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We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a 300 $\ensuremath{\mu}\mathrm{m}$ long intrinsic Josephson junction stack consisting of $N=700$ junctions. The junctions in the stack are combined with $M$ segments where we assume that inside a segment all junctions behave identically. Most simulations are for $M=20$. For not too high bath temperatures there is the appearance of a hot spot at high-bias currents. In terms of electromagnetic properties, robust standing-wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to $\ensuremath{\pi}$-kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with $M$, on the basis of simulations with $M$ between 10 and 350.

We investigated the normal-state resistivity ${\ensuremath{\rho}}_{xx}(T)$ and the Hall effect in Zn- and Co-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single-crystalline microbridges. A crossover temperature ${T}^{*}$ was observed in the temperature dependency of the longitudinal resistivity ${\ensuremath{\rho}}_{xx}(T)$, which separates ${\ensuremath{\rho}}_{xx}(T)$ into temperature-linear and temperature-nonlinear regions. Above ${T}^{*}$, the carriers in ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and Co-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{1.94}{\mathrm{Co}}_{0.06}{\mathrm{As}}_{2}$ demonstrate electronlike behavior and an anomalous nonlinear magnetic field dependence of the Hall voltage with a sign reversal. By contrast, the Zn-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{1.94}{\mathrm{Zn}}_{0.06}{\mathrm{As}}_{2}$ behaves like a hole type and the Hall coefficient is independent of the magnetic field. The field-induced sign reversal of the Hall coefficient of undoped and Co-doped samples depends on the field modification on the mobility and hole/electron concentration ratio. The ${T}^{2}$-dependent Hall angle of a nonmagnetic Zn-doped crystal is observed as a nearly parallel shift from that of the impurity-free crystal in the low-temperature region, indicating that the Zn induces a weak change of the spinons excitations, while increasing the number of scattering centers. The Co works as a nonmagnetic impurity as well, while it provides both spinons excitations and impurity scattering.

In our experiments, fully oxygenated ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals were annealed in flowing oxygen at various temperatures and quenched in liquid nitrogen to become oxygen deficient. These single crystals, with transition temperatures ranging from 35 K to 82 K, were then fabricated into mesalike devices along c axis with $a\ensuremath{-}b$ plane dimensions down to 2 $\ensuremath{\mu}\mathrm{m}.$ We measured dc current-voltage $(I\ensuremath{-}V)$ curves, and observed typical intrinsic Josephson effects such as hysteresis, voltage jumps, etc., in all oxygen deficient samples. However, the oxygen contents and $a\ensuremath{-}b$ plane sizes of the samples greatly affected the appearances of the $I\ensuremath{-}V$ characteristics, supplying important information on the intrinsic Josephson effects. Microwave responses were also explored for possible high-frequency applications.

Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions which are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation., Comment: accepted in PRB

Within the time-dependent Ginzburg-Landau theory we study the dynamic properties of current-carrying superconducting strips in the presence of a perpendicular magnetic field. We found pronounced voltage peaks as a function of the magnetic field, the amplitude of which depends both on sample dimensions and external parameters. These voltage oscillations are a consequence of moving vortices, which undergo alternating static and dynamic phases. At higher fields or for high currents, the continuous motion of vortices is responsible for the monotonic background on which the resistance oscillations due to the entry of additional vortices are superimposed. Mechanisms for such vortex-assisted resistance oscillations are discussed. Qualitative changes in the magnetoresistance curves are observed in the presence of random defects, which affect the dynamics of vortices in the system.

At high enough input power in stacks of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent terahertz (THz) emission. We investigate the interplay of hot spots and standing electromagnetic waves by low-temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. Standing electromagnetic wave patterns and THz emission are observed for a disk-shaped sample. The growth of a hot spot with increasing input power is monitored by small detector junctions surrounding a large rectangular mesa. For two rectangular mesas equipped with two current injectors and one arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot-spot position. The results support the picture of the hot spot acting as a reflective termination of the cavity, formed by the cold part of the mesa.

The effect of chemical ordering of a half-metallic full-Heusler alloy ${\text{Co}}_{2}\text{MnSi}$ (CMS) on interlayer exchange coupling (IEC) behavior was investigated in Co-Mn-Si(20 nm)/Cr(1.2 nm)/Co-Mn-Si(7 nm) trilayers. Two types of experimental investigations were employed, namely, the variations in IEC with changing the annealing temperature of the bottom CMS from room temperature to $500\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and with changing the Co concentration in top and bottom Co-Mn-Si layers. The $90\ifmmode^\circ\else\textdegree\fi{}$ coupling parameter ${J}_{2}$ and the bilinear coupling parameter ${J}_{1}$ were estimated from the numerical simulations of magnetization curves. Only the strong $90\ifmmode^\circ\else\textdegree\fi{}$ coupling with almost the same strength was observed in both the completely $B2$ ordered and in a mixture of ordered $L{2}_{1}$ and $B2$ structures. The degree of $B2$ ordering as well as the strength of $90\ifmmode^\circ\else\textdegree\fi{}$ coupling was observed to decrease simultaneously with the Co concentration in Co-Mn-Si and $180\ifmmode^\circ\else\textdegree\fi{}$ coupling appeared in highly Co antisite defective $(A2)$ structures, i.e., for the Co concentration over $73\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. This observation showed a clear relationship between the degree of $B2$ ordering and the IEC behavior in full-Heusler Co-Mn-Si-based structures.

We demonstrate the operation of a deterministic fluxon ratchet made of a stack of 30 intrinsic ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ Josephson junctions. The ratchet has the shape of a gear with 20 asymmetric teeth (periods). It produces a rectified voltage of about $100\text{ }\ensuremath{\mu}\text{V}$ at a 12 GHz drive frequency. The effect of coupling between intrinsic junctions, i.e., the mode of fluxon motion for ratchet operation, has been studied within the framework of the two-dimensional coupled sine-Gordon equations. Further, we used low-temperature scanning laser microscopy to demonstrate that voltage rectification indeed is due to directed fluxon motion, in agreement with numerical simulations.

We measured the Josephson vortex flow resistance as a function of magnetic field applied parallel to the $ab$ planes using annealed $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ intrinsic Josephson junctions having high anisotropy $(\ensuremath{\sim}40)$ by oxygen content reduction. Periodic oscillations were observed in magnetic fields above $45--58\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, corresponding to dense-dilute boundary for Josephson vortex lattice. The observed period of oscillations, which agrees well with the increase of one fluxon per two junctions $({H}_{p}={\ensuremath{\Phi}}_{0}∕2Ls)$, may correspond to formation of a triangular lattice of Josephson vortices as has been reported by Ooi et al. for highly anisotropic $(\ensuremath{\geqslant}200)$ Bi-2212 intrinsic Josephson junctions.

We investigate both experimentally and theoretically transport properties of the Josephson vortices (JV) with a finite bias current along the $c$ axis of the intrinsic Josephson junctions in layered high-${T}_{c}$ superconductor of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. Employing the perturbed sine-Gordon equation and integrating a novel characterization factor of the difference between current distributions on neighboring layers $\ensuremath{\Delta}J$, we are successful in explaining new phenomena such as (1) oscillation with period ${H}_{p}$ corresponding to adding one JV per layer, (2) its peak splitting, and (3) reverse of the peaks and valleys with increase of current in JV flow resistance as a function of magnetic field observed in our experiments. Additionally, our simulation shows concrete evidence for the sample size dependent phase transition between ${H}_{p}∕2$-period and ${H}_{p}$-period oscillations. The novel phenomena are expected to be useful for designing practical devices in experiments.

We have experimentally investigated the fluxon dynamics in a narrow ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ stack with junction length $L\ensuremath{\sim}1.8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. As an evidence of high-frequency excitation by a collective cavity mode, under an (in-plane) external magnetic field, the current-voltage characteristics show prominent Fiske steps with the corresponding resonance frequencies of $75--305\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. Further study of flux-flow resistance oscillation with various $c$-axis currents clarifies the correlation with Fiske steps by distinguishing two different regions, i.e., the static flux-flow region at low bias current level and dynamic Fiske step region at high bias current level.