Department of Physics and Astronomy, The University of Tennessee, Knoxville
I will introduce an extension of the Kitaev honeycomb model by including four-spin interactions that preserve the local gauge structure and hence the integrability of the original model. This extended model emerges naturally from generic time reversal invariant perturbations to the Kitaev honeycomb model. We will see that the model has a rich phase diagram containing five distinct vison crystals, as well as a symmetric π-flux spin liquid with a Fermi surface of Majorana fermions and a sequence of Lifshitz transitions. I will also discuss possible experimental signatures and, in particular, present finite-temperature Monte Carlo calculations of the specific heat and the static vison structure factor. Finally, we will see how different topologically ordered Z2 quantum spin liquids with abelian and non-abelian anyons emerge naturally from this model, complementing the liquids with Chern numbers equal to 0, 1 and -1 that appear in the Kitaev honeycomb model.
The t-J model has been argued to be of fundamental importance for understanding strongly correlated matter, including the high Tc superconductors. To tackle the difficulties of strong coupling and non-canonical nature in this model, we have recently developed the extremely correlated Fermi liquid (ECFL) theory, benchmarking with dynamical mean field theory in infinite dimensions. In this talk, I will present the application of the ECFL theory into studying the t-t’-J model in one dimension (1-d) combining with density matrix renormalization group, and two dimensions (2-d). In 1-d, the DMRG method gives essentially exact numerical results, which are shown to compare quite well with the ECFL method. I will present the strong momentum dependence of self-energy and clear signature of spin-charge separation in spectral function from both methods. In 2-d, I will show the doping and next neighbor hopping dependent quasi-particle weight, the high thermal sensitivity of spectral function, temperature and doping dependence of DC resistivity, optical conductivity and non-resonant Raman susceptibilities in three geometries. These results are exciting because they are consistent to experimental findings on correlated material.
Complex oxides have gotten lots of attentions due to diverse electronic/magnetic phases and their tunability by external stimuli such as strain, magnetic fields, electric fields, etc. In addition, mixed ionic and electronic conducting behaviors in complex oxides have been applied for many energy devices such as solid oxide fuel cells and electrochemical sensors, where redox reactions and catalytic activity at the interfaces of gas-solid and solid-liquid play critical roles for the performance. The primary purpose of this presentation is to address ion-exchange behaviors that can be used for controlling phase in epitaxial complex oxides. As a model system, we chose oxygen sponge SrFe1-xCoxO3-δ (x = 0.2 and 0.5 in this work) grown by pulsed laser deposition. Example 1 is reversible redox reaction in SrFei0.8Co0.2O3-δ (0 ≤ δ ≤ 0.5) in ambient pressure, which is a typical condition for operating energy devices. Using real-time x-ray reflectivity and diffraction techniques, we probed reversible structural phase transitions including cycling experiments. From spectroscopic methods, we confirmed the transitions lead changes in optical conductivity and valence state of transition metals. Example 2 is to show oxygen tug-of-war at the oxide interfaces. By capping different oxides such as a hole-doped manganite and a titanate on SrFe0.5Co0.5O2.5, we observed either oxygen absorption or desorption in SrFe0.5Co0.5O2.5. In case of the absorption, we confirmed ferromagnetism, which is not from the capping materials. Example 3 is about selective reduction in SrFe0.5Co0.5O2.5. We observed drastic change in optical conductivity spectra and stabilization of unique divalent cobalt ions. We will also address the role of iron ions.
Understanding the properties of strongly correlated models is a challenging problem since many analytical tools cannot be applied due to the complicated nature of the problems. Here, we apply quantum Monte Carlo techniques in order to tackle one of the most important and unknown phenomena of many materials: the mechanism of superconductivity. We study the Hubbard-Holstein model, which is one of the most simple theoretical models including strong correlation and electron-phonon coupling. By using twist-averaging boundary conditions we eliminate finite-size errors in the Monte Carlo simulations. With this useful tool, we first solve the Hubbard-Holstein model by variational Monte Carlo by reporting the phase diagram of the model for different phonon frequencies at half-filling. Then we investigate the phase separation away from half-filling. Finally, we try to attack the problem with an essentially unbiased method based on the auxiliary field quantum Monte Carlo technique and accelerated Langevin dynamics. By curing the sign problem via Cauchy integration in the complex plane where the auxiliary fields have been defined, we report the effect of the electron-phonon coupling on some observables such as magnetization, spin and charge structure factors. We show preliminary results that are already meaningful to understand the nature of the transition between magnetism and charge order in the model at half-filling.
We will review recent inelastic experiments in the triangular lattice S = 1/2 Heisenberg antiferromagnet, Ba3CoSb2O9 [1-5], revealing large deviations from the dynamical spin structure factor S(q,ω) obtained from non-linear spin wave theory (NSWT). We will see that, while NSWT works very well inside the magnetic field induced magnetization plateau (up-up-down phase) [5], it fails at zero magnetic field (120-degree ordering). This observation strongly suggests that the failure of a semiclassical treatment is due to strong quantum fluctuations, which are indeed expected for frustrated 2D antiferromagnets.
In an attempt of finding alternative ways of modelling the S(q,ω) of ordered low dimensional antiferromagnets, we will derive the zero temperature S(q,ω) of the triangular lattice Heisenberg model using a Schwinger Boson approach that includes the Gaussian fluctuations (1/N correction) around the saddle point solution [5]. While the ground state of this model exhibits a well-known 120-degree magnetic ordering, the excitation spectrum revealed by S(q,ω) has a strong quantum character, which is not captured by low-order 1/S expansions. The low-energy magnons consist of two-spinon bound states confined by the gauge fluctuations of the auxiliary fields. This composite nature of the single-magnon modes is accompanied by a continuum of high-energy spinon modes, which extends up to three times the single-magnon bandwidth.
The synthesis of films and heterostructures with atomic precision has revealed numerous new and unexpected phenomena. The near order of magnitude enhancement of the superconducting properties of FeSe in the ultrathin 2D limit is an example of such a surprise. In this talk I will discuss the latest efforts in combining chalcogenide and oxide molecular beam epitaxy (MBE) with in situ angle resolved photoemission spectroscopy (ARPES) to determine the origins of the Tc enhancement. Through deposition of FeSe on different oxide materials and interrogation of the electronic structure, we show how charge transfer and cross-interface electron-phonon coupling cooperatively boost the superconducting properties to record breaking Tc's for this class of materials. With photon energy dependent ARPES measurements we confirm the signatures of cross-interface electron-phonon coupling within the data are indeed intrinsic to the underlying physics of sample. By growing different oxides with specific surface terminations, we learn how to control both the charge transfer to the monolayer FeSe and the strength of the electron phonon coupling, further confirming their relation to Tc. These results not only advance our understanding of the enhanced superconducting properties but show the power of combining film growth and characterization into a multi-modal playground for investigating quantum materials.
Magnetism is a fascinating phenomenon: it is rooted in relativistic quantum mechanics and yet an integral component of the technologies we use every day. In magnetic insulators, where atomic-scale magnetic dipoles carried by electrons are closely bound to a crystal lattice, novel phases of matter with no classical analogues are possible. Chief among these phases are spin-liquids, in which strong fluctuations of magnetic dipoles preclude conventional magnetic order even for temperatures low compared to the average interaction between spins. Such exotic magnetic matter is of great fundamental interest because it features a wealth of coherence and entanglement phenomena – the hallmarks of the quantum world – and is often amenable to theoretical and computational predictions. In this talk, I will present experimental research that brings together materials chemistry, neutron scattering and computer modeling to understand the magnetic excitations in a range of frustrated oxide compounds with kagome and pyrochlore lattice structures. My talk will emphasize the importance of neutron scattering instrumentation to probe complex materials behavior in which chemical disorder, geometrical frustration and quantum fluctuations interplay to stabilize – or destroy – spin-liquid physics. In particular, I will provide several examples in which the presence of a continuous excitation spectrum, usually associated with the existence of fractional excitations out of a spin-liquid ground-state, is misleading and can in fact be explained without invoking fractionalization. Supported by the U.S. Department of Energy under award DE-SC-0018660 and the National Science Foundation under award NSF-DMR-1750186.
Two-dimensional melting is one of the most fascinating and poorly understood phase transitions in nature. Theoretical investigations often point to a two-step melting scenario involving unbinding of topological defects at two distinct temperatures. Here we report on a novel melting transition of a charge-ordered K-Sn alloy monolayer on a silicon substrate. Melting starts with short-range positional fluctuations in the K sublattice while maintaining long-range order, followed by longer-range K diffusion over small domains, and ultimately resulting in a molten sublattice. Concomitantly, the charge-order of the Sn host lattice collapses in a multi-step process with both displacive and order-disorder transition characteristics. Our combined experimental and theoretical analysis provides a rare insight into the atomistic processes of a multi-step melting transition of a two-dimensional materials system.
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In recent years, iridates have been recognized as an ideal material platform where the emergent many-body phenomena mesh with AF functionalities. As the prototype, Sr2IrO4 is a quasi-two-dimensional AF Mott insulator of pseudospin-half electrons that exhibits remarkable phenomenological analogy to the high-Tc cuprates. Meanwhile, significant magnetic responses of the AF order in both bulk and thin film samples have been demonstrated, including metamagnetism, magnetoresistance (MR), and anisotropic magnetoresistance (AMR). The high controllability of the AF order is largely related to the unique electronic structure that the strong spin-orbit coupling (SOC) of Ir stabilizes the pseudospin-half Kramer doublet and hence makes no contribution to the magnetic anisotropy. In this study, we directly engage with the pseudo Jahn-Teller distortion and drive the AF order of Sr2IrO4 by applying continuously tunable uniaxial strain. The application of uniaxial strain is seen to modulate the pseudospin-lattice coupling and control the stability of different AF spin structures, leading to highly efficient magnetoelastic control of the Jeff = 1/2 AF order and the associated electronic properties.
Perovskite oxide heterostructures are well known for hosting emergent properties, which are absent in the bulk of their constituents, owing to the interplay between various broken symmetries at the interface. I shall talk about the spin-chirality-driven Hall effect, known as the topological Hall effect, at the interface between manganite (La1−xSrxMnO3) and iridate (SrIrO3) compounds. Broken structural inversion symmetry at the interface and strong spin-orbit coupling of the iridate produces a sizeable Dzyaloshinskii-Moriya interaction (DMI). Monte Carlo calculations reveal a Skyrmion crystal (SkX) phase that is stabilized within a range of applied magnetic fields, due to an interplay between the ferromagnetic double-exchange hopping at the manganite layer, the DMI and the external field. A phase diagram, in the temperature versus magnetic field plane, shows that the topological Hall response is significant in the SkX phase at low temperatures. Reference: N. Mohanta, E. Dagotto, and S. Okamoto, Phys. Rev. B 100, 064429 (2019).
The high-Tc pairing mechanism remains an important outstanding problem in condensed matter physics, and the dome-like dependence of the superconducting transition temperature Tc on the charge carrier concentration holds important clues about its origin. Here I will discuss recent dynamic cluster quantum Monte Carlo calculations of single- and multi-band Hubbard models with varying degrees of hole concentration corresponding to weakly doped to highly overdoped. Specifically, I will discuss what one can learn from these calculations regarding the underlying physics that gives rise to the decrease of Tc at the under- and overdoped ends of the Tc dome, as well as the appearance of a second Tc dome in the extremely overdoped cuprates as Cu2+ varies to Cu3+.
Abstract Forthcoming.