Condensed Matter
Thursday, August 30, 2018
11:00 AM
Physics Building, Room 313

"Available"


Special Condensed Matter Seminar
Friday, August 31, 2018
3:30 PM
Physics Building, Room 204
Note special date.
Note special time.
Note special room.

Hitesh J. Changlani
[Host: Bellave Shivaram]
Florida State University
"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 nontrivial 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 and on the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum manybody systems.
I present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the frustrated kagome geometry, which has seen a flurry of research activity owing to several nearideal 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 spin1 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 wellstudied spin1/2 case [2]. I explain the existence of an exactly solvable point in the XXZHeisenberg model for the ratio of Ising to transverse coupling $J_z/J=1/2$ [3]. This point in the phase diagram, previously unreported in the literature, has "threecoloring" 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, under preparation.


Condensed Matter
Thursday, September 6, 2018
11:00 AM
Physics Building, Room 313

"Available"


Condensed Matter
Thursday, September 13, 2018
11:00 AM
Physics Building, Room 313

Available


Condensed Matter
Thursday, September 20, 2018
11:00 AM
Physics Building, Room 313

Nirmal Ghimire
[Host: Bellave Shivaram]
George Mason University
""A materialsdriven approach to the novel topological states of matter""

ABSTRACT:
Materials in condensed matter have recently been testbeds for several exotic particles, predicted but never realized, in high energy physics. The examples are skyrmions observed in magnetic textures. Weyl fermions in the low energy electronic excitations of Weyl semimetals and Majorana fermions in topological superconductors. These discoveries have not only allowed access to the fundamental physics of the rare particles but also driven large interest in the application of such exotic states to future technologies such as spin based electronics and quantum computation. Discoveries of topological states in materials have largely benefited from the precision of the electronic structure calculations in the weakly correlated systems. In the first part of this talk, I will discuss our resent results on two such predicted materials – 1) NbAs, one of the first generation Weyl semimetals [13] and 2) Pd_{3}Pb, a novel topological material hosting multiple Dirac points and surface states [4]. While calculations are pretty accurate in weakly correlated systems, the topological states in presence of strong electron correlations are still not well understood. As such, materials can take a lead in this field. In the second part of the talk, I will briefly highlight our recent efforts in this area, driven by specific materials design criteria. As an illustration, I will discuss our study on the chirallattice antiferromagnet CoNb_{3}S_{6} that has topological character in the electronic band structure, and manifests an unusually large anomalous Hall effect [5].
[1] N. J. Ghimire et al. J. Phys.: Condens. Matter 27, 152201 (2015).
[2] Y. Luo et al. Phys. Rev. B 92, 205134 (2015)
[3] P. J. W. Moll et al., Nat. Communs. 7, 12492 (2016).
[4] N. J. Ghimire et al., Phys. Rev. Materials 2, 081201(R) (2018)
[5] N. J. Ghimire et al., Nat. Communs. 9, 3280 (2018)


Condensed Matter
Thursday, September 27, 2018
11:00 AM
Physics Building, Room 313

Ed Barnes
[Host: Israel Klich]
Virginia Tech
"Toward the next quantum revolution: controlling physical systems and taming decoherence"

ABSTRACT:
Recent years have witnessed enormous progress toward harnessing the power of quantum mechanics and integrating it into novel technologies capable of performing tasks far beyond presentday capabilities. Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information in a process known as decoherence. I will present recent progress in understanding and modeling the effects of multiple noise sources on the evolution of a quantum bit and show how this can be used to develop new ways to slow down decoherence. I will then describe a new general theory for dynamically combatting decoherence by driving quantum bits in such a way that noise effects destructively interfere and cancel out, enabling the high level of control needed to realize quantum information technologies.


Condensed Matter
Thursday, October 4, 2018
3:30 PM
Physics Building, Room 204
Note special time.
Note special room.

Reserved for Special Colloquium


Condensed Matter
Thursday, October 11, 2018
11:00 AM
Physics Building, Room 313

Alex Levchenko
[Host: Dmytro Pesin]
University of WisconsinMadison
"Transport in Strongly Correlated 2D Electron Fluids"

ABSTRACT:
In this talk I plan to overview measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities with an emphasis on magnetoresistance and drag resistance. As many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium we will concentrate on physics associated with strong correlation effects and develop hydrodynamic theory of transport. We will apply these ideas to composite fermions of quantum Hall bilayers in hydrodynamic regime.


Condensed Matter
Thursday, October 18, 2018
11:00 AM
Physics Building, Room 313

Erhai Zhao
[Host: Bellave Shivaram]
George Mason University
"Competing orders in a quantum spin model with longrange interactions"

ABSTRACT:
Quantum spin liquids evade longrange magnetic order down to absolute zero temperature. These anarchic, yet highly entangled states break no symmetry but have remarkable properties such as fractional excitations. In this talk, I will first give an example of spin liquid using a compass model relevant to recently discovered honeycomb antiferromagnet NaNi2BiO6. Then I will introduce a new model, the dipolar Heisenberg model, motivated by recent experiments on artificial manyspin systems based on interacting dipoles. I will argue that longrange magnetic order can be suppressed by simply tuning the direction of the dipoles using an external field. The classical, semiclassical, and quantum phase diagram of this frustrated spin model will be presented to show an extended region where the ground state is a quantum paramagnet. By comparing to DMRG, I will argue that it is likely a quantum spin liquid.


Condensed Matter
Thursday, October 25, 2018
11:00 AM
Physics Building, Room 313

"Available"


Condensed Matter
Thursday, November 1, 2018
11:00 AM
Physics Building, Room 313

Available


Condensed Matter
Thursday, November 8, 2018
11:00 AM
Physics Building, Room 313

Arnab Banerjee
[Host: Bellave Shivaram]
Oak Ridge National Laboratory
"TBA"


Condensed Matter
Thursday, November 15, 2018
11:00 AM
Physics Building, Room 313

"Available"


Condensed Matter
Thursday, November 29, 2018
11:00 AM
Physics Building, Room 313

"Available"


Condensed Matter
Thursday, December 6, 2018
11:00 AM
Physics Building, Room 313

Available



