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Register a new userThe electronic specific heat of Ba1-xKxFe2As2 from 2K to 380K
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Using a differential technique, we have measured the specific heats of polycrystalline Ba1-xKxFe2As2 samples with x=0, 0.1 and 0.3, between 2K and 380K and in magnetic fields 0 to 13 Tesla. From this data we have determined the electronic specific heat coefficient, gamma, over the entire range for the three samples. The most heavily doped sample (x=0.3) exhibits a large superconducting anomaly Delta gamma(Tc)~48mJ/molK^2 at Tc=35K, and we determine the energy gap, condensation energy, superfluid density and coherence length. In the normal state for the x=0.3 sample, gamma~47 mJ/molK^2 is constant from Tc to 380K. In the parent compound (x=0) there is a large almost first order anomaly at the spin density wave (SDW) transition at To=136K. This anomaly is smaller and broader for x=0.1. At low T, gamma is strongly reduced by the SDW gap for both x=0 and 0.1, but above To, gamma for all three samples are similar.
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Using a high-resolution differential technique we have determined the electronic specific heat coefficient gamma(T) of Ba1-xKxFe2As2 with x=0 to 1.0, at temperatures (T) from 2K to 380K and in magnetic fields H=0 to 13T. In the normal state gamma_n(x,T) increases strongly with x at low temperature, compatible with a mass renormalisation ~12 at x=1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x=0.2 to 1, with transition temperatures and condensation energies peaking sharply at x=0.4. Superconducting coherence lengths xi_{ab}~20{AA} and xi_c~3{AA} are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T-dependences of gamma_s(T,H) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad knee and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at T_{c,s} and a T-dependence gamma_s(T)proptosqrt{1-T/T_{c,s}}. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T-dependence Delta_s(T)propto(1-T/T_{c,s})^{0.75} near T_{c,s}. We estimate band fractions and T=0 gaps from 3-band alpha-model fits to our data and compare the x-dependences of the band fractions with spectroscopic studies of the Fermi surface.
The pairing symmetry in the iron-based superconductor Ba1-xKxFe2As2 may change from nodeless s-wave near x~0.4 and Tc>30 K, to nodal (either d-wave or s-wave) at x=1 and Tc<4 K. Theoretical interest has been focused on this possibility, where in the transition region both order parameters would be present and time reversal symmetry would be broken. We report specific heat in magnetic fields down to 0.4 K of three single crystals, free of low temperature magnetic anomalies, of heavily overdoped Ba1-xKxFe2As2, x= 0.91, 0.88, and 0.81, Tc(mid) ~ 5.6, 7.2 and 13 K and Hc2 ~ 4.5, 6, and 20 T respectively. The data can be analyzed in a two gap scenario, Delta2/Delta1 ~ 4, with the field dependence of gamma (=C/T as T->0) showing an S-shape vs H, with the suppression of the lower gap by 1 T and gamma ~ H**1/2 overall. Although such a non-linear gamma vs H is consistent with deep minima or nodes in the gap, it is not clear evidence for one, or both, of the gaps being nodal. Following the established analysis of the specific heat of d-wave cuprate superconductors containing line nodes, we present the specific heat/H**1/2 vs T/H**1/2 of these Ba1-xKxFe2As2 samples which all, due to the absence of magnetic impurities, convincingly show the scaling for line node behavior for the larger gap. There is however no clear observation of the nodal behavior C ~ alpha*T**2 in zero field at low temperatures, with alpha ~ 2 mJ/molK**3 being consistent with the data. This, with the scaling, leaves the possibility of extreme anisotropy in a nodeless larger gap, Delta2, such that the scaling works for fields above 0.25 to 0.5 T (0.2 to 0.4 K in temperature units), where this an estimate for the size of the deep minima in the Delta2 ~ 20-25 K gap. Thus, the change from nodeless to nodal gaps in Ba1-xKxFe2As2 may be closer to the KFe2As2 endpoint than x=0.91.
We report the results of a systematic investigation of the phase diagram of the iron-based superconductor, Ba1-xKxFe2As2, from x = 0 to x = 1.0 using high resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of Deltax <~ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1-xKxFe2As2 crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with space group Fmmm and a striped antiferromagnet (AF) with space group Fcmmm. The transition temperature falls from a maximum of 139K in the undoped compound to 0K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3) and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 falling to 3 K at x = 1.0. We discuss reasons for the suppression of the spin-density-wave order and the electron-hole asymmetry in the phase diagram.
We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium-doping, falling to zero at x <~ 0.3. However, unlike Ba(Fe1xCox)2As2, the two transitions are first-order and coincident over the entire phase diagram, with a biquadratic coupling of the two order parameters. The phase diagram is refined showing that the onset of superconductivity is at x = 0.133 with all three phases coexisting until x >~ 0.24.
The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by inelastic X-ray scattering measurement at room temperature. K doping induces the softening and broadening of phonon modes in the energy range E = 10-15 meV. Analysis with a Born-von Karman force-constant model indicates that the softening results from reduced interatomic force constants around (Ba,K) sites following the displacement of divalent Ba by monovalent K. The phonon broadening may be explained by the local distortions induced by the K substitution. Extra phonon modes are observed around the wave vector q = (0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either from the local disorder induced by K doping or from electron-phonon coupling.
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