While ferromagnets have been known and exploited for millennia, antiferromagnets were only discovered in the 1930s. The elusive nature indicates antiferromagnets’ unique properties: At large scale, due to the absence of global magnetization, antiferromagnets may appear to behave like any non-magnetic material; At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. We study this exciting possibility in an antiferromagnetic Axion insulator, even-layered MnBi2Te4 flakes. We report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions.

Reference:

A. Gao, et al.  “Layer Hall effect in a 2D topological axion antiferromagnet.” Nature 595, 521 (2021).

In one spatial dimension, enhanced thermal and quantum fluctuations should preclude the existence of any long range ordered superfluid phase of matter.  Instead, the quantum liquid should be described at low energies by an emergent hydrodynamic framework known as Tomonaga-Luttinger liquid theory.  In this talk I will present details on some complimentary experimental and theoretical searches for this behavior in helium-4 including: (1) pressure driven superflow through nanopores, and (2) the excitation spectrum of a confined superfluid inside nano-engineered porous silica-based structures. For flow experiments, we have devised a framework that is able to quantitatively describe dissipation at the nanoscale leading to predictions for the critical velocity borne out by recent superflow measurements in nanopores.  In confined porous media, with radii reduced via pre-plating with rare gases, I will discuss ab initio simulations of phase and density correlations inside the pore that are in agreement with recent neutron scattering measurements.   Taken together, these results indicate significant progress towards the experimental observation of a truly one-dimensional quantum liquid.

This work was supported by the NSF through grants DMR-1809027 and DMR-1808440.  

Materials manipulation via ion or laser beams can achieve precisely tuned atomic geometries that are necessary, e.g. to engineer interactions between defects in quantum materials and for fabricating novel electronic devices with nanoscale dimensions. In addition, such beams are also used to characterize and probe materials properties by means of electronic and optical excitations. I will discuss recent quantum- mechanical first-principles predictions for electron dynamics and the subsequent ionic motion that follows after an excitation of the electronic system. Using real-time time-dependent density functional theory we simulated the underlying ultrafast time scales of electron dynamics in semiconductors and metals. Examples include long-lived electronic excitations in proton, electron, and laser irradiated bulk semiconductors that facilitate diffusion of point defects, such as oxygen vacancies in MgO. We compare such bulk simulations to aluminum surfaces under irradiation, for which we quantify electron emission, charge capture, and pre-equilibrium effects that are unique to thin films or two-dimensional materials. Limitations and possible extensions of the theoretical description will be included in the discussion.

Spintronics-based technology, which uses spins to represent and propagate information, holds promise to realize devices that surpass the current CMOS transistor technology in power, density and speed. For example, magnetic random-access memory (MRAM) based on magnetic tunnel junctions were identified as promising non-volatile memory but its use has been limited. A second generation MRAM-based on spin transfer torque has reduced currents. However, next generation MRAM based on pure spin currents may provide even more energy efficiency. My research is focused on developing power-efficient ways to generate, propagate and manipulate spins via pure spin currents. In order to develop such pure spin current technologies, the development of new materials such as topological insulators must come hand in hand with the development of new devices. In this talk, I will discuss (i) low damping ferromagnetic insulating thin films for achieving efficient spin current generation in spintronic devices, (ii) spin current generation in these films and large spin-charge interconversion in neighboring layers, (iii) spin interactions in ferromagnetic insulator/topological insulator heterostructures. Together these results lay the foundation for new energy-efficient pure spin current-based electronics.

Reference:

1. Li, P. et al. Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Lett. 21, 1, 84 (2021).

2. Li, P. et al. Switching magnetization utilizing topological surface state. Science Advances 5, eaaw3415 (2019).

In this talk, I will discuss rich topological behavior in two related models – the Majorana wire and a Su-Schrieffer-Heeger ladder- in the presence of potential energy landscapes. An introduction of the two models and of techniques that directly provide information on edge-state properties will form the starting point for obtaining topological phase diagrams in these models. In the case of these systems subject to a quasiperiodic potential, a beautiful topological phase diagram emerges mimicking Hofstadter’s butterfly patterns. In the case of disordered potential landscapes, Anderson localization physics informs the behavior of the disordered topological phase diagram.  Finally, I will discuss the possible implementation of this physics in a variety of experimental systems, including solid state, cold atomic and electro-mechanical settings. 

Amorphous oxide semiconductors (AOS)—ternary or quaternary oxides of post-transition metals—have attracted a lot of attention due to high carrier mobility which is an order of magnitude larger than that of amorphous silicon (a-Si:H). Unlike Si-based semiconductors, AOS exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. However, the properties of AOS are extremely sensitive to deposition conditions, oxygen stoichiometry, and metal composition, rendering the available research data inconsistent or hard to reproduce, thus, hampering further progress. Moreover, owing to the weak metal-oxygen bonding as well as many degrees of freedom in disordered materials, defects in AOS have the structural, thermal, and electronic characteristics that differ fundamentally from those in the crystalline transparent conducting oxides.

To navigate the large parameter space for AOS materials, computationally-intensive ab-initio Molecular Dynamics simulations followed by comprehensive structural analysis and accurate Density-Functional calculations, are performed for several AOS families. Integrated with systematic experimental measurements, the results provide microscopic understanding of complex relationships between the morphology, carrier generation, and electron transport across the crystalline-amorphous transition and help derive versatile design principles for next-generation transparent amorphous semiconductors with a combination of properties not achievable in Si-based architectures.

Magnetic tweezers allow the user to apply force and torque to magnetic beads attached to single DNA molecules, and to observe the resulting changes in DNA extension. This technique, however, is limited to measuring a single degree of freedom: the distance between the magnetic bead and the microscope slide surface. Total internal reflection fluorescence microscopy enables visualization of single molecules that have been tagged with fluorescent dyes. By combining TIRF microscopy and magnetic tweezers, we can simultaneously manipulate DNA molecules and use fluorescence to detect additional parameters, such as the presence of a protein or orthogonal changes in the DNA structure. We have recently installed a custom MT-TIRF instrument. In this talk I will discuss the instrument design, the physics underlying the two techniques, and how we plan to utilize them together to extract more information from our systems of interest.

I will describe how the geometry of the band structure of metals manifests itself in their optical and transport properties. I particular, I will show that the natural optical activity of metals, equivalent to the so-called dynamic chiral magnetic effect, stems from the intrinsic magnetic moments of quasiparticles, and demonstrate that these magnetic moments can be of both intrinsic and extrinsic origin. I will then discuss optical Hall response of chiral crystals in the presence of a DC transport current – the gyrotropic Hall effect – and show that it is related to the Berry curvature dipole. The latter fact makes the gyrotropic Hall effect a diagnostic tool for topological properties of three-dimensional chiral metals. If time permits, I will discuss how to observe the chiral magnetic effect in Weyl semimetals using the heating effect of a transport electric field.

High harmonic generation (HHG) is an extreme non-linear phenomenon where strong laser-field pulses interact with a medium to produce coherent and high-frequency harmonics of the incident light.  Since its first observation in solids in 2011, pioneering theoretical studies have clarified some of the details of the microscopic mechanism behind this phenomenon, like the role of intra- and inter-band transitions, the contribution of the transition dipole moments to the even and odd harmonic peaks, effects of broken symmetry, etc. However, the role of electron correlation effects in the HHG in strongly correlated materials is much less understood. This talk will discuss the role of these effects in the high-harmonic (HH) spectra of solids, using time-dependent density-functional theory and dynamical mean-field theory, for the examples of semiconductor ZnO, perovskites BaTiO3 and BiFeO3 and transition-metal oxide VO2. It is found that correlation effects significantly modify the HH spectrum of all systems, in particular through the ultrafast modification of the electronic spectrum in ZnO. In the case of BaTiO3, correlation effects generate "super-harmonics" – periodic enhancements and suppressions of specific harmonic orders that depend on the correlation strength. Memory effects in HHG were found to lead to a further extension of the harmonic cutoff. For the HH spectrum of VO2,  we find correlation-induced higher harmonics, in good agreement with experimental data. The obtained results shed light on the role of electron correlations in the HH spectrum in complex materials and may help pave the way for future advancements in the field of ultrafast science and attosecond physics.

Quantum materials such as topological, 2D and nanostructured materials have attracted tremendous attention due to their exotic quantum properties. In this presentation, we will talk about our findings of chemical doping effect in inducing magnetism and superconductivity in chalcogenide compounds. Several interesting physical properties have been observed such as the anomalous Hall effect in Cr-doped Sb2Te3, metamagnetic behavior in Fe-doped Bi2Se3, and the coexistence of ferromagnetism and superconductivity in Nb-doped Bi2Se3 single crystals. We will also present results of the synthesis and characterization of superconducting TaS2 nanowires. Our approach includes the synthesis of 1D charge-density-wave (CDW) TaS3 nanostructure precursors followed by the nondestructive and controlled adjustment of the S composition. TaS3 nanowires show the canonical CDW behavior, but the converted TaS2 nanowires show superconductivity and vortex avalanche behaviors at temperatures below its Tc ~ 3.8 K, which is about three times higher than that of 2D bulk TaS2 crystals. Physical properties of polycrystalline Zn1-xCrxTe samples will also be presented. The samples show metallicity and ferromagnetic behavior for higher Cr doping concentration. Furthermore, optical transparency in the visible light range of these polycrystalline ferromagnetic Zn1-xCrxTe was found to be 40% - 85% for Cr doping concentration up to x = 0.18.