given. We check the quantitative validity of our theory by comparison to numerical transfer matrix computations.

Thursday, October 9th 2014

1:25 pm:

Problems in Human Motion Planning

Moving through a densely-populated environment can be surprisingly hard, owing to the problem of congestion. Learning to deal with congestion in crowds and in networks is a long-standing and urgently-studied problem, one that can be equally well described at the level of dense, correlated matter or at the level of game-theoretical decision making. In this talk I describe two related problems associated with motion planning in congested environments. In the first part I consider a description of pedestrian crowds as densely-packed repulsive particles, and I address the question: what is the form of the pedestrian-pedestrian interaction law? In the second part of the talk I examine a simple model of a traffic network and study how inefficiency in the traffic flow arises from "selfish" decision-making. Analysis of the model reveals a surprising connection between Nash equilibria from game theory and percolative phase transitions from statistical physics.

Thursday, October 16th 2014

1:25 pm:

Thursday, October 23rd 2014

1:25 pm:

Thursday, October 30th 2014

1:25 pm:

Phase transitions are fascinating phenomena in nature with consequences

ranging from the large scale structure of the universe to exotic quantum phases

at low temperatures. Many realistic systems contain impurities, defects and

other forms of quenched disorder. This talk explores the consequences of such

randomness on the properties of phase transitions.

At zero-temperature quantum phase transitions, randomness can have

particularly peculiar and strong effects. Often, rare strong disorder fluctuations and the rare spatial regions that support them dominate the physics close to the transition. They give rise to strong singularities in the free energy, the so-called quantum-Griffiths singularities. In some systems such as metallic magnets, the effects of are fluctuations can be even stronger, leading to a destruction of the phase transition by smearing. These general results are illustrated using experiments in transition metal alloys and heavy fermion systems.

Thursday, November 6th 2014

1:25 pm:

Complex oxides with ABO3 perovskite structure have been of scientific nterests for a long time due to their ability to display a wide-range of phenomena. Recent advances in thin film growth approaches have enabled the growth of this material class in thin film and heterostructure forms with structural quality, which has now become similar to that of the conventional semiconductors. However the grand challenge in the field is to obtain these materials with the high level of stoichiometric and defect control. In this talk, we will present our group’s effort to address these challenges and to utilize stoichiometry defects as a new degree of freedom to control material’s physical phenomena using the hybrid molecular beam epitaxy (MBE) approach with the focus to understand and control novel electronic and magnetic ground states in defect-managed oxide thin films and heterostructures.

Thursday, November 13th 2014

1:25 pm:

In this talk I will show some of our recent work involving spatial patterning of energy band gaps from the nanoscale down to atomic dimensions. Within various two-dimensional materials, these methods can be used to control charge and spins, funnel excitons, and create new lower-dimensional topologically confined states. I’ll present some examples from strain-textured monolayer dichalcogenides, valley-polarized boundary states in molecular graphene, and spin-polarized surface states emerging from a bulk band insulator.

Thursday, November 20th 2014

1:25 pm:

Recent advances in technology made available thin ferroelectric films and ferroelectric-paraelectric multilayers of very high quality. At the same time our understanding of properties of these systems is far from being complete. This is evidenced, in particular, by difficulties in interpretation of various anomalies observed at ferroelectric phase transitions in these systems. Experimentally, they are different from the phase transition anomalies in high quality bulk systems but the reasons of the differences are debatable. The aim of the talk is to discuss some specific features of ferroelectric phase transitions in thin films and multilayers which are visible now but, to a considerable extent, are not fully taken into account by the existing theory. The discussion is limited to answering the question about character of the ferroelectric state forming just after the transition. Three possibilities are considered: single-domain, multi-domain, two-phase. First, thin films with electrodes and on a substrate will be discussed. Incomplete screening of the depolarizing field in the electrodes and difference in properties of the surface layer and the bulk implies formation of domains at the phase transition for realistic parameters of the system. Clamping by the substrate may lead also to formation of two-phase state. This phenomenon is practically unexplored for thin films on substrates. Second, ferroelectric-paraelectric multilayers will be discussed which prove to be much more challenging for theoretical study than the thin films. This is because of lack of periodicity in the polarization distribution despite of periodicity of the structure.

Thursday, November 27th 2014

1:25 pm:

Thursday, December 4th 2014

1:25 pm:

The recent discovery of superconductivity in iron based materials are a subject of intensive investigations and controversy. They serve as an excellent test of the current capabilities for predicting the physical properties of correlated materials. We will describe some of the LDA+DMFT based Studies for various spectroscopies: optical conductivity, photoemission and inelastic neutron scattering and will compare them with experiments. We will argue that these materials are Hunds metals, a new class of strongly correlated materials where the correlations are controlled by the strength of the Hunds rule coupling J rather than by the Hubbard U, and will place these materials in the broader context of searching for other high temperature superconductors.

Thursday, December 11th 2014

1:25 pm:

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.