Condensed Matter Seminars

Condensed
Matter Seminars are in Room NPB 2205 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 


Speaker 
David Tanner (UF) 

Title 
Superfluid and normalfluid 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 highTc superconductivity” to Zeitschrift für Physik B. The resulting bombshell changed condensedmatter 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 (farinfrared to ultraviolet) have been analyzed using KramersKronig analysis to obtain the optical conductivity, σ(ω), and (by integration of the real part of the conductivity) the spectral weight of low and midenergy excitations. For the KramersKronig analysis to give reliable results, accurate highfrequency extrapolations, based on xray 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 zerofrequency deltafunction conductivity of the superconductor is seen. The strength of this delta function gives the superfluid density, ρ_{s}. There are two ways to measure ρ_{s}, using either the low energy spectral weight or by examination of the imaginary part, σ_{2}(ω); both estimates show that 98% of the abplane 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 abplane lowenergy spectral weight in the superfluid. The rest remains in finitefrequency, 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. 

Host 

January 30 


Speaker 
Larry Engelhardt (Francis Marion University) 

Title 
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 nonFermi liquid fixed point explored via RG 

Abstract 
Three dimensional systems where the noninteracting 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 nonFermi 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 nonFermi 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 noninteracting 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 
Twocoil 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 lowfrequency measurement technique, around 50 kHz. The “twocoil” 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 electrondoped cuprates and BiSrCaCuO that is underdoped by oxygen removal. Also, if you want to know whether cuprates are essentially twodimensional materials with very weak interlayer coupling, then you need to grow a film one or two unit cells thick, so it is honestly twodimensional, 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 twocoil 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 


Speaker 
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 semimetalsuperconductor 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 SmB_{6}, can support pairdensitywave transitions at which a different type of supersymmetry emerges. By mirror symmetry, a duality of supersymmetric theories similar to particlevortex 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 22 (Wednesday, 10:00 am) Note the date and time. 


Speaker 
Maxim Korshunov (Kirensky Institute of Physics) 

Title 
Superconductivity, spinresonance peak, and disorder in Febased materials 

Abstract 
The symmetry and structure of the superconducting gap in Febased superconductors is the most fundamental issue in the rapidly developing field of unconventional multiband superconductivity. I will show how spin fluctuations lead to the signchanging gap in multiband models and discuss experimental signatures of the resulting superconducting state. In particular, emergence of the spinresonance peak in the inelastic neutron scattering, the disorderinduced transition between different gap structures, and the effect of impurity scattering on the spinresonance 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. 


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Title 
Progress and prospects in understanding the ironbased superconductors: a status report 

Abstract 
In the first part of my talk I will review our current understanding of the ironbased 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 ironbased superconductivity like recently observed BCSBEC 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 ironbased superconductors like CaKFe_{4}As_{4} 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 typeII multiferroic MnWO_{4} (MWO), NPD reveals an increase in the temperature range of the multiferroic MWO spinspiral phase for nanosized particles. Polarized neutron reflectometry (PNR) is sensitive to nuclear and magnetic scattering length densities in thin films. The double perovskite Ba_{2}FeMoO_{6} (BFMO) and the distorted perovskite BiMnO_{3} (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 BerezinskiiKosterlitzThouless transition of Fermi superfluids: An interplay of spinorbit coupling, Zeeman fields and interactions. 

Abstract 
We investigate the BerezinskiiKosterlitzThouless (BKT) transition in a twodimensional (2D) Fermi system with spinorbit coupling (SOC), as a function of the twobody 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 RashbaDresselhaus (ERD) and the Rashbaonly (RO) cases. We show that in the ERD case, at fixed nonzero Zeeman field, the BKT transition temperature T_{BKT} 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 phasefluctuation action that describes elliptic vortices and antivortices, 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 spinorbit 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 