Physics Home

Condensed Matter Seminars
Spring 2017

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

Contact: Yasu Takano or K. Muttalib

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January 9      

 

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January 16 (No seminar - Martin Luther King Jr. Day)       

 

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January 23    

 

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David Tanner (UF)

 

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Superfluid and normal-fluid densities in the cuprate superconductors

 

Abstract

It was on 17 April 1986 when Bednorz and Mueller of the IBM Zürich laboratories sent a paper about “possible high-Tc superconductivity” to Zeitschrift für Physik B.  The resulting bombshell changed condensed-matter physics forever.  Experimenters and theorists developed methods to measure and calculate in ways that were much improved over prior years.  However, despite 30 years of intense study, the description of these materials remains incomplte.  I’ll discuss what infrared spectroscopy can tell us about these materails.  Measurements for a number of cuprate families of optical reflectance over a wide spectral range (far-infrared to ultraviolet) have been analyzed using Kramers-Kronig analysis to obtain the optical conductivity, σ(ω), and (by integration of the real part of the conductivity) the spectral weight of low- and mid-energy excitations.  For the Kramers-Kronig analysis to give reliable results, accurate high-frequency extrapolations, based on x-ray atomic scattering functions, were used.  When the optical conductivities of the normal and superconducting states are compared, a transfer of spectral weight from finite frequencies to the zero-frequency delta-function conductivity of the superconductor is seen.  The strength of this delta function gives the superfluid density, ρs.  There are two ways to measure ρs, using either the low energy spectral weight or by examination of the imaginary part, σ2(ω); both estimates show that 98% of the ab-plane superfluid density comes from low energy scales, below about 0.15 eV.  Moreover, there is a notable difference between clean metallic superconductors and the cuprates.   In the former, the superfluid density is essentially equal to the conduction electron density.  The cuprates, in contrast, have only about 20% of the ab-plane low-energy spectral weight in the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials and in materials where stripe charge order is seen, the superfluid fraction is even smaller.  The consequences of this observation for the electronic structure will be addressed.

 

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January 30        

 

Speaker

Larry Engelhardt (Francis Marion University)

 

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Quantum Monte Carlo simulations for magnetic molecules

 

Abstract

Many condensed matter systems can be described in terms of interacting quantum spins. I will discuss some of the numerical/computational methods that can be used to simulate systems of interacting quantum spins, with a particular emphasis on quantum Monte Carlo (QMC) simulations. I will discuss the advantages and limitations of QMC simulations, and I will show examples of how QMC simulations can be used to analyze complex magnetic molecules.

 

Host

Meisel


February 6    

 

Speaker

Rahul Nandkishore (U of Colorado, Boulder)

 

Title

Disorder driven destruction of a controlled non-Fermi liquid fixed point explored via RG

 

Abstract

Three dimensional systems where the non-interacting band structure hosts a quadratic band crossing at the Fermi level provide an interesting playground for condensed matter physics. Forty years ago, Abrikosov argued that when Coulomb interactions are added to this system, then the ground state should provide a rare example of a non-Fermi liquid which can be described in a controlled fashion through an epsilon expansion renormalization group. In this work we explore the  consequences of adding disorder to the system. We introduce a controlled renormalization group procedure using which we explore the interplay of disorder and Coulomb interactions. We show that disorder destroys the Abrikosov non-Fermi liquid phase, and that the system flows inevitably to strong disorder where the analysis is no longer controlled. Extrapolating the flow to strong disorder we conclude that the long wavelength physics of the interacting system can be described using a non-interacting sigma model description. We conclude with a discussion of general principles revealed for the analysis of gapless semiconductors, and the connections to the theory of many body localization.

 

Host

Maslov


February 13       

 

Speaker

Tom Lemberger (Ohio State University)

 

Title

Two-coil measurements of superfluid density (magnetic penetration depth) and coherence length in superconducting

 

Abstract

A few weeks ago, David Tanner told you all about spectroscopic measurements of superfluid density in cuprates, why superfluid density is important, and what puzzles it presents to us. This talk is about superfluid densities determined with a low-frequency measurement technique, around 50 kHz. The “two-coil” technique is restricted to study of thin films, but this is an advantage in that there are interesting superconducting compounds that exist only as films grown on substrates – the substrate stabilizes them. Such materials include certain electron-doped cuprates and BiSrCaCuO that is underdoped by oxygen removal. Also, if you want to know whether cuprates are essentially two-dimensional materials with very weak interlayer coupling, then you need to grow a film one or two unit cells thick, so it is honestly two-dimensional, and see how it behaves compared with thick samples. We’ve done this and the results are interesting. Recently, we’ve devised a phenomenological way to use the two-coil technique to determine the superconducting coherence length. It involves cranking the applied ac field up to where it creates lots of vortices and antivortices in the sample film. In this measurement, “high” field means tens of gauss. No need to go to Tallahassee to measure the upper critical field at 30 tesla or so. The underlying theory for this technique is lacking.

 

Host

Tanner


February 20  

 

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Joseph Maciejko (University of Alberta)

 

Title

Superconducting Dirac fermions and mirror symmetry

 

Abstract

In the presence of strong interactions, the gapless boundary modes of topological insulators/superconductors are potential platforms for observing exotic types of quantum criticality. It was recently shown that the semimetal-superconductor quantum phase transition for a single 2D Dirac fermion, such as found on the surface of a 3D topological insulator, exhibits an emergent supersymmetry with the Dirac fermions and Cooper pairs behaving as superpartners. In this talk I will argue that the surface of a topological insulator with three degenerate Dirac cones, such as the (111) surface of SmB6, can support pair-density-wave transitions at which a different type of supersymmetry emerges. By mirror symmetry, a duality of supersymmetric theories similar to particle-vortex duality, this novel critical point is dual to supersymmetric quantum electrodynamics, a theory of bosonic and fermionic matter fields interacting with dynamical gauge bosons and fermionic gauginos.

 

Host

Maslov


February 27    

 

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March 6 (No Seminar - UF Spring Break Week)

 

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March 13 (No Seminar - APS March Meeting in New Orlean)      

 

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March 20       

 

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Yasuo Yoshida (ISSP, Univ. Tokyo)

 

Title

Atomic-scale visualization of surface-assisted orbital order in the heavy fermion compound CeCoIn5

 

Abstract

Orbital-mediated quantum phenomena have proved over the past decade to be far-reaching and complex, as exemplified by exotic orbital orders, nontrivial orbital-fluctuation-mediated superconductivity, orbital Kondo effect, and multipole-moment ordering. To understand electron, spin, and orbital correlations in these phenomena, it is crucial to have a direct access to real-space orbital texture, but so far orbital-sensitive probes have shown rather limited functionality. Recent progress in scanning tunneling microscopy (STM) has enabled orbital-selective tunneling by fine-tuning the tip-sample distance (TSD). We exploit the orbital sensitivity of STM to unveil a surface-assisted cobalt d-orbital order in the heavy fermion compound CeCoIn5. We find that at a small TSD, cobalt atoms in STM topographies take on dumbbell shapes alternatingly aligned in the [100] and [010] directions on a cleaved (001) surface. Domain boundaries of this ordered structure, which are localized within a terrace, denote two-dimensionality of the ordered structure. First-principles calculations show that the structure is a consequence of a staggered dxz-dyz orbital order assisted by surface termination. This novel surface-assisted orbital ordering seems to be ubiquitous in transition metal oxides, heavy fermion superconductors and other materials, but has been overlooked until now.

 

Host

Takano


March 22  (Wednesday, 10:00 am) Note the date and time.      

 

Speaker

Maxim Korshunov (Kirensky Institute of Physics)

 

Title

Superconductivity, spin-resonance peak, and disorder in Fe-based materials

 

Abstract

The symmetry and structure of the superconducting gap in Fe-based superconductors is the most fundamental issue in the rapidly developing field of unconventional multiband superconductivity. I will show how spin fluctuations lead to the sign-changing gap in multiband models and discuss experimental signatures of the resulting superconducting state. In particular, emergence of the spin-resonance peak in the inelastic neutron scattering, the disorder-induced transition between different gap structures, and the effect of impurity scattering on the spin-resonance peak.

 

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Hirschfeld


March 27       

 

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April 3    

 

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April 6 (Thursday, 1:00 pm, Room 2165) Note the date, time, and room.    

 

Speaker

Ilya Eremin (Ruhr-Universität Bochum)

 

Title

Progress and prospects in understanding the iron-based superconductors: a status report

 

Abstract

In the first part of my talk I will review our current understanding of the iron-based superconductors and the most likely mechanisms of the superconducting instability in these systems. In the second half of the talk I will analyze some peculiar features in the physics of iron-based superconductivity like recently observed BCS-BEC crossover in FeSe and its possible fingerprints in Ba112 doped with K. Finally, I will discuss the electronic and superconducting properties of the recently discovered novel iron-based superconductors like CaKFe4As4 and ThFeAsN.

 

Host

Hirschfeld


April 10      

 

Speaker

Daniel Pajerowski (Oak Ridge National Laboratory)

 

Title

Some magnetic neutron scattering

 

Abstract

Neutron scattering is a general term that encompasses a wide array of techniques and science. I'll start with a short introduction to the field of neutron scattering before moving onto some specific condensed matter cases that I am currently a part of. Neutron powder diffraction (NPD) measures the nuclear and magnetic lattice. In the prototypical type-II multiferroic MnWO4 (MWO), NPD reveals an increase in the temperature range of the multiferroic MWO spin-spiral phase for nanosized particles. Polarized neutron reflectometry (PNR) is sensitive to nuclear and magnetic scattering length densities in thin films. The double perovskite Ba2FeMoO6 (BFMO) and the distorted perovskite BiMnO3 (BMO) are both found to have reduced magnetization in films as compared to crystals as elucidated by PNR, but with different causes. Inelastic neutron spectroscopy (INS) probes correlation functions at finite time scales. The spin chain Nickel Bifluoride Chloropyridine Tetrafluoroborate (NBCT) has been suggested to be in the vicinity of a quantum critical point (QCP) from magnetization and specific heat data, and INS probes the Hamiltonian in a quite direct manner by measuring the propagating modes.

 

Host

Meisel


April 17      

 

Speaker

Carlos Sá de Melo (Georgia Tech)

 

Title

Uncloaking topological phases in the Berezinskii-Kosterlitz-Thouless transition of Fermi superfluids: An interplay of spin-orbit coupling, Zeeman fields and interactions.

 

Abstract

We investigate the Berezinskii-Kosterlitz-Thouless (BKT) transition in a two-dimensional (2D) Fermi system with spin-orbit coupling (SOC), as a function of the two-body binding energy and a perpendicular Zeeman field [1]. By including a generic form of the SOC, as a function of Rashba and Dresselhaus terms, we study the evolution between the equal Rashba-Dresselhaus (ERD) and the Rashba-only (RO) cases. We show that in the ERD case, at fixed non-zero Zeeman field, the BKT transition temperature TBKT is increased by the effect of the SOC for all values of the binding energy. We also find a significant increase in the value of the Clogston limit compared to the case without SOC. Furthermore, we demonstrate that the superfluid density tensor becomes anisotropic (except in the RO case), leading to an anisotropic phase-fluctuation action that describes elliptic vortices and anti-vortices, which become circular in the RO limit. This deformation constitutes an important experimental signature for superfluidity in a 2D Fermi system with ERD SOC. In addition, we show that the anisotropic sound velocity exhibit anomalies at low temperatures in the vicinity of quantum phase transitions between topologically distinct uniform superfluid phases. Finally, we mention new results of more exotic superfluid phases which involve the coupling of “charge” and “spin” vortices [3] induced by the presence of spin-orbit and Zeeman fields.

[1] Jeroen P. A. Devreese, Jacques Tempere, and Carlos A. R. Sá de Melo, Phys. Rev. Lett. 113, 165304 (2014). [2] Jeroen P. A. Devreese, Jacques Tempere, and Carlos A. R. Sá de Melo, Phys. Rev. A 92, 043618 (2015). [3] Jeroen P. A. Devreese, Jacques Tempere, and Carlos A. R. Sá de Melo, in preparation, to appear at the ArXiv (2017).

 

Host

Hirschfeld