Single crystals of BaFe$_2$As$_2$ and (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated and can have dimensions as large as 3 x 3 x 0.2 mm$^3$. The BaFe$_2$As$_2$ single crystals manifest a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat and the anisotropic magnetic susceptibility. In the (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near $T_c$ is small, but resolvable, giving $ΔC_p/γT_c \approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by only $\sim 4$ K when applied along the c-axis and by $\sim 2$ K when applied perpendicular to the c-axis. The ratio of the anisotropic upper critical fields, $γ= H_{c2}^{\perp c} / H_{c2}^{\| c}$, varies between 2.5 and 3.5 for temperatures down to $\sim 2$ K below $T_c$.
We report normal and superconducting properties of the Rashba-type noncentrosymmetric com- pound CaIrSi3, using single crystalline samples with nearly 100% superconducting volume fraction. The electronic density of states revealed by the hard x-ray photoemission spectroscopy can be well explained by the relativistic first-principle band calculation. This indicates that strong spin-orbit interaction indeed affects the electronic states of this compound. The obtained H - T phase diagram exhibits only approximately 10% anisotropy, indicating that the superconducting properties are almost three di…
We present a detailed study of RNi1-xBi2+-y (R = La-Nd, Sm, Gd-Dy) single crystals by measurements of stoichiometry and temperature dependent magnetic susceptibility, magnetization, and electrical resistivity. This series forms with partial Ni occupancy as well as a variable Bi occupancy. For R = Ce-Nd, Sm, Gd-Dy, the RNi1-xBi2+-y compounds show local-moment like behavior and order antiferromagnetically at low temperatures. Determination of anisotropies as well as antiferromagnetic ordering temperatures for RNi1-xBi2+-y (R = Ce-Nd, Sm, Gd-Dy) have been made. Although crystalline samples from t…
Using high-resolution dilatometry, we study the thermodynamic response of the lattice parameters to superconducting order in a self-flux grown Ba(Fe0.92Co0.08)2As2 single crystal. The uniaxial pressure dependencies of the critical temperature of Tc, calculated using our thermal expansion and specific heat data via the Ehrenfest relation, are found to be quite large and very anisotropic (dTc/dpa = 3.1(1) K/GPa and dTc/dpc = - 7.0(2) K/GPa). Our results show that there is a strong coupling of the c/a ratio to superconducting order, which demonstrates that Tc is far from the optimal value. A surp…
Majorana Kramers pairs emerged on surfaces of time-reversal-invariant topological crystalline superconductors show the Ising anisotropy to an applied magnetic field. We clarify that crystalline symmetry uniquely determines the direction of the Majorana Ising spin for given irreducible representations of pair potential, deriving constraints to topological invariants. Besides, necessary conditions for nontrivial topological invariants protected by the n-fold rotational symmetry are shown.
We investigate the anisotropic nature of magnetocrystalline coupling between the crystallographic and skyrmion crystal (SKX) lattices in the chiral magnet MnSi by magnetic field-angle resolved resonant ultrasound spectroscopy. Abrupt changes are observed in the elastic moduli and attenuation when the magnetic field is parallel to the [011] crystallographic direction. These observations are interpreted in a phenomenological Ginzburg-Landau theory that identifies switching of the SKX orientation to be the result of an anisotropic magnetocrystalline coupling potential. Our paper sheds new light o…
Single-crystal synchrotron x-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave-vector at low temperatures, indicating an anisotropic two-dimensional magnetoelastic coupling. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. In addition, we find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the doma…
We theoretically investigate the crystalline anisotropy of topological phase transitions in phase-controlled planar Josephson junctions (JJs) subject to spin-orbit coupling and in-plane magnetic fields. It is shown how topological superconductivity (TS) is affected by the interplay between the magnetic field and the orientation of the junction with respect to its crystallographic axes. This interplay can be used to electrically tune between different symmetry classes in a controlled fashion and thereby optimize the stability and localization of Majorana bound states in planar Josephson junctio…
Ambient-pressure-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$ with a superconducting transition temperature $T_{c}\sim$3K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline $c$-axis and the $ab$-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field $H_{c2}^{\perp}(T)$, the in-plane upper critical field $H_{c2}^{\parallel}(T)$, as well as the angular dependence of $H_{c2}$ at fixed temperatures for ambient-pressure-grown LaO$_{0.5}$F$_{0…
Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the…
We present superconducting properties of single crystalline Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ by measuring magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal fish-tail hysteresis loop at high temperatures and relatively high critical current density above $J_{c}=10^{5}$ A/cm$^{2}$ at low temperatures. Upper critical field determined by resistive transition is anisotropic with anisotropic parameter $\sim$ 3.5. Hall effect measurements indicate that Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ is a multiband system…
$\require{mediawiki-texvc}$The locally non-centrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P6_{3}$/$mmc$ with lattice parameters $a$ = 4.2445(4) $Å$ and $c$ = 8.9513(18) $Å$. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at T$_c$ $\sim$2.2 K, in agreement with previous reports on polycrystalline sam…
Topological crystalline metals/semimetals (TCMs) have stimulated a great research interest, which broaden the classification of topological phases and provide a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide Pb$_{1/3}$TaS$_2$. Systematic measurements indicate that Pb$_{1/3}$TaS$_2$ is a quasi-two-dimensional (q-2D) type-II superconductor ({\em T}$_c \approx$ 2.8 K) with a significantly enhanced anisotropy of upper critical field ($γ_{H_{c2}}$ = $H_{c2}^{ab…
Single crystalline samples of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x < 0.12$ have been grown and characterized via microscopic, thermodynamic and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high temperature tetragonal to low temperature orthorhombic) and magnetic (high temperature non magnetic to low temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per percent Co. In addition, for $x \ge 0.038$ superconductivity is stabilized, rising to a maximum $T_c$ of approximately 23 K for $x \app…
Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Add…
The free energy, non-gradient terms of the Ginzburg-Landau expansion, and the jump of the specific heat of a multiband anisotropic-gap clean BCS superconductor are derived in the framework of a separable-kernel approximation. Results for a two-band superconductor, d-wave superconductor, and some recent models for MgB_2 are derived as special cases.
Single crystalline, single phase CaKFe$_{4}$As$_{4}$ has been grown out of a high temperature, quaternary melt. Temperature dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization and specific heat, combined with field dependent measurements of electrical resistivity and field and pressure dependent measurements of magnetization indicate that CaKFe$_{4}$As$_{4}$ is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, $T_c$ = 35.0 $\pm$ 0.2 K. Other than super…
We show that the density of states and the thermodynamic properties of a 2D d-wave superconductor in the vortex state with applied magnetic field $\bf H$ in the plane depend on the angle between $\bf H$ and the order parameter nodes. Within a semiclassical treatment of the extended quasiparticle states, we obtain fourfold oscillations of the specific heat, measurement of which provides a simple probe of gap symmetry. The frequency dependence of the density of states and the temperature dependence of thermodynamic properties obey different power laws for field in the nodal and anti-nodal direct…
High resolution magnetization measurements performed in a high quality Ba_(1-x)K_xFe2As2 single crystal allowed to determine the diamagnetism induced above the superconducting transition by thermally activated Cooper pairs. These data, obtained with magnetic fields applied along and transverse to the crystal ab layers, demonstrate experimentally that the superconducting transition of iron pnictides may be explained at a phenomenological level in terms of the Gaussian Ginzburg-Landau approach for three-dimensional anisotropic superconductors.
Based on weakly-coupled anisotropic BCS theory, the dependence of energy gap and specific heat on temperature are evaluated for superconductor MgB$_2$, and the results are compared with experimental data.We show that the weak coupling anisotropic BCS theory describes thermodynamic experimental data with high precision 3-6%.
We present anisotropic thermal expansion measurements on single crystalline ErNi2B2C. All three, superconducting, antiferromagnetic and weak ferromagnetic phase transitions are unambiguously distinguished in the data. Anisotropic uniaxial pressure dependencies of the transitions are estimated based on the Ehrenfest relation, leading to a conclusion, in particular, that weak ferromagnetic state will be suppressed by small, order of several kbar, hydrostatic pressure.
Polarization-dependent x-ray absorption spectroscopy at the B 1s edge of single-crystalline Mg(x)Al(1-x)B(2) reveals a strongly anisotropic electronic structure near the Fermi energy. Comparing spectra for superconducting compounds (x=0.9, 1.0) with those for the non-superconductor x=0.0 gives direct evidence on the importance of an in-plane spectral feature crossing E_F for the superconducting properties of the diborides. Good agreement is found with the projected B 2p density of states from LDA band structure calculations.
We present a detailed study of single crystalline K$_{2}$Cr$_{3}$As$_{3}$ and analyze its thermodynamic and transport properties, anisotropic $H_{c2}(T)$, and initial pressure-dependence of $T_{c}$. In zero-field, the temperature-dependent resistivity is metallic. Deviation from a linear temperature-dependence is evident below 100 K and a $T^{3}$-dependence is roughly followed from just above $T_{c}$ ($\sim$10 K) to $\sim$ 40 K. Anisotropic $H_{c2}(T)$ data were measured up to 140 kOe with field applied along and perpendicular to the rod-like crystals. For the applied field perpendicular to th…
The density of states of a 2D d-wave superconductor in the vortex state with applied magnetic field $\bf H$ in the plane is shown to exhibit fourfold oscillations as a function of the angle of the field with respect to the crystal axes. We find further that the frequency dependence of the density of states and the temperature dependence of transport coefficients obey different power laws, thus leading to different magnetic scaling functions, for field in the nodal and anti-nodal direction. We discuss the consequences of this anisotropy for measurements of the specific heat.
We analyze the thermodynamical properties, at finite density and nonzero temperature, of the (1+1)-dimensional chiral Gross-Neveu model (the NJL_2 model), using the exact inhomogeneous (crystalline) condensate solutions to the gap equation. The continuous chiral symmetry of the model plays a crucial role, and the thermodynamics leads to a broken phase with a periodic spiral condensate, the "chiral spiral", as a thermodynamically preferred limit of the more general "twisted kink crystal" solution of the gap equation. This situation should be contrasted with the Gross-Neveu model, which has a di…
We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally high fluorine concentration by using a CsCl flux method. Comparing to conventional flux methods, this method can introduce about double the amount of fluorine into the oxygen site. The obtained single crystal shows the highest superconducting transition temperature (Tc = 57.5 K) in single crystalline iron pnictides. In addition, the residual resistivity ratio (RRR) is almost three times as large as that of previously reported single crystals. This suggests that our single crystals are suitable for investigation of the i…
We present a comprehensive study of the binary intermetallic superconductor, BaSn5. High-quality single crystalline BaSn5 was grown out of Sn flux. Detailed thermodynamic and transport measurements were performed to study BaSn5's normal and superconducting state properties. This material appears to be a strongly coupled, multiband superconductor. Hc2(T)is almost isotropic. De Haas-van Alphen oscillations were observed and two effective masses were estimated from the FFT spectra. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of ~ -0.053 K/kbar.
We discuss the possibility that in finite density QCD an anisotropic phase is realized. This case might arise for quarks with different chemical potential and/or different masses. In this phase crystalline structures may be formed. We consider this possibility and we describe, in the context of an effective lagrangian, the corresponding phonons as the Nambu-Goldstone bosons associated to the breaking of the space symmetries.
