Session F1: Superconducting and Strongly Correlated Materials

Features of Charge Density Wave Energy Gap and Electronic Dispersion of $2H-\textrm{TaS}_2$ and Other Layered Transition Metal Dichalcogenides.

We present Angle Resolved Photoemission Spectroscopy (ARPES) study of $2H-\textrm{TaS}_2$, a canonical incommensurate Charge Density Wave (CDW) material, and compare to similar layered transition metal dichalcogenides such as $2H-\textrm{NbSe}_2$. Similarities were observed in the preferential appearance of CDW energy gap in the vicinity of some specific high symmetry points on the Fermi surface, the particle-hole asymmetry of this gap and the existence of the pseudogap behavior above CDW transition temperature. However, in contrast to $2H-\textrm{NbSe}_2$, where the gap opens up only in specific momentum locations in the vicinity of high symmetry points, we observed a non-zero gap for all momentum locations for the case of $2H-\textrm{TaS}_2$. Since the conventional Fermi surface nesting model was unable to explain these similarities and differences, we use a tight binding model with emphasis on orbital selectivity and strong electron-phonon coupling. Further analysis of the electronic dispersion showed pronounced many body renormalization in the system and revealed the phononic mechanism behind momentum and temperature dependent features of the band structure. We suggest that this model can be generalized for a broader class of incommensurate charge density wave materials.   

Identifying a forward scattering superconductor through pump-probe spectroscopy

Understanding the mechanism behind high temperature superconductivity has been one of the challenges in condensed matter physics. The particular momentum structure of the electron-boson coupling plays a crucial role in the formation of Cooper pairs, which can lead to the enhanced superconductivity. One such example is electron-phonon interaction that is peaked in the forward direction. This interaction has been suggested as an essential ingredient for enhanced superconductivity observed in FeSe monolayers on STO substrates. We study the superconducting state of a system dominated by forward scattering using non-equilibrium methods and contrast its behavior against the standard isotropic BCS case. An analysis of the electron's dynamics in the pump-driven non-equilibrium state reveals that the superconducting order in the forward-focused case is robust and persistent against the pump-induced perturbations. The superconducting order parameter also exhibits a non-uniform melting in momentum space, which could be measured by time-resolved ARPES. We show that this behavior is in sharp contrast to the isotropic interaction case and propose that time-resolved approaches are a potentially powerful tool to differentiate the nature of the dominant coupling in correlated materials.   

Superconductivity and Electrical transport in NbTiN Films

In this work, we systematically investigate the superconducting properties of Niobium Titanium Nitride (NbTiN) films with different geometries, which at low temperature remain superconducting. NbTiN films with a few nm thickness are widely used in devices such as superconductor-insulator-superconductor (SIS) mixers, superconducting cavities and resonators and superconducting nanowire single-photon detectors (SNSPD). In all these applications, films of varying dimensions are required to achieve optimal performance. The initial thickness of our samples was 125 nm deposited on Silicon substrate. We change the width/length of our samples using lithography and ion milling. We measure and analyze the thickness and width dependence of various superconducting properties of these films. This work was supported by the US Department of Energy through Grant No. DE-FG02-99ER45763.   

Transverse Voltage in Superconducting Films: Hysteresis

Transverse voltages have been observed in many superconducting films near their transition temperatures. Explanations have ranged from vortex motion, to inhomogeneity, and even anyons. We have looked at a variety of Nb based superconductors (Nb, NbZr, and NbCN) in thin film form (thicknesses from 25-100 nm) to look for the claimed hysteresis from the vortex model put forward by Glazman in 1986. Hysteresis has been observed, and the degree can be varied by the sweep rate of the current (from 20-200 mA/s). We can tell this is not a thermal hysteresis as the sweep up peak does not change, but only the sweep down peak. For slow sweep rates ($\approx$ 7mA/s), we observe oscillations in the Transverse voltage vs current as the current is decreased. The open question is exactly how to compare this hysteresis to that predicted by Glazman.   

Pressure effects on the superconductivity of Tl-2223 crystals

This study investigates the application of high pressure on the superconducting properties of a thallium-based cuprate namely Tl$_{2}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{10 +\delta}$ (Tl-2223). The superconducting transition temperature (T$_{c}$) and the critical current density (J$_{c}$) were studied by applying 0.8 GPa of pressure. The pressure was applied in a piston-cylinder-cell (PCC), using Pb as manometer and Daphne 7373 oil as the pressure transmitting medium. For estimating the J$_{c}$, we used the Bean’s critical state formula on the magnetic hysteresis curves obtained at 10 K and 20 K. Both the T$_{c}$ and J$_{c}$ shifted with pressure, clearly indicating that pressure is another tool to control properties of quantum materials.   

Flux Vortex Explosion in Superconductors

Flux vortices are a very interesting phenomenon in condensed matter physics. A flux vortex (fluxon) is a quantized amount of magnetic flux that is contained within a swirling super-current. They undergo many curious effects and various regimes. We investigated an interfacial superconductor and observed a transition in the current-resistance and temperature-resistance which indicates Likharev vortex explosion. This explosion occurs as the coherence length within the material becomes greater than the size of the sample itself. Effectively, this chokes the super-current and causes the vortex to grow and then explode when it exceeds the bounds of the material..   

Synthesis of Ruddlesden-Popper Strontium Iridate Epitaxial Thin Films

We investigated the growth conditions conducive to synthesize Ruddlesden-Popper type SrIrO3, Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, and Sr$_{\mathrm{3}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$epitaxial thin films via pulsed laser deposition (PLD). Many factors influence the thermodynamic interactions of the deposition and therefore, determines the material phase that is created. Through a systematic review of these growth conditions, we constructed a growth phase diagram that maps out conditions that enable stable formation of strontium iridate phases. We synthesized these phases with a single Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ target and by varying the O$_{\mathrm{2}}$ chamber pressure and the substrate temperature. These films allow for the analysis of magnetic properties of the material through vibrating sample magnetometry (VSM) and other methods. Our findings demonstrate the control of the thermodynamic stability of different epitaxial layered structure of the complex Ruddlesden-Popper family.   

High pressure structural parameters and equation of state of osmium to 207 GPa

The most incompressible transition metal osmium (Os) has been studied under high pressure. There is significant interest in Os because of the structural anomalies attributed to topological transitions in the Fermi surface for valence electrons in the hexagonal close-packed phase. We report on measurements of structural parameters and equation of state on Os metal to a pressure of 207 GPa at ambient temperature using platinum as a pressure standard. We obtained angle-dispersive X-ray diffraction data at a synchrotron source with closely spaced pressure intervals to observe any discontinuities or anomalies in the axial c/a ratio at high pressures. Rietveld refinements of X-ray diffraction data show a slowly varying axial ratio (c/a) with a broad minimum at 75 GPa. Our data do not provide any evidence of anomalous behavior in the c/a ratio in Os at 25 or 150 GPa as have been reported in previous studies. Our experimental results are in agreement with theoretical calculations that do not predict any anomalous behavior in c/a ratio in Os under extreme conditions. We present an equation of state for Os to 207 GPa (V/V0 $=$ 0.761) at ambient temperature and compare our results with the previously published data.