Search results for " superconductivity"
showing 10 items of 319 documents
Optimal operation of transition-edge sensors on ballistic membranes
2007
We calculate the operating parameters of a transition edge sensor that is mounted on a thin dielectric membrane with the assumption that the phononic heat transport in the membrane is ballistic. Our treatment uses the correct phonon modes from elasticity theory (Lamb-modes), and spans the transition from 3D to 2D behavior. The phonon cooling power and conductance have a global minimum as function of membrane thickness, which leads to an optimal value for the membrane thickness with respect to noise equivalent power at a fixed operating temperature. The energy resolution of a calorimeter will not be affected strongly, but, somewhat counterintuitively, the effective time constant can be reduc…
Highly tunable NbTiN Josephson junctions fabricated with focused helium ion beam
2023
We demonstrate a "direct writing" method for the fabrication of planar Josephson junctions from high quality superconducting niobium titanium nitride (NbTiN) thin films using focused He-ion beam irradiation. Compared to the materials previously used in such processing, YBCO and MgB$_2$, NbTiN has much better mechanical and electrical properties, as well as good corrosion resistance. We show that we can control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, and that this controllable critical temperature suppression combined with the high spatial resolution and position control of the He-ion beam in a helium ion microscope enables us to successful…
Superconductivity provides a giant enhancement to the spin battery effect
2021
We develop a theory of the spin battery effect in superconductor/ferromagnetic insulator (SC/FI) systems taking into account the magnetic proximity effect. We demonstrate that the spin-energy mixing enabled by the superconductivity leads to the enhancement of spin accumulation by several orders of magnitude relative to the normal state. This finding can explain the recently observed giant inverse spin Hall effect generated by thermal magnons in the SC/FI system. We suggest a non-local electrical detection scheme which can directly probe the spin accumulation driven by the magnetization dynamics. We predict a giant Seebeck effect converting the magnon temperature bias into the non-local volt…
Chiral pair density wave states generated by spin supercurrents
2021
We report that spin supercurrents in magnetic superconductors and superconductor/ferromagnetic insulator bilayers can induce the Dzyaloshinskii-Moriya interaction which strength is proportional to the superconducting order parameter amplitude. This effect leads to the existence of inhomogeneous parity-breaking ground states combining the chiral magnetic helix and the pair density wave orders. The formation of such states takes place via the penetration of chiral domain walls at the threshold temperature below the superconducting transition. We find regimes with both the single and the re-entrant transitions into the inhomogeneous states with decreasing temperature. The predicted hybrid chir…
Enhanced Superconductivity and Suppression of Charge-density Wave Order in 2H-TaS$_2$ in the Two-dimensional Limit
2017
As superconductors are thinned down to the 2D limit, their critical temperature $T_c$ typically decreases. Here we report the opposite behavior, a substantial enhancement of $T_c$ with decreasing thickness, in 2D crystalline superconductor 2H-TaS$_2$. Remarkably, in the monolayer limit, $T_c$ increases to 3.4 K compared to 0.8 K in the bulk. Accompanying this trend in superconductivity, we observe suppression of the charge-density wave (CDW) transition with decreasing thickness. To explain these trends, we perform electronic structure calculations showing that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which contributes t…
Low-Frequency Imaginary Impedance at the Superconducting Transition of 2H-NbSe2
2019
The superconducting transition leads to a sharp resistance drop in a temperature interval that can be a small fraction of the critical temperature T$_c$. A superconductor exactly at T$_c$ is thus very sensitive to all kinds of thermal perturbations, including the heat dissipated by the measurement current. We show that the interaction between electrical and thermal currents leads to a sizeable imaginary impedance at frequencies of order of tens of Hz at the resistive transition of single crystals of the layered material 2H-NbSe$_2$. We explain the result using models developed for transition edge sensors. By measuring under magnetic fields and at high currents, we find that the imaginary im…
Giant negative magnetoresistance driven by spin-orbit coupling at the LAO/STO interface
2014
The LAO/STO interface hosts a two-dimensional electron system that is unusually sensitive to the application of an in-plane magnetic field. Low-temperature experiments have revealed a giant negative magnetoresistance (dropping by 70\%), attributed to a magnetic-field induced transition between interacting phases of conduction electrons with Kondo-screened magnetic impurities. Here we report on experiments over a broad temperature range, showing the persistence of the magnetoresistance up to the 20~K range --- indicative of a single-particle mechanism. Motivated by a striking correspondence between the temperature and carrier density dependence of our magnetoresistance measurements we propos…
Tuning interchain ferromagnetic instability in A2Cr3As3 ternary arsenides by chemical pressure and uniaxial strain
2020
We analyze the effects of chemical pressure induced by alkali metal substitution and uniaxial strain on magnetism in the A2Cr3As3 (A = Na, K, Rb, Cs) family of ternary arsenides with quasi-one dimensional structure. Within the framework of the density functional theory, we predict that the non-magnetic phase is very close to a 3D collinear ferrimagnetic state, which realizes in the regime of moderate correlations, such tendency being common to all the members of the family with very small variations due to the different interchain ferromagnetic coupling. We uncover that the stability of such interchain ferromagnetic coupling has a non-monotonic behavior with increasing the cation size, bein…
Dynamics of Collective Modes in an unconventional Charge Density Wave system BaNi$_{2}$As$_{2}$
2021
AbstractBaNi2As2 is a non-magnetic analogue of BaFe2As2, the parent compound of a prototype pnictide high-temperature superconductor, displaying superconductivity already at ambient pressure. Recent diffraction studies demonstrated the existence of two types of periodic lattice distortions above and below the triclinic phase transition, suggesting the existence of an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon doping results in a sixfold increase in the superconducting transition temperature and enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here, we apply time-resolved optical spectroscopy to investigate collective d…
The Composite Operator Method (COM)
2011
The Composite Operator Method (COM) is formulated, its internals illustrated in detail and some of its most successful applications reported. COM endorses the emergence, in strongly correlated systems (SCS), of composite operators, optimally deals with their unusual features and implements algebra constraints, and other relevant symmetries, in order to properly compute the unconventional properties of SCS.