Physics Home

Condensed Matter Seminars
Fall 2018

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

Contact: Yasu Takano or Dmitrii Maslov

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August 27      

 

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September 3 (No seminar – Labor Day)     

 

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September 10      

 

Speaker

Naween Anand (NHMFL)

 

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Evolution of magnetic phases and emergent magnetic monopoles in the spin ice Ho2Ti2O7

 

Abstract

The presence of geometric and magnetic frustration in spin-ice Ho2Ti2O7 (HTO) leads to a macroscopically degenerate ground state with a magnetic phase exhibiting short-range spin-ice correlation among the Holmium ions. This macroscopic degeneracy gets lifted and the residual entropy is released when external field is applied along certain crystallographic directions, leading to specific long-range ordered phases.

Developing such ordered phase or transition from one to another magnetic phase in HTO leads to a dynamical transient state with a continuous loop of creating and annihilating emergent magnetic monopoles. Torque magnetometry provides a unique technique to study the evolution of such magnetic phases and associated magneto-crystalline anisotropy. The technique has much higher sensitivity and any small change in the order parameter with varying magnetic field or the crystallographic orientation is subsequently reflected as the changed slope and curvature of the data. A phenomenological model for spin flips has been proposed to map out this rich magnetic phase diagram of spin-ice HTO. A direct comparison between single crystals and thin films of varying thicknesses in HTO torque data has helped us in understanding the possible tuning of spin-ice physics in thin films. Other magnetometric techniques, such as tunnel diode oscilloscope (TDO), which provides a scaled measurement of dynamical susceptibility, and standard DC magnetization measurement through MPMS technique strongly support the torque results and model predictions. This thorough study makes a solid stride towards developing another complementary measurement technique for studying a rich magnetic phase diagram, possibly a much more effective tool for thin film studies where neutron scattering technique faces serious limitations.

 

Host

David Tanner


September 17      

 

Speaker

Assel Aitkaliyeva (UF, Materials Science)

 

Title

Establishing defect-property relationship for low dimensional materials

 

Abstract

The radiation response of low dimensional materials (LDMs) deviates dramatically from bulk materials with three-dimensional (3D) bonding networks. In bulk solids, energy deposited by the energetic particles can dissipate in all three-dimensions, unlike in their low dimensional counterparts. The less efficient energy dissipation mechanism in LDMs can lead to increase in local kinetic energy after the impact. This poses important questions such as: how tolerant are LDMs to extreme photon and particle fluxes and what are the mechanisms governing their radiation response? This contribution seeks to gain understanding of the radiation tolerance of various LDMs. We demonstrate that irradiation can be used to manipulate physical properties of LDMs in a controllable manner. The work to be discussed includes previously conducted ion irradiation of carbon nanotubes and graphene and experiments planned on other LDM systems. The PI’s group specializes in: 1) establishing defect formation and evolution mechanisms in LDMs subjected to irradiation, 2) quantifying radiation tolerance of LDMs, and 3) developing the requisite fundamental knowledge base needed to understand defect-property relationship in this unique class of materials.

 

Host

Mark Meisel


September 24      

 

Speaker

Yuxuan Wang (UF)

 

Title

Second-order topological superconductors

 

Abstract

Conventional topological superconductors are fully gapped in the bulk but host gapless Majorana modes on their boundaries. We instead focus on a new class of superconductors, second-order topological superconductors, that have gapped, topological surfaces and gapless Majorana modes instead on lower-dimensional boundaries, i.e., corners of a two-dimensional system or hinges for a three-dimensional system. Here we propose two general scenarios in which second-order topological superconductivity can be realized spontaneously with weak-pairing instabilities. First, we show that px+ipy-wave pairing in a (doped) Dirac semimetal in two dimensions with four mirror symmetric Dirac nodes realizes second-order topological superconductivity. Second, we show that p+id pairing on an ordinary spin-degenerate Fermi surface realizes second-order topological superconductivity as well. In the latter case we find that the topological invariants describing the system can be written using simple formulae involving only the low-energy properties of the Fermi surfaces and superconducting pairing. In both cases we show that these exotic superconducting states can be intrinsically realized in a metallic system with electronic interactions. For the latter case we also show it can be induced by proximity effect in a superconducting heterostructure.

 

Host

Dmitrii Maslov


October 1      

 

Speaker

Andreas Kreisel (Univ. Leipzig)

 

Title

Magnon-phonon interaction in multiferroic h-YMnO3

 

Abstract

The multiferroic material YMnO3 is known to show a large spin lattice coupling such that the spin and lattice degrees of freedom influence various physical properties. Here, we discuss a Heisenberg model describing the low-temperature magnetically ordered phase and the implications of the resulting magnon-phonon interactions from an single-ion magnetostriction mechanism. Especially, an avoided crossing between magnon and phonon modes appears. This can be used to distinguish possible magnetic ground state configurations which otherwise show an identical response in neutron scattering experiments. A comparison to various experimental results from inelastic neutron scattering allows us to exclude several of the earlier proposed models. The combined information from a polarization analysis and the observation of a field-induced magnon splitting points to a magnetic ground state of the P6'3c'm symmetry such that an effective dynamic model relevant for the multiferroic hexagonal manganites is obtained.

 

Host

Peter Hirschfeld


October 8      

 

Speaker

Leonid Levitov (MIT)

 

Title

Long-lived excitations and tomographic dynamics in 2D electron fluids

 

Abstract

This talk will discuss the peculiar collective behavior in two-dimensional Fermi gases originating from the head-on carrier-carrier collisions. Such collisions dominate in 2D Fermi systems at cold temperatures T << TF owing to the interplay of Pauli blocking and momentum conservation. The predominantly head-on character of two-body collisions results in a large number of modes with anomalously long lifetimes, associated with the odd-parity harmonics of the electron momentum distribution. The slow modes give rise to a “tomographic” transport regime dominated by fermionic jets, featuring an unusual hierarchy of time scales and scale-dependent transport coefficients with nontrivial fractional scaling dimensions.


Host

Dmitrii Maslov


October 15      

 

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October 22      

 

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Hitesh Changlani (FSU)

 

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The mother of all states of the kagome quantum antiferromagnet

 

Abstract

Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically non-trivial properties. Among these are geometrically frustrated magnets, which harbor spin liquid phases with fractional excitations. On the experimental front, this has motivated the search for new low dimensional quantum materials. On the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum many-body systems.

I present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the kagome geometry, which has seen a flurry of research activity owing to several near-ideal material realizations. On the theoretical front, the kagome problem has a rich history and poses multiple theoretical puzzles which continue to baffle the community. First, I present a study of the spin-1 antiferromagnet, where our numerical calculations indicate that the ground state is a trimerized valence bond (simplex) solid with a spin gap [1], contrary to previous proposals. I show evidence from recent experiments that support our findings but also pose new questions. The second part of the talk follows from an unexpected outcome of my general investigations in the area for the well-studied spin-1/2 case [2]. I explain the existence of an exactly solvable point in the XXZ-Heisenberg model for the ratio of Ising to transverse coupling Jz/J=–1/2 [3]. This point in the phase diagram has "three-coloring" states as its exact quantum ground states and is macroscopically degenerate. It exists for all magnetizations and is the origin or "mother" of many of the observed phases of the kagome antiferromagnet. I revisit aspects of the contentious and experimentally relevant Heisenberg case and discuss its relationship to the newly discovered point [3,4].

[1] H. J. Changlani, A.M. Lauchli, Phys. Rev. B 91, 100407(R) (2015). [2] K. Kumar, H. J. Changlani, B. K. Clark, E. Fradkin, Phys. Rev. B 94, 134410 (2016). [3] H. J. Changlani, D. Kochkov, K. Kumar, B. K. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018). [4] H. J. Changlani, S. Pujari, C.M. Chung, B. K. Clark, arXiv.1808.08633, under review (2018).

 

Host

Yuxuan Wang


October 29      

 

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November 5      

 

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Madhab Neupane (UCF)

 

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Experimental realization of topological insulator and beyond

 

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A three-dimensional (3D) topological insulator (TI) is a crystalline solid, which is an insulator in the bulk but features spin-polarized Dirac electron states on its surface. In 2007, the first 3D TI was discovered in a bismuth-based compound. The discovery of the first TI tremendously accelerated research into phases of matter characterized by nontrivial topological invariants. Not only did the 3D TI itself attract great research interest, it also inspired the prediction of a range of new topological phases of matter. The primary examples are the topological Kondo insulator, the topological 3D Dirac, Weyl and nodal-line semimetals, the topological crystalline insulator and the topological superconductor. Each of these phases was predicted to exhibit surface states with unique properties protected by a non-trivial topological invariant. In this talk, I will discuss the experimental realization of these new phases of matter in real materials through momentum-, spin- and time-resolved photoemission spectroscopy. The unusual properties of the protected topological surface states can lead to future applications in spintronics and quantum computation.

 

Host

Yoonseok Lee


November 12 (No seminar – Veterans Day)     

 

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November 19 (Monday of Thanksgiving Week)       

 

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Ilya Eremin (Univ. Bochum)

 

Title

Magnetic skyrmions at topological insulator surfaces and in proximity to a superconductor

 

Abstract

Ferromagnets in contact with a topological insulator have become appealing candidates for spintronics due to the Dirac surface states, which exhibit spin-momentum locking. Bilayer Bi2Se3-EuS structures, for instance, show a finite magnetization at the interface at temperatures well exceeding the Curie temperature of bulk EuS. Here we determine theoretically the effective magnetic interactions at a topological insulator-ferromagnet interface above the magnetic ordering temperature. We show, by integrating out the Dirac fermion fluctuations, that an effective Dzyaloshinskii-Moriya interaction and magnetic charging interaction emerge. As a result, individual magnetic skyrmions and extended skyrmion lattices can form at interfaces of ferromagnets and topological insulators, the first indications of which have been very recently observed experimentally. In the second part of my talk I will analyze a hybrid heterostructure with magnetic skyrmions inside a chiral ferromagnet interfaced by a thin superconducting film via an insulating barrier. The barrier prevents the electronic transport between the superconductor and the chiral magnet, such that the coupling can only occur through the magnetic fields generated by these materials. We find that Pearl vortices are generated spontaneously in the superconductor within the skyrmion radius, while anti-Pearl vortices compensating the magnetic moment of the Pearl vortices are generated outside of the skyrmion radius,, forming an energetically stable topological hybrid structure. Finally, we analyze the interplay of skyrmion and vortex lattices and their mutual feedback on each other. In particular, we argue that the size of the skyrmions will be greatly affected by the presence of the vortices, offering another prospect of manipulating the skyrmionic size by the proximity to a superconductor.

 

Host

Peter Hirschfeld


November 26 (Monday after Thanksgiving Holiday)        

 

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December 3      

 

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Physics Home

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

x

January 7      

 

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Daniel Agterberg (Univ. Wisconsin-Milwaukee)

 

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TBA

 

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Peter Hirschfeld


January 14       

 

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Stuart Brown (UCLA)

 

Title

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

 

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Host

Mark Meisel


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

 

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January 28      

 

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February 4   

 

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February 11      

 

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February 18  

 

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February 25      

 

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

 

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March 11      

 

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Lilia Boeri (Univ. Rome—La Sapienza)

 

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Peter Hirschfeld


March 18       

 

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March 25       

 

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April 1    

 

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Phil Allen (Stony Brook U.)

 

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Peter Hirschfeld, Dmitrii Maslov


April 8      

 

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April 15      

 

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April 22      

 

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