Possible f-wave superconductivity in Sr2_2RuO4_4?

Until recently it has been believed that the superconductivity in Sr$_2$RuO$_4$ is described by p-wave pairing. However, both the recent specific heat and the magnetic penetration depth measurements on the purest single crystals of Sr$_2$RuO$_4$ appear to be explained more consistently in terms of f-wave superconductivity. In order to further this hypothesis, we study theoretically the thermodynamics and thermal conductivity of f-wave superconductors in a planar magnetic field. We find the simple expressions for these quantities when $H \ll H_{c2}$ and $T \ll T_{c}$, which should be readily accessible experimentally.Comment: 6 pages, 2 figure

Critical Current of the Spin-Triplet Superconducting Phase in Sr2_2RuO4_4

There have been two different proposals for the spin-triplet order parameter of the superconducting phase in Sr$_2$RuO$_4$; an $f$-wave order parameter and the multigap model where some of the bands have the line node. In an effort to propose an experiment that can distinguish two cases, we study the behavior of the supercurrent and compute the critical current for these order parameters when the sample is a thin film with the thickness $d \ll \xi$ where $\xi$ is the coherence length. It is found that the supercurrent behaves very differently in two models. This will serve as a sharp test for the identification of the correct order parameter.Comment: 4 pages, 1 figur

Threshold electric field in unconventional density waves

As it is well known most of charge density wave (CDW) and spin density wave (SDW) exhibit the nonlinear transport with well defined threshold electric field E_T. Here we study theoretically the threshold electric field of unconventional density waves. We find that the threshold field increases monotonically with temperature without divergent behaviour at T_c, unlike the one in conventional CDW. The present result in the 3D weak pinning limit appears to describe rather well the threshold electric field observed recently in the low-temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4.Comment: 4 pages, 2 figure

Thermal conductivity in B- and C- phase of UPt_3

Although the superconductivity in UPt_3 is one of the most well studied, there are still lingering questions about the nodal directions in the B and C phase in the presence of a magnetic field. Limiting ourselves to the low temperature regime (T<<Delta(0)), we study the magnetothermal conductivity with in semiclassical approximation using Volovik's approach. The angular dependence of the magnetothermal conductivity for an arbitrary field direction should clarify the nodal structure in UPt_3.Comment: 4 pages, 5 figure

Electronic Quantum Monte Carlo Calculations of Atomic Forces, Vibrations, and Anharmonicities

Atomic forces are calculated for first-row monohydrides and carbon monoxide within electronic quantum Monte Carlo (QMC). Accurate and efficient forces are achieved by using an improved method for moving variational parameters in variational QMC. Newton's method with singular value decomposition (SVD) is combined with steepest descent (SD) updates along directions rejected by the SVD, after initial SD steps. Dissociation energies in variational and diffusion QMC agree well with experiment. The atomic forces agree quantitatively with potential energy surfaces, demonstrating the accuracy of this force procedure. The harmonic vibrational frequencies and anharmonicity constants, derived from the QMC energies and atomic forces, also agree well with experimental values.Comment: 6 pages, 2 figures; updated conten

Novel vortex lattice transition in d-wave superconductors

We study the vortex state in a magnetic field parallel to the $c$ axis in the framework of the extended Ginzburg Landau equation. We find the vortex acquires a fourfold modulation proportional to $\cos(4\phi)$ where $\phi$ is the angle ${\bf r}$ makes with the $a$-axis. This term gives rise to an attractive interaction between two vortices when they are aligned parallel to $(1,1,0)$ or $(1,-1,0)$. We predict the first order vortex lattice transition at $B=H_{cr}\sim \kappa^{-1} H_{c2}(t)$ from triangular into the square lattice tilted by $45^\circ$ from the $a$ axis. This gives the critical field $H_{cr}$ a few Tesla for YBCO and Bi2212 monocrystals at low temperatures ($T\leq 10 K$).Comment: 6 pages, 4 figure

Neural-Network Vector Controller for Permanent-Magnet Synchronous Motor Drives: Simulated and Hardware-Validated Results

This paper focuses on current control in a permanentmagnet synchronous motor (PMSM). The paper has two main objectives: The first objective is to develop a neural-network (NN) vector controller to overcome the decoupling inaccuracy problem associated with conventional PI-based vector-control methods. The NN is developed using the full dynamic equation of a PMSM, and trained to implement optimal control based on approximate dynamic programming. The second objective is to evaluate the robust and adaptive performance of the NN controller against that of the conventional standard vector controller under motor parameter variation and dynamic control conditions by (a) simulating the behavior of a PMSM typically used in realistic electric vehicle applications and (b) building an experimental system for hardware validation as well as combined hardware and simulation evaluation. The results demonstrate that the NN controller outperforms conventional vector controllers in both simulation and hardware implementation

Low-lying excitations around a single vortex in a d-wave superconductor

A full quantum-mechanical treatment of the Bogoliubov-de Gennes equation for a single vortex in a d-wave superconductor is presented. First, we find low-energy states extended in four diagonal directions, which have no counterpart in a vortex of s-wave superconductors. The four-fold symmetry is due to 'quantum effect', which is enhanced when $p_{F}\xi$ is small. Second, for $p_{F}\xi \sim 1$, a peak with a large energy gap $E_{0}\sim \Delta$ is found in the density of states, which is due to the formation of the lowest bound states.Comment: 7pages, Revte

Helstrom Theorem by No-Signaling Condition

We prove a special case of Helstrom theorem by using no-signaling condition in the special theory of relativity that faster-than-light communication is impossible.Comment: Minor corrections (A reference added, discussion part deleted, typos in equations corrected), 2 pages, RevTe