Condensed Matter Seminars
Spring 2019

Condensed Matter Seminars are in Room NPB 2205
on Mondays @ 4:05 pm t0 4:55 pm

Contact: Yasu Takano or Dmitrii Maslov

January 7      

Speaker

Daniel Agterberg (Univ. Wisconsin-Milwaukee)

Title

Topologically protected Bogoliubov Fermi surfaces

Abstract

It is commonly believed that, in the absence of disorder or an external magnetic field, there are three possible types of superconducting excitation gaps: The gap is nodeless, it has point nodes, or it has line nodes. Here, we show that, for an even-parity nodal superconducting state which spontaneously breaks time-reversal symmetry, the low-energy excitation spectrum generally does not belong to any of these categories; instead, it has extended Bogoliubov Fermi surfaces. These Fermi surfaces are topologically protected from being gapped by a non-trivial Z2 invariant. In this talk, I will discuss the physical origin, topological protection, and energetic stability of these Bogoliubov Fermi surfaces, using chiral superconductivity in j=3/2 fermions as a representative example. 

Host

Peter Hirschfeld

January 8, Tuesday (Note special date)     

Speaker

Lucia Steinke (Texas A&M Univ.)

Title

Superconductivity at quantum critical points and in proximity with topological boundary states

Abstract

Collective quantum effects can be enhanced when two quantum phenomena combine in one material: in quantum critical systems, the critical fluctuations of a quantum phase transition at zero temperature can mediate superconducting pairing and lead to enhanced transition temperatures, or even be required to observe superconductivity at all. In the first part of my talk, I will show how we studied this connection between quantum criticality and superconductivity at ultra-low temperatures, and how we solved experimental challenges encountered at these extreme conditions.

In the second half of my talk, I will discuss a potentially new route to topological boundary states, similar to quantum Hall edges, found on the surface of the half-Heusler compound HfNiSn. These states appear to originate from strong electronic correlations, and they form even in the absence of external magnetic fields. Oscillations in the magnetoresistance suggest quantum interference with coherence lengths up to 1 μm, at only moderately low temperatures up to 80 K. The combination of quantum Hall edges or similar chiral one-dimensional states and superconductors is particularly attractive, as such junctions are expected to host the elusive Majorana fermions that seem a promising choice for future qubits, and the chiral nature of quantum Hall edge states could enable braiding operations between them, providing a possible platform for topological quantum computing. Our first tests of metal deposition on HfNiSn single crystals show promising results, where proximity to superconducting tin or niobium leads to conductance steps close to the quantized value of ~ 0.5 e²/h expected for Majorana fermions. We further observe a clear disruption of quantum interference patterns at the superconducting transition, and magnetoresistance features associated with the critical field that can be traced up to approximately 80 K.

Host

James Hamlin

January 14       

Speaker

Stuart Brown (UCLA)

Title

Tuning the magnetic fluctuations and superconducting ground state of Sr2RuO4: NMR studies under uniaxial strain

Abstract

Sr2RuO4 is a correlated multiband system that undergoes a transition to a superconducting state at Tc=1.45 K. A longstanding question relates to order parameter symmetry, with many experiments interpreted as consistent with a chiral odd-parity state. Application of compressive uniaxial strain results in a remarkable increase in transition temperature, 1.45 K to 3.5 K, which has been tentatively linked to a van Hove singularity. ¹⁷O NMR spectroscopy of the normal state, while subject to in situ uniaxial strain, is interpreted as consistent with a Lifshitz transition coincident with the maximum Tc, as well as enhanced ferromagnetic fluctuations and proximity to a quantum phase transition. The spin susceptibility is observed to significantly decrease in the superconducting state, which was also studied in variable strain conditions. The results invite a re-evaluation of the order parameter symmetry, as well as the role of ferromagnetic fluctuations in stabilizing the ground state.

Host

Mark Meisel

January 16, Wednesday (Note special date)      

Speaker

Elizabeth Green (Dresden High Magnetic Field Laboratory)

Title

Fermi surface topology change in Nd-doped CeCoIn5

Abstract

CeCoIn5, one of the most well-known heavy fermion compounds, exhibits a novel field-induced superconducting state above 10 T for B||c known as the Q-phase. Recent neutron scattering measurements have demonstrated that a similar Q-vector appears at zero applied magnetic field for the 5% Nd-doped CeCoIn5 [1] which has initiated intense theoretical and experimental work on this doping series. In this talk I will present our de Haas-van Alphen (dHvA) effect measurements which show a drastic Fermi-surface reconstruction between 2 and 5% Nd-doping levels is responsible for the emergence of this unconventional superconducting state. The cylindrical Fermi surface develops a quasi-three-dimensional topology with increased doping levels thus reducing the likelihood of an enhanced nesting scenario, previously given as a possible explanation for the Q-phase. Effective masses remain unchanged up to 10% Nd, indicating the presence of a spin density wave type of quantum critical point. In addition, I will present evidence that by substituting Ce with Nd the electronic pairing potential is altered. These results highlight the need for additional experiments and further theoretical calculations to accurately model this unique system.

[1] S. Raymond et al., JPSJ 83, 013707 (2014).

Host

Mark Meisel

January 21 (No seminar - Martin Luther King Jr. Day)      

Speaker

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January 22, Tuesday, 10:40 AM (Note special date and early time)      

Speaker

Felix Büttner (MIT)

Title

Dynamic imaging of chiral and topological excitations in magnetism

Abstract

Magnetic materials like Fe, Ni, and Co are among the oldest and best studied systems in materials science. Yet, the delicate balance of correlated quantum interactions and long-range classical fields give rise to a richness that we are just starting to reveal. Here, I will discuss two emerging topics that are of particular interest: chiral soliton dynamics and non-equilibrium physics. The key to explore these new territories of materials physics is direct time-resolved imaging, which can be realized via time-resolved x-ray holography [1] at synchrotrons and free electron lasers with unprecedented resolution and depth of information.

In magnetism, strong chiral interactions can lead to the formation of localized chiral solitons with non-trivial topology, so-called skyrmions. Skyrmions can move like particles but their dynamics exhibit fascinating topological signatures, such as gyration [1], inertia [1], the skyrmion Hall effect [2], and topological damping [3]. The properties of skyrmions are intricately linked to the underlying material. For example, room-temperature skyrmions are abundant in ultrathin ferromagnetic films sandwiched between dissimilar materials with high spin-orbit interactions [1-4]. In these materials, skyrmion size and stability are determined by long-range stray fields while quantum interactions at the interfaces lead to efficient and deterministic skyrmion motion and skyrmion nucleation by electrical currents [2,4]. I will discuss these physics based on quantitative theoretical modeling [3] and show that there is a new class of materials, chiral ferri- and antiferromagnets, with a small phase pocket for purely quantum-stabilized skyrmions. These quantum-stabilized skyrmions are insensitive to classical fields, their size can be smaller than 10 nm at room temperature, and their dynamics is ultrafast and without topological signatures [3]. I will show how high resolution x-ray holography led to the discovery of these skyrmions in the material predicted by the model [5]. As an outlook, I will present first results of skyrmion dynamics in the far-from-equilibrium state after optical excitation with extremely bright femtosecond laser pulses and discuss the perspective to image these and other ultrafast processes at new free electron laser sources.

[1] Büttner et al., Nat. Phys. 11, 225 (2015). [2] Litzius et al., Nat. Phys.13, 170 (2017).[3] Büttner et al., Sci. Rep. 8, 4464 (2018). [4] Büttner et al., Nat. Nanotech. 12, 1040 (2017). [5] Caretta, Mann, Büttner et al., Nat. Nanotech. 13, 1154 (2018).

Host

Amlan Biswas

January 23, Wednesday (Note special date)      

Speaker

Rasul Gazizulin (Institut Néel)

Title

On-chip mechanical thermometry at millikelvin temperatures

Abstract

With the aim of measuring on-chip the phonon temperature of a mechanical nano-oscillator, we have set up an optomechanical system where a long mechanical beam is coupled to a microwave cavity. The experiment is mounted on the Grenoble nuclear demagnetisation cryostat able to reach temperatures below 1 mK. We report on the first measurements of this set-up at dilution-refrigerator temperatures, and discuss future possibilities for ultra-cold nanomechanics where cooling relies only on cryogenic technologies. No active cooling techniques are used in our experiments, in which we explore the limits of "brute force" cooling.

Host

Yasu Takano

January 24, Thursday, 10:40 AM (Note special date and early time)

Speaker

Mohammad Hamidian (Swayable)

Title

Directly imaging the super states of nature

Abstract

A superconductor is a homogeneous quantum condensate of Cooper pairs, each formed by binding two electrons into a zero-spin, zero-momentum eigenstate. In 1964 Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) proposed an alternative ground state wavefunction of Cooper pairs formed under a magnetic field, thus arriving at a new super state of electronic matter. The resulting pairs carry momentum Q requiring the superfluid density to modulate with wavevector Q.

The last 40 years have seen a proliferation of novel and exotic superconductors. Despite decades of effort, however, FFLO-like states remained unobserved. The challenge to detect modulated superfluids has become particularly urgent because of implications for the theory of high temperature superconductivity, and in particular for the class of materials with some of the highest transition temperatures. The presence of a spontaneously generated modulated super state in the cuprate compounds, referred to as a Pair Density Wave (PDW), provides a possible missing link to unify our understanding of the cuprate phase diagram and ultimately the mechanism for high temperature superconductivity.

Over the last decade we have engineered novel instruments for electronic structure imaging to discover exotic phases of quantum matter. I will describe the development of nanometer-resolution scanned Josephson tunneling microscopy (SJTM) to directly image Cooper-pair density and the first visualization of a modulated superfluid. I will go on to describe how we are also converging on the existence of PDW state in the high field regime of the cuprates. By using sub-unit-cell magnetic field subtraction imaging we uncovered eight unit-cell density modulations in superconducting vortex halos, the long predicted PDW signature in cuprates. Our new visualization techniques for pair condensates, furthermore, opens the prospect of condensate visualization in other unconventional superconductors, and will be especially advantageous in the study of topological superconductors.

Host

Andrew Rinzler

January 28      

Speaker

Seminar by a biophysics faculty candidate. See https://www.phys.ufl.edu/wp/index.php/events/ for details.

Title

Abstract

Host

January 29, Tuesday, 10:40 AM (Note special date and early time)

Speaker

Xiao-Xiao Zhang (Kavli Institute, Cornell Univ.)

Title

Controlling light-matter interaction in 2D semiconductors

Abstract

Monolayer transition metal dichalcogenides (TMDCs) in the MX2 class (M=Mo, W; X=S, Se, Te) hold exceptionally strong light-matter interaction. The bright emission at the direct-gap from the K/K’ valleys has facilitated many studies on the underlying many-body and valley-spin physics, as well as opened up the prospect for various optoelectronic devices. Interestingly, in addition to these bright exciton states, lower-energy dark excitons are expected to exist in certain TMDC compounds (e.g., WS2 and WSe2), which arise from spin splitting in conduction bands. In this talk, I will present two experimental approaches to probe, and further control these optically-inactive dark excitons in monolayer WSe2. First, I will provide an indirect probe of the dark states by analyzing the temperature dependence of time-resolved photoluminescence. The readily observable bright exciton population exhibits thermal activation, thus allowing us to infer information about the lower-energy dark state. I will then show how we can control the radiative properties and directly brighten these spin-forbidden dark excitons with the application of an in-plane magnetic field. Both the dark excitons and dark trions (charged excitons) are identified, and their precise energy levels are determined. Most significantly, much increased emission lifetime and valley lifetime were exhibited by these dark excitons, thus providing new opportunities for the study of highly correlated Bosonic interactions and of spin-valley physics. In this context, I will present experimental results on how these long-lived dark excitons assist the easy formation of biexcitons. The potentials of these dark states to serve as long-lived valley information carriers will also be discussed.

Host

Art Hebard

January 30, Wednesday (Biophysics theory seminar)   

Speaker

Assaf Amitai (MIT)

Title

Geometry and stochastic dynamics in biological systems

Abstract

The interaction of proteins with chromatin regulates many cellular functions. Most DNA-binding proteins interact both non-specifically and transiently with many chromatin sites, as well as specifically and more stably with cognate binding sites. These interactions and chromatin structure are important in governing protein dynamics. I will show that the dynamics of proteins is determined by the 3d organization of chromatin in the nucleus.

By analyzing the motion of CTCF, a zinc-finger DNA binding protein observed using microscopy methods, we found that it interacts with small nuclear domains. These domains, composed of RNA, are central in guiding CTCF to find its cognate binding site and in determining the organization of chromatin.

In the second part of the talk, I will describe our advances in studying the immune response to flu proteins. Using coarse-grained molecular dynamics simulations and modeling the immune response, we show that nanoparticles presenting flu proteins at unique geometries and densities can direct the immune response, and lead to the creation of antibodies of high breadth. These can form the basis for a universal flu vaccine.

Host

Steve Hagen

January 31, Thursday, 1:40 PM (Note special date and early time)

Speaker

Matteo Mitrano (U. Illinois, Urbana-Champaign)

Title

Emergent nonequilibrium dynamics of complex solids

Abstract

Ultrafast optical excitation, especially when resonant to specific lattice modes, has recently emerged as a powerful means to control complex solids and their phase transitions. One of the most ambitious applications of these optical methods is the possibility to manipulate the lattice and the electronic interactions to bring about nonequilibrium superconductivity at temperatures far above the thermodynamic critical temperature Tc. In this talk, I will discuss how midinfrared optical excitation of the BCS-like superconductor K3C60 led to an emergent nonequilibrium superconducting-like phase above its equilibrium Tc. This light-induced state is suppressed by external pressure, as expected for a conventional BCS superconductor, but its microscopic origin is still unclear. In order to further our understanding of these phenomena, it is necessary to go beyond ultrafast optics and instead probe electronic excitations at finite momentum. I will report the recent observation of collective, finite-momentum dynamics of a charge order condensate in the cuprate La2-xBaxCuO4 using next-generation time-resolved X-ray scattering methods. Finally, I will discuss how ultrafast electron and X-ray scattering will enable the observation of novel nonlinear effects in the charge, spin and lattice response of complex solids.

Host

Yoonseok Lee

February 4 (Biophysics theory seminar)   

Speaker

Yizeng Li (Johns Hopkins Univ.)

Title

Why hydraulic resistance matters in cell biology

Abstract

Cells in vivo live in diverse physical environments that provide mechanical and biochemical cues for cells to migrate. For example, cell migration on two-dimensional surfaces is mostly driven by forces from actin polymerization and focal adhesions (actin-driven), whereas cells in confined geometries can be driven by water permeation (water-driven). While the factors that affect actin-driven cell migration have been extensively studied, little is known about the water-driven mechanism, not to mention the relation between these two. In this talk, I will first introduce the concept of actin-driven and water-driven cell migrations. I will then present a physiology- and physics-based cell migration model that I have developed. This is the first model that unifies the two distinct mechanisms of cell migration in a single mathematical framework and satisfies energy identity. The model shows that the potential mechanism of migration one cell chooses depends on the external hydraulic resistance in the environment. It further predicts that higher external hydraulic resistance increases the speed of water-driven cell migration. This prediction is supported by multiple sets of experimental data which I will also discuss. I will conclude the talk by discussing the implications of hydraulic resistance in health and medicine.

Host

David Tanner

February 5, Tuesday, 10:40 AM (Note special date and early time)

Speaker

Fereshte Ghahari (NIST, Gaithersburg)

Title

Visualizing graphene quantum dots: Topology and interaction driven phenomena

Abstract

Recent progress in creating graphene quantum dots (QDs) with fixed build-in potentials has offered a new platform to visualize and probe the confined electronic states. In this talk, I describe scanning tunneling spectroscopy measurements of the energy spectrum of graphene QDs as a function of energy, spatial position, and magnetic field. In zero field, the charge carriers are confined by oblique Klein scattering at the p-n junction boundary giving rise to a series of quasi-bound single particle states. Applying a weak magnetic field, we observe a giant and discontinuous change in the energy of time-reversed angular-momentum states, which manifests itself as the appearance of “new” resonances in the tunneling density of states. This behavior corresponds to the on/off switching of a π-Berry phase when a weak critical magnetic field is reached. With increased applied magnetic field, the QD states can be confined even further as they condense into highly degenerate Landau levels providing the first spatial visualization of the interplay between spatial and magnetic confinement. This is observed as formation of the seminal wedding-cake structures of concentric compressible and incompressible density rings in strong magnetic fields.

Host

Yoonseok Lee

February 11 (Biophysics theory seminar)     

Speaker

Purushottam Dixit (Columbia Univ.)

Title

Memory and heterogeneity in mammalian signaling networks

Abstract

Many human sensory systems operate on a logarithmic scale; i. e. they respond to relative changes in external stimuli. At the organism level, logarithmic (relative) sensing is achieved by complex neurophysiological pathways. In contrast, the ability of mammalian cells to perform relative sensing remains unknown. For the first time, in a combined computational and experimental study, we demonstrate the ability of the growth factor (GF) activated signaling network in mammalian cells to sense relative changes in GF concentrations. Moreover, we show that the relative sensing mechanisms is based on a novel form of cell memory, where cells “remember” background ligand levels by tuning the concentration of ligand-cognate receptors on the cell surface. Computational analysis suggests that the novel memory mechanism and logarithmic sensing are robust to parameter variations and applicable to several mammalian signaling pathways.

Next, we study cell population heterogeneity in the network activity downstream of GF stimulation. Population heterogeneity arises due to cell-to-cell variability in network parameters such as reaction rates and species concentrations. Notably, heterogeneity in parameters has important functional consequences, for example, emergence of drug resistant subpopulations in tumors. The multivariate distribution of network parameters can have an arbitrary shape and its inference from data remains mathematically challenging. In this talk, I will present a maximum entropy based approach and apply it to study heterogeneity in GF activated pathways. I will also discuss possible applications that will allow identification of drug resistance subpopulations in tumors.

Host

Dmitrii Maslov

February 18  

Speaker

Seminar by a biophysics faculty candidate. See https://www.phys.ufl.edu/wp/index.php/events/ for details.

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February 25      

Speaker

(Tentatively reserved.)

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March 4 (No Seminar - UF Spring Break Week, APS March Meeting in Boston)

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March 11, 10:40 AM (Note the early time)      

Speaker

Hiroki Ikegami (RIKEN)

Title

Symmetry breaking and topology in superfluid ³He

Abstract

Quantum phenomena observed in the p-wave superfluid ³He are intimately related to symmetry breaking and topology, and their understanding has significant implications not only on condensed matter physics but also on particle physics and cosmology. In this talk, I will show novel phenomena arising from symmetry breaking and non-trivial topology of superfluid ³He investigated by electrons trapped below a free surface. In particular, I will present the first direct demonstration of time-reversal symmetry breaking in one of the superfluid phases called ³He-A by showing that electrons immersed in superfluid ³He-A exhibit an anomalous Hall effect [1,2] and the direct observation of Majorana surface states formed at a free surface of topological ³He-B phase [3-5].

[1] H. Ikegami, Y. Tsutsumi, and K. Kono, Science 341, 59-62 (2013). [2] O. Shevtsov and J. A. Sauls, Phys. Rev. B 94, 064511 (2016). [3] H. Ikegami, S. B. Chung, and K. Kono, J. Phys. Soc. Jpn. 82, 124607 (2013). [4] H. Ikegami and K. Kono, published online in J. Low Temp. Phys. DOI: 10.1007/s10909-018-2069-y (arXiv:1805.11231). [5] Y. Tsutsumi, Phys. Rev. Lett. 118, 145301 (2017).

Host

Yoonseok Lee

March 11      

Speaker

Lilia Boeri (Univ. Rome—La Sapienza)

Title

TBA

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Peter Hirschfeld

March 18       

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March 25       

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April 1    

Speaker

Phil Allen (Stony Brook U.)

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TBA

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Peter Hirschfeld, Dmitrii Maslov

April 8      

Speaker

Hrvoje Petek (Univ. Pittsburgh)

Title

Ultrafast spectroscopy and microscopy of plasmonic excitations in metals

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Chris Stanton

April 15      

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April 22      

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Yasuyuki Nakajima (UCF)

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Yasu Takano

Condensed Matter Seminars
Fall 2019

Condensed Matter Seminars are in Room NPB 2205
on Mondays @ 4:05 pm t0 4:55 pm

Contact: Yasu Takano or Dmitrii Maslov

August 26      

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September 2 (No seminar – Labor Day)     

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September 9      

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September 16      

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September 23      

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September 30      

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October 7      

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October 14      

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October 21      

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October 28      

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November 4      

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November 11 (No seminar – Veterans Day)     

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November 18        

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November 25 (Monday of Thanksgiving Week)        

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December 2 (Monday after Thanksgiving Holiday)      

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December 9      

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