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Condensed Matter Seminars

Summer/Fall 2017

The seminars are held every Friday in the seminar room of the Joint Institute for Advanced Materials (Room 147).

Date Speaker Title

July 7

Zhentao Wang (UT)

Resistivity Minimum in Highly Frustrated Itinerant Magnets

July 14

Timmy Ramirez Cuesta (SNS-ORNL)

Neutrons and Numbers: Studying materials and processes with VISION and VirtuES

July 21

Zhiling Dun (UT)

The tripod kagome lattice: a new playground for geometrical frustration

July 28

Ward Plummer (LSU)

Surface and bulk properties of BaMnSb2, a topological semimetal with non-trivial Berry phase

August 4

No Seminar

NA

August 11

Luigi Sangaletti (Università Cattolica del Sacro Cuore)

Interface effects at all-oxide epitaxial heterojunction probed by photoemission spectroscopies

August 18

Jie Ma (Shanghai Jiao Tong University)

Neutron Scattering Study on the Quantum Effect in Ba3CoSb2O9

August 25

Jian Shen (Fudan University)

Towards all-in-one spintronics: Manipulating electronic phase separation in complex oxides

September 1

Lin Hao (UT)

Two-dimensional Jeff=1/2 Antiferromagnetism unraveled from interlayer coupling and controlled under external magnetic field

September 8

Phil Pincus (UT)

Screening in concentrated electrolyte solutions

September 22

Hidemaro Suwa (UT)

Enhanced controllability at proximity of hidden SU(2) symmetry in quasi two dimensions

September 29

Fangfei Ming (UT)

Realization of a hole-doped Mott insulator on a triangular silicon lattice

October 6

Anjana Samarakoon (University of Virginia)

Comprehensive study of the dynamics of a classical Kitaev spin liquid

October 13

Tao Hong (Quantum Condensed Matter Division, ORNL)

Exotic spin dynamics in a two-dimensional quantum antiferromagnet near the quantum critical point

October 20

Niravkumar D. Patel (UT)

Magnetic and pairing tendencies in quasi 1D multi-orbital models

October 27

Michel Gingras (University of Waterloo and Canadian Institute for Advanced Research)

Rare-Earth Pyrochlore Oxides – a Fascinating Playground for the Experimental and Theoretical Study of Frustrated Magnetism in Three Dimensions

November 3

Paige Kelley (UT/ORNL)

Suppression of Magnetic Order Near the Kitaev Quantum Spin Liquid

November 10

Yi Zhang (LSU)

Localization in Energy Materials

November 17

 

 

December 1

Jacek Herbych (UT)

Quantum distillation: how to make cold atoms cold

December 8

Shang-Shun Zhang (UT)

Paring from strong repulsion in triangular lattice Hubbard model


Abstracts
July 7

Zhentao Wang (UT)

Resistivity Minimum in Highly Frustrated Itinerant Magnets

Metals with magnetic impurities exhibit a minimum in the temperature dependence of the resistivity. The Kondo effect explains this minimum by the spin flip scattering between conduction electrons and the local impurities. Surprisingly, several compounds including a dense periodic array of magnetic impurities with large spin or with strong easy-axis anisotropy, also exhibit a resistivity minimum despite the suppression of the Kondo effect (spin flip scattering). Motivated this observation, we study the case when the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction dominates over the Kondo screening. It is found that frustrated RKKY interaction stabilizes a liquid like spin state, which extends down to temperatures well below the RKKY interaction scale. The crossover into this state is characterized by spin structure factor enhancement at wave vectors smaller than twice the Fermi wave vector magnitude. The corresponding enhancement of electron scattering generates a resistivity minimum without Kondo effect.


July 14

Timmy Ramirez Cuesta (SNS-ORNL)

Neutrons and Numbers: Studying materials and processes with VISION and VirtuES

Molecular spectroscopy is a very powerful tool to study the dynamical properties of solid, liquid and gases. Inelastic Neutron scattering is a very powerful tool to study hydrogen-containing materials. With the development of neutron spallation sources, and the use of epithermal neutrons, inelastic neutron scattering can measure the vibrational spectra of materials on the whole range of vibrational motions (0-4400 cm−1) and effectively opening up the field of neutron spectroscopy [1]. INS is a technique that was mostly used to study hydrogen-containing materials due to the high cross section of hydrogen [2].

The recently commissioned VISION spectrometer at the SNS in Oak Ridge Tennessee has an increased overall flux at low energy transfers up to 4000 times over its predecessors and it has unprecedented sensitivity. I will examine the limits of what is now possible in INS thanks to VISION. From the determination of INS spectra of publishable quality in minutes (for samples in the gram quantity range) [3], measuring the signal of samples in the milligram range to the direct determination of the signal of 2 mmol of C02 adsorbed on functionalized catalysts [4].

Sample environment developments are a crucial part of an effective neutron scattering program, at VISION we have developed the world’s largest single crystal diamond anvil cell and measured the INS spectra of 1 mm3 of a HMB sample. Gas handling experiments are trivial to perform. A sample changer designed for VISION is being built, it is a whole new concept that will allow continuous operation for large number of samples (hundreds at a time) that will enhance the mail-in program for sample measurement. Recently, a simultaneous Raman and INS center-stick has been developed and tested in VISION measuring simultaneously the rotational spectra of hydrogen in the gas, liquid and in the solid state as function of the relative para-ortho hydrogen concentrations. We also have in-situ dielectric spectroscopy capabilities. There is a catalysis cell and gas handling equipment that is currently being built to perform in-situ chemical reactions.

Finally, the major challenges that we are facing will be discussed, in particular methods to automate data analysis and interpretation through computer modelling [5]. We have recently commissioned VirtuES (VIRTUal Experiments in Spectroscopy), March 2016, a computer cluster dedicated to analyse VISION data. We are developing the software to maximize the potential of the technique by generation of automated VDoS, generation of thermodynamic data, creation of databases etc.

References:
[1] Mitchell PCH, Parker SF, Ramirez-Cuesta A, Tomkinson J. Vibrational Spectroscopy with Neutrons, with applications in Chemistry, Biology, Materials Science and Catalysis. London: World Scientific; 2005.
[2] AJ Ramirez-Cuesta, MO Jones, WIF David, Materials Today, 12, 2009, 54-61.
[3] Jalarvo, N., Gourdon, O., Ehlers, G., Tyagi, M., Kumar, S. K., Dobbs, K. D., … Crawford, M. K. (2014). The Journal of Physical Chemistry C, 118(10), 5579–5592. doi:10.1021/jp412228r
[4] T. J. Bandosz, M. Seredych, E. Rodriguez-Castellon, Y. Cheng, L. L. Daemen, and A. J. Ramirez-Cuesta, Carbon 96, 856 (2016).


July 21

Zhiling Dun (UT)

The tripod kagome lattice: a new playground for geometrical frustration

Finding new kagome lattice-containing compounds with spin-type variability has been an experimental challenge for realizing the exotic states predicted theoretically. Recently, we discovered such a new kagome compound family, A2RE3Sb3O14 (A = Mg, Zn; RE = rare-earth element), by partial ion substitution in the pyrochlore. These compounds feature a hitherto unstudied structure, namely the “tripod kagome lattice”. In this talk, I shall demonstrate that due to the unique tripod-like spin anisotropies and a large variability of the rare-earth spin sets, the complex interplay between crystal field splitting and spin-spin interactions in the tripod kagome lattice leads to various exotic states. These include a dipolar spin order, a magnetic charge order, and possible quantum spin liquid states. In addition, the unique geometry of the “tripod” effectively enhances quantum fluctuations for systems with non-Kramers ions, which turns a classical frustrated Ising magnet (i.e. Mg2Ho3Sb3O14) into a quantum kagome ice.
References:
[1] Z. L. Dun, J. Trinh, K. Li, M. Lee, K. W. Chen, R. Baumbach, Y. F. Hu, Y. X. Wang, E. S. Choi, B. S. Shastry, A. P. Ramirez, and H. D. Zhou, Phys. Rev. Lett. 116, 157201 (2016).
[2] Z. L. Dun, J. Trinh, M. Lee, E. S. Choi, K. Li, Y. F. Hu, Y. X. Wang, N. Blanc, A. P. Ramirez, H. D. Zhou, Phys. Rev. B 95, 104439, (2017).


July 28

Ward Plummer, LSU

Surface and bulk properties of BaMnSb2, a topological semimetal with non-trivial Berry phase

Among topological materials, experimental study of topological semimetals that host Dirac or Weyl fermions has just begun, even though these topological concepts were proposed nearly a century ago by Dirac and Weyl. The Dirac/Weyl fermions should be massless with extremely high mobility and non-trivial Berry phase, characteristics that are highly desirable for applications. Our bulk studies show that the magnetic-semimetal BaMnSb2 exhibits nearly zero mass fermions with high mobility and a non-trivial Berry phase. The Shubnikov-de Hass (SdH) oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the non-trivial Berry phase. [1]. What is unique is the magnetic order, ferromagnetic along the ab plane but antiferromagnetically coupled along the c direction, indicting the system should be Weyl type due to time-reversal symmetry breaking. Theory shows that the spin order is very fragile, so it is expected that the application of magnetic field or uniaxial pressure could drive the material to be a type-II Weyl semimetal. The need for high quality samples will be illustrated.
The surface properties were determined using LEED and STM/STS [2], revealing a persistent 2x1 reconstruction and two Ba terminated surface. STS measurements indicate that the surfaces are semiconducting not semimetallic.

Work done in collaboration with Rongying Jin (LSU) and Zheng Gai (CNMS, ORNL).
* Funded by the National Science Foundation.
1) “Non-Trivial Berry Phase in Magnetic BaMnSb2 Semimetal”, Silu Huang, Jisun Kim, W. A. Shelton, E. W. Plummer, Rongying Jin, PNAS (2017).
2) Experiments conducted at CNMS (ORNL) by Zheng Gai, Kun Zhao, and Qiang Zhang.


August 11

Luigi Sangaletti (Università Cattolica del Sacro Cuore))

Interface effects at all-oxide epitaxial heterojunction probed by photoemission spectroscopies

Recent advances in the growth of epitaxial oxide thin films have fostered a steady increase of research on perovskite oxide heterojunctions, which are now produced with unprecedented quality. Applications of these ultra-thin interfaces in the field of electronics, photon harvesting, photovoltaics and photocatalysis strongly rely on the capability to master band gap engineering at the nanoscale. X-ray photoemission spectroscopies (XPS) are playing a key role in the investigation of electronic and structural properties of all-oxide heterointerfaces [1]. Core level and valence band XPS can be combined to probe the band gap alignment [2]. The use of tunable light sources allow to change the in-depth sensitivity, with the possibility to profile the band-gap close to the interface and compare the results with bulk electronic states [3].

Furthermore, angle-resolved XPS spectra can probe the local order around the photoemitting atom, but through suitable modeling these data can also be used to track cation interdiffusion across the interfaces [4]. Finally, the spectral weight enhancement obtained by tuning the photon energy [5], has disclosed unexpected possibilities in the study of band dispersion at buried interfaces. Here, the combination of these techniques is focused on perovskite oxide layers (in particular LaAlO3) grown on SrTiO3, as these systems can host a two dimensional electron gas (2DEG) at the interface and display magnetic ordering and superconductivity effects, disclosing possible applications in the next-generation nanoelectronic devices. Electron spectroscopy results add important details to the physics of these systems, displaying a far richer scenario with respect to the bare electronic reconstruction. In particular, origin and signatures of the 2DEG are discussed in connection with cation interdiffusion and surface cation substoichiometry.

[1] A Giampietri, G Drera, L Sangaletti, Advanced Materials Interfaces, 4 (2017) 1700144
[2] G Drera, G Salvinelli, A Brinkman, et al. PRB B 87 (2013) 075435
[3] G Drera, G Salvinelli, F Bondino, et al., PRB B 90 (2014) 035124
[4] G Salvinelli, G Drera, A Giampietri, L Sangaletti ACS-AMI 7 (2015), 25648
[5] G Drera, F Banfi, FF Canova, P Borghetti, L Sangaletti, et al. APL 98 (2011), 052907


August 18

Jie Ma (Shanghai Jiao Tong University)

Neutron Scattering Study on the Quantum Effect in Ba3CoSb2O9

Ba3CoSb2O9 is a spin-1/2 triangular-lattice antiferromagnet and has attracted a lot of attention in the past decade. Since Co2+ ions form the idea triangle, the Dzyaloshinskii-Moriya effect is absent in the highly symmetric hexagonal lattice, and this compound is recognized as an ideal material to study the interplay between frustration, low-dimensionality, and strong quantum fluctuations. A striking quantum phenomena is the transition from an ambient field non-collinear 120∘ spin structure into a collinear up-up-down (uud) state with applied magnetic field, a magnetization plateau at one-third of its saturation value 𝑀 = 𝑀s/3. We applied neutron scattering techniques to study the non-collinear 120∘ (0T), intermediate state (0T< H <9.8T) and the collinear uud states (9.8T< H <16T), and found that although the spin wave theory couldn’t explain the spin-wave with 120∘ structure, the uud phase was simulated quantitatively, which indicated an intrinsic quantum mechanical origin for anomalous zero-field spin dynamics.


August 25

Jian Shen (Fudan University)

Towards all-in-one spintronics: Manipulating electronic phase separation in complex oxides

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. Investigation of EPS phenomena is not only important for understanding the strong electronic correlations in these materials, but also very useful for tuning their physical properties. Most work in studying EPS has focused on observing EPS and understanding its formation mechanism. The natural appearance of EPS domains, as expected, is totally random in terms of their size and spatial distribution. In order to control the EPS and thus the physical properties of the complex oxides, in recent years we have developed several methods to control the shape, density, location, size, spatial distribution and even the very existence of the EPS domains in manganites. As a result, we are able to order array the EPS domains and finely tune the corresponding physical properties. It is hoped that these abilities will allow us to soon design spintronic devices by patterning EPS domains in one material. These new spintronic devices do not have chemical interfaces and thus are expected to have high spin transport efficiency.


September 1

Lin Hao (UT)

Two-dimensional Jeff=1/2 Antiferromagnetism unraveled from interlayer coupling and controlled under external magnetic field

A two-dimensional (2D) lattice formed of IrO6 octahedra emerged as a novel playground for some of the most outstanding and challenging problems in condensed matter physics, such as metal-insulator transition and quntum magnetism. A notable example is the confined 2D SrIrO3 perovskite layers in iridate Ruddlesden-Popper (RP) phases, in which dimensionality, structure, effective electron-electron correlation and spin-orbit coupling entangle each other and lead to a rich phase diagram. The investigation, unfortunately, has been hindered due to limitation of available bulk compounds with rigid layering structures. Experimentally, epitaxial atomic layering may enable more structural tunabilities and offer remarkable opportunity to fully understand the complex diagram and shed light on the hidden physics. In this talk, I will show an experimental investigation of the 2D Jeff = 1/2 antiferromagnetism by artificially varying the interlayer exchange coupling in superlattices [1]. Both effective electron-electron correlation and magnetic ordering display dimensionality-dependence unobtainable in bulk RP phase. Resonant x-ray magnetic scattering revealed a switchable sign of the interlayer exchange coupling locked to the octahedral rotation pattern. With diminishing interlayer coupling, the results show realization of a 2D antiferromagnet at finite temperatures stabilized by spin anisotropy. The 2D antiferromagnetic order stability shows a dramatic increase under magnetic field effect due to a hidden symmetry. These findings demonstrate a powerful route to discover and realize novel 2D quantum magnets by heteroepitaxial engineering.

[1] L. Hao, D. Meyers, C. Frederick, G. Fabbris, J. Yang, N. Traynor, L. Horak, D. Kriegner, Y. Choi, J.-W. Kim, D. Haskel, P.J. Ryan, M.P.M. Dean, J. Liu, Phys. Rev. Lett. 119, 027204 (2017).


September 8

Phil Pincus (UT)

Screening in concentrated electrolyte solutions

Recent surface force experiments suggest that the Debye screening length in aqueous solutions is not monotonic in electrolyte concentration. I shall review the fundamentals of Debye-Huckel theory and discuss some possible scenarios to understand the experimental observations.


September 22

Hidemaro Suwa (UT)

Enhanced controllability at proximity of hidden SU(2) symmetry in quasi two dimensions

Continuous symmetries cannot be spontaneously broken at finite temperature in pure two-dimensional systems with neighboring interactions. For example, the isotropic Heisenberg spins with the SU(2) symmetry can exhibit a long-range order only at zero temperature. The susceptibility of ordering magnetization exponentially increases toward zero temperature. As a result, small perturbation, such as an interlayer coupling, anisotropy, and external magnetic field, to the isotropic spins in two dimensions is able to trigger enhanced response, which provides a great opportunity for controllable devices.

In this talk, I will first explain the critical phenomena in quasi-two-dimensional systems. The transition or crossover temperature logarithmically increases as a function of a perturbative term, showing an infinite slope. Then I apply this theory to the confined two-dimensional SrIrO3 perovskite layers consisting of IrO6 octahedra, where the low-energy physics is described by Jeff=1/2 antiferromagnets. In spite of the existence of significant spin-orbit coupling which produces the anisotropic and Dzyaloshinskii-Moriya interactions, remarkably, the system has a hidden SU(2) symmetry. Tiny uniform magnetic field drastically increases the crossover temperature to the ordered state. This material shows an excellent figure of merit in the combination of the quasi-two-dimensional physics and the hidden SU(2) symmetry.


September 29

Fangfei Ming (UT)

Realization of a hole-doped Mott insulator on a triangular silicon lattice

The physics of doped Mott insulators is at the heart of some of the most exotic physical phenomena in materials research including insulator-metal transitions, colossal magneto-resistance, and high-temperature superconductivity in layered perovskite compounds. These phenomena often emerge as a function of carrier doping and are rooted in the strongly correlated motion of the charge carriers and their coupling to lattice and magnetic excitations of the crystal. Advances in this field would greatly benefit from the availability of new material systems with similar richness of physical phenomena, ideally those that are less complex in structure and composition, and highly ordered. Here we show that such a system can be realized on a silicon platform. Adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half-filled dangling bond orbitals. Modulation hole-doping of these dangling bonds unveils clear hallmarks of Mott physics, such as spectral weight transfer and the formation of quasi-particle states at the Fermi level, well-defined Fermi contour segments, and a sharp singularity in the density of states. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials. It suggests that exotic quantum matter phases can be housed and engineered on silicon-based materials platforms.


anjana samarakoon
October 6

Anjana Samarakoon (University of Virginia)

Comprehensive study of the dynamics of a classical Kitaev spin liquid

We study the spin- S Kitaev model in the classical (S→∞) limit using Monte Carlo simulations combined with semi-classical spin dynamics. We discuss differences and similarities in the dynamical structure factors of the spin-1/2 and the classical Kitaev liquids. Interestingly, the low-temperature and low-energy spectrum of the classical model exhibits a finite energy peak, which is the precursor of the one produced by the Majorana modes of the S = 1/2 model. The classical peak is spectrally narrowed compared to the quantum result, and can be explained by magnon excitations within fluctuating one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of magnons propagating in one-dimensional manifolds. Moreover, we show that the momentum space distribution of the low-energy spectral weight of the S = 1/2 model follows the momentum space distribution of zero modes of the classical model.


tao hong
October 13

Tao Hong (Quantum Condensed Matter Division, ORNL)

Exotic spin dynamics in a two-dimensional quantum antiferromagnet near the quantum critical point

Inelastic neutron scattering (INS) is a powerful tool to probe the exotic quantum phenomena in low-dimensional quantum spin systems. In this talk, I will first introduce a novel spin-1/2 two-dimensional coupled ladder antiferromagnet (dimethylammonium) (3,5-dimethylpyridinium) CuBr4 (C9H18N2CuBr4), abbreviated as DLCB, near the quantum critical point at zero field and pressure [1, 2]. After that, I will present the recent INS studies on DLCB. In conjunction with theoretical calculations, our results show the exotic quantum effects in spin dynamics including evidences of the field-induced spontaneous (T=0 K) magnon decay in an applied transverse magnetic field [3] and observation of the Higgs amplitude mode [4], which is characterized by fluctuation of amplitude of the order parameter and not predicted by the linear spin-wave theory. Our work provides much-needed experimental insights to the understanding of these quantum many-body effects in low-dimensional antiferromagnets.

[1] F. Awwadi et al., Inorg. Chem. 47, 9327 (2008).
[2] T. Hong et al., Phys. Rev. B 89, 174432 (2014).
[3] T. Hong et al., Nat. Commun. 8, 15148 (2017).
[4] T. Hong et al., Nat. Phys. 13, 638 (2017).


Niravkumar D. Patel
October 20

Niravkumar D. Patel (UT)

Magnetic and pairing tendencies in quasi 1D multi-orbital models

The recent discovery of superconductivity under high pressure in the two-leg ladder compound BaFe2S3 opens a broad avenue of research because it represents the first report of pairing tendencies in a quasi-one-dimensional iron-based high-critical-temperature superconductor. Similarly, as in the case of the cuprates, ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts, particularly if several orbitals are active. As a first step, we derive a two-orbital Hubbard model from first principles that describes individual ladders of BaFe2S3. The model is then studied with the density matrix renormalization group technique. Three main results are found: (i) at half-filling, ferromagnetic order emerges as the dominant magnetic pattern along the rungs of the ladder, and antiferromagnetic order along the legs, in excellent agreement with neutron experiments; (ii) with hole doping, pairs form in the intermediate/strong coupling regime, as found by studying the binding energy of two holes doped on the half-filled system; (iii) projector analysis of the ground state show that same orbital rung and diagonal pairs are most probable. In addition, recently pairing tendencies were also found by our group in a 1D chain of two-orbitals (that can be mapped into a single-orbital two-leg ladder). The analysis of pair-pair correlations show that pairs are formed involving inter-orbital singlets on neighboring sites and different orbitals. In addition, pairing tendencies can be enhanced by increasing the Hund coupling and also by adding an inter-orbital nearest-neighbor antiferromagnetic Heisenberg coupling. These results clearly suggest that magnetic fluctuations are crucial to superconductivity in the iron-based ladder superconductors. These exciting new results pave the way for our understanding of pairing in the iron family of high-Tc materials.


Michel Gingras
October 27

Michel Gingras (University of Waterloo and Canadian Institute for Advanced Research)

Rare-Earth Pyrochlore Oxides – a Fascinating Playground for the Experimental and Theoretical Study of Frustrated Magnetism in Three Dimensions

Frustration is a ubiquitous phenomenon in condensed matter physics, and in science in general. One can even read about it on Wikipedia: http://en.wikipedia.org/wiki/Geometrically_frustrated_magnet. In simple terms, frustration arises when a system cannot, due to competing interactions, minimize its total classical ground state energy by minimizing the energy between interacting degrees of freedom, pair by pair. There has been in recent years an explosion of experimental and theoretical activities directed at the study of geometrically frustrated magnetic systems. The motivation stems from the hope that highly frustrated magnetic systems may give rise to exotic quantum and classical phenomena. Experimentally, materials where magnetic moments (spins) reside on a three-dimensional pyrochlore lattice of corner-sharing tetrahedra have proven to be the host of unusual and intriguing behaviors induced by frustration. Examples include the spin ice phenomenology, order-by-disorder, persistent spin dynamics down to absolute zero temperature and, potentially, quantum spin liquid in so-called quantum spin ice materials. In this talk, I will review some of the exotic phenomena that have been uncovered in rare-earth magnetic pyrochlore oxide materials over the past twenty years, some of them now well-understood, others less or not much at all.


Paige Kelley
November 3

Paige Kelley (UT/ORNL)

Suppression of Magnetic Order Near the Kitaev Quantum Spin Liquid

The quasi-2D honeycomb magnetic insulator α-RuCl3 has recently been investigated as a candidate to host the quantum spin liquid ground state of the 2D Kitaev model. However, fragile long-range magnetic order arises in α-RuCl3 from non-Kitaev terms in the Hamiltonian. In this talk I will describe two approaches to suppressing the zigzag antiferromagnetic order in α-RuCl3: the application of a magnetic field in the honeycomb plane, and the introduction of spin vacancies via chemical substitution. Inelastic neutron scattering in these magnetically disordered limits will be discussed.


November 10

Yi Zhang (LSU)

Localization in Energy Materials

Disorder is ubiquitous in materials and can drastically affect their properties. In particular it can induce localization of electrons and lead to a metal-insulator transition, which is known as the Anderson localization transition. Anderson localization is not only interesting from a scientific point of view, but also has been proposed to play a critical role in materials that can be used for the harvesting and efficient use of energy. Combining two recently developed methods, the effective disorder Hamiltonian method[1] and the typical medium dynamical cluster approximation[2], has opened the door to investigate localization in such energy materials with first principles simulations. In this talk, I will present our recent studies of localization in the Fe based superconductor KxFe2-ySe2[3], the diluted magnetic semi-conductor Ga(Mn,N)[4] and the intermediated-band photovoltaic Ti doped Si.

[1] T. Berlijn, D. Volja, and W. Ku PRL 106, 077005 (2011)
[2] C. E. Ekuma, H. Terletska, K.-M. Tam, Z.-Y. Meng, J. Moreno, and M. Jarrell, PRB 89, 081107 (2014)
[3] Y. Zhang, H. Terletska, C. Moore, C. Ekuma, K.-M. Tam, T. Berlijn, W. Ku, J. Moreno, and M. Jarrell, PRB 92, 205111 (2015)
[4] Y. Zhang, R. Nelson, E. Siddiqui, K-M Tam, U. Yu, T. Berlijn, W. Ku, NS Vidhyadhiraja, J. Moreno, and M Jarrell, PRB 94, 224208 (2016)


December 1

Jacek Herbych (UT)

Quantum distillation: how to make cold atoms cold

In recent years, experiments focusing on the nonequilibrium transport properties of ultra-cold atomic gases in optical lattices were pushed forward, ranging from the few-body to the many-body regime [1-3]. Not surprisingly, many unusual and sometimes counter intuitive phenomena exist in the transient dynamics of nonequilibrium problems, such as prethermalization, the dynamical quasi-condensation of hard-core bosons, or the quantum distillation mechanism. In parallel, theoretical understanding [4] of the nonequilibrium transport properties of strongly interacting systems in low dimensions has become a very active field of research.

Although the optical lattices have emerged as ideal simulators for the Hubbard model, there is one clear disadvantage of such a setup: the ENERGY SCALE [5]. The energy scale of solids is typically of the order of kelvin, where in atomic system it is nanokelvin. This minuscule temperature scale makes it difficult to address exotic phases, such as d-wave superconductivity in the high-temperature superconducting cuprates.

In my presentation, I will focus on one aspect of transient nonequilibrium mass transport, namely the quantum distillation [6-7] in a system of interacting fermions. Quantum distillation is the dynamical spatial separation of the lattice gas into one portion that carries predominantly interaction energy and another one that carries mostly kinetic energy. Such phenomenon was proposed as a possible cooling mechanism for, both, bosons and fermions. Dynamical filtering leads to the formation of low entropy (and therefore low temperature) regions consisting of doubly occupied sites.

I present a numerical investigation of the quantum distillation process within one- and quasi-one-dimensional lattice geometries. As a main result, we show that this phenomenon is not limited to the chains that were previously studied [6]. Interestingly, there are additional dynamical processes on the two-leg ladder such as density oscillations and self-trapping of defects that lead to a less efficient distillation process. An investigation of the time evolution starting from product states provides an explanation for this behavior. Initial product states are also considered, since in optical lattice experiments such states are often used as the initial setup. I propose configurations that lead to a fast and efficient quantum distillation.

[1] I. Bloch, Nature Physics 1, 23 (2005).
[2] I. Bloch, J. Dalibard, and S. Nascimbène, Nature Physics 8, 267 (2012).
[3] I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys. 80, 885 (2008).
[4] A. Polkovnikov, K. Sengupta, A. Silva, and M. Vengalattore, Rev. Mod. Phys. 83, 863 (2011).
[5] D. C. McKay and B. DeMarco, Rep. Prog. Phys. 74, 054401 (2011).
[6] F. Heidrich-Meisner, S. R. Manmana, M. Rigol, A. Muramatsu, A. E. Feiguin, and E. Dagotto, Phys. Rev. A 80, 041603 (2009).
[7] J. Herbrych, A. E. Feiguin, E. Dagotto, and F. Heidrich-Meisner, arXiv: cond-mat/1707.01792 (2017).


shang-shun zhang
December 8

Shang-Shun Zhang (UT)

Paring from strong repulsion in triangular lattice Hubbard model

The generation of an effective attraction between electrons out of the bare Coulomb repulsion is a long sought-after goal of the condensed matter community. In this talk I will describe a paring mechanism between holes in the dilute limit of doped frustrated Mott insulators. We will see that magnons provide a strong glue in the infinitely repulsive limit of triangular lattice Hubbard model. The strongly attractive hole-magnon interaction is a manifestation of a “counter-Nagaoka” theorem: the single-hole kinetic energy is minimized for an antiferromagnetically ordered state. We will show that the resulting hole-magnon glue is strong enough to bind a second hole and form a hole-hole-magnon three-body bound state. Remarkably, the binding energy of this “composite Cooper pair” is rather strong, while its effective mass still has a moderate value, giving rise to relatively high transition temperature for superconductivity. We will discuss possible experimental realizations of this new mechanism.