Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different from the anisotropy of the penetration depth gamma_{lambda} = lambda_c/lambda_{ab}. With increasing difference between gamma_H and gamma_{lambda}, the equilibrium orientation of the crystal relative to the applied field may shift from theta = pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta = 0 for large enough gamma_H. These effects are expected to take place in MgB_2.Comment: 4 pages, 3 fig

Temperature-dependent Hc2H_{c2} anisotropy in MgB2_2 as inferred from measurements on polycrystals

We present data on temperature-dependent anisotropy of the upper critical field of MgB$_2$ obtained from the analysis of measurements on high purity, low resistivity polycrystals. The anisotropy decreases in a monotonic fashion with increase of temperature

Pressure induced superconductivity in CaFe2_2As2_2

CaFe$_2$As$_2$ has been found to be exceptionally sensitive to the application of hydrostatic pressure and superconductivity has been found to exist in a narrow pressure region that appears to be at the interface between two different phase transitions. The pressure - temperature ($P - T$) phase diagram of CaFe$_2$As$_2$ reveals that this stoichiometric, highly ordered, compound can be easily tuned to reveal all the salient features associated with FeAs-based superconductivity without introducing any disorder. Whereas at ambient pressure CaFe$_2$As$_2$ does not superconduct for $T > 1.8$ K and manifests a first order structural phase transition near $T \approx 170$ K, the application of $\sim 5$ kbar hydrostatic pressure fully suppresses the resistive signature of the structural phase transition and instead superconductivity is detected for $T < 12$ K. For $P \ge 5.5$ kbar a different transition is detected, one associated with a clear reduction in resistivity and for $P > 8.6$ kbar superconductivity is no longer detected. This higher pressure transition temperature increases rapidly with increasing pressure, exceeding 300 K by $P \sim 17$ kbar. The low temperature, superconducting dome is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity, and possibly the associated fluctuations, too dramatically

Field-Dependent Hall Effect in Single Crystal Heavy Fermion YbAgGe below 1K

We report the results of a low temperature (T >= 50 mK) and high field (H <= 180 kOe) study of the Hall resistivity in single crystals of YbAgGe, a heavy fermion compound that demonstrates field-induced non-Fermi-liquid behavior near its field-induced quantum critical point. Distinct features in the anisotropic, field-dependent Hall resistivity sharpen on cooling down and at the base temperature are close to the respective critical fields for the field-induced quantum critical point. The field range of the non-Fermi-liquid region decreases on cooling but remains finite at the base temperature with no indication of its conversion to a point for T -> 0. At the base temperature, the functional form of the field-dependent Hall coefficient is field direction dependent and complex beyond existing simple models thus reflecting the multi-component Fermi surface of the material and its non-trivial modification at the quantum critical point

Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2

We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02 < x < 0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and non-reconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05, close to the highest superconducting T_c. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism

Scanning Tunneling Microscopy in the superconductor LaSb2

We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb$_2$. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling s-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find T$_c$ of 1.2 K by following the tunneling conductance with temperature.Comment: 5 pages, 4 figure

Local superconducting density of states of ErNi2B2C

We present local tunnelling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C. The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.Comment: 9 pages, 3 figure

NMR Study of the New Magnetic Superconductor CaK(Fe$0.951Ni0.049)4As4: Microscopic Coexistence of Hedgehog Spin-vortex Crystal and Superconductivity

Coexistence of a new-type antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by $^{75}$As nuclear magnetic resonance study on single-crystalline CaK(Fe$_{0.951}$Ni$_{0.049}$)$_4$As$_4$. The hedgehog SVC order is clearly demonstrated by the direct observation of the internal magnetic induction along the $c$ axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature $T_{\rm N}$ $\sim$ 52 K. The nuclear spin-lattice relaxation rate 1/$T_1$ shows a distinct decrease below $T_{\rm c}$ $\sim$ 10 K, providing also unambiguous evidence for the microscopic coexistence. Furthermore, based on the analysis of the 1/$T_1$ data, the hedgehog SVC-type spin correlations are found to be enhanced below $T$ $\sim$ 150 K in the paramagnetic state. These results indicate the hedgehog SVC-type spin correlations play an important role for the appearance of SC in the new magnetic superconductor.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. B rapid communicatio

Magnetoresistivity and Complete Hc2(T)H_{c2}(T) in MgB2MgB_2

Detailed magneto-transport data on dense wires of $MgB_2$ are reported for applied magnetic fields up to 18 T. The temperature and field dependencies of the electrical resistivity are consistent with $MgB_2$ behaving like a simple metal and following a generalized form of Kohler's rule. In addition, given the generally high $T_c$ values and narrow resistive transition widths associated with $MgB_2$ synthesized in this manner, combined with applied magnetic fields of up to 18 T, an accurate and complete $H_{c2}(T)$ curve could be determined. This curve agrees well with curves determined from lower field measurements on sintered pellets and wires of $MgB_2$. $H_{c2}(T)$ is linear in $T$ over a wide range of temperature (7 K $\le~T~\le$ 32 K) and has an upward curvature for $T$ close to $T_c$. These features are similar to other high $\kappa$, clean limit, boron-bearing intermetallics: $YNi_2B_2C$ and $LuNi_2B_2C$.Comment: minor changes in styl

Magnetic field-induced quantum critical point in YbPtIn and YbPt0.98_{0.98}In single crystals

Detailed anisotropic (H$\parallel$ab and H$\parallel$c) resistivity and specific heat measurements were performed on online-grown YbPtIn and solution-grown YbPt$_{0.98}$In single crystals for temperatures down to 0.4 K, and fields up to 140 kG; H$\parallel$ab Hall resistivity was also measured on the YbPt$_{0.98}$In system for the same temperature and field ranges. All these measurements indicate that the small change in stoichiometry between the two compounds drastically affects their ordering temperatures (T$_{ord}\approx3.4$ K in YbPtIn, and $\sim2.2$ K in YbPt$_{0.98}$In). Furthermore, a field-induced quantum critical point is apparent in each of these heavy fermion systems, with the corresponding critical field values of YbPt$_{0.98}$In (H$^{ab}_c$ around 35-45 kG and H$^{c}_c\approx120$ kG) also reduced compared to the analogous values for YbPtIn (H$^{ab}_c\approx60$ kG and H$^{c}_c>140$ kG