arXiv subjects

R. Lortz

Publications and source records attributed to R. Lortz.

36 records · Page 2Linked to original sources

arXiv2010

Multi-band Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8

Sub-Kelvin scanning tunnelling spectroscopy in the Chevrel Phases SnMo6S8 and PbMo6S8 reveals two distinct superconducting gaps with Delta_1 = 3 meV, Delta_2 ~ 1.0 meV and Delta_1 = 3.1 meV, Delta_2 ~ 1.4 meV respectively. The gap distribution is strongly anisotropic, with Delta_2 predominantly seen when scanning across unit-cell steps on the (001) sample surface. The spectra are well-fitted by an anisotropic two-band BCS s-wave gap function. Our spectroscopic data are confirmed by electronic heat capacity measurements which also provide evidence for a twin-gap scenario.

cond-mat.supr-con↗

arXiv2010

A second superconducting energy gap of Nb_3Sn observed by breakjunction point-contact spectroscopy

We report on investigations of the superconducting energy gap of the A15 superconductor Nb_3Sn by point-contact spectroscopy of breakjunctions. The voltage-dependent differential conductance dI/dV reveals features of a second energy gap besides the energy gap known from previous tunnel measurements with maxima at Δ_1=3.92 +/- 0.16 meV and Δ_2=0.85 +/- 0.17 meV as derived from a histogram summarizing the data of more than 60 contacts. These findings are the first spectroscopic evidence that Nb_3Sn belongs to the class of two-band superconductors and they are in line with low-temperature specific-heat measurements on Nb_3Sn.

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arXiv2009

Fingerprint of dynamical charge/spin correlations in the tunneling spectra of colossal magnetoresistive manganites

We present temperature-dependent scanning tunneling spectroscopy measurements on $La_{1-x}Ca_{x}MO_{3}$ ($x\sim0.33$) films with different degrees of biaxial strain. A depletion in normalized conductance around the Fermi level is observed both above and below the insulator-to-metal transition temperature $T_{MI}$, for weakly as well as highly-strained films. This pseudogap-like depletion globally narrows on cooling. The zero-bias conductance decreases on cooling in the insulating phase, reaches a minimum close to $T_{MI}$ and increases on cooling in the metallic phase, following the trend of macroscopic conductivity. These results support a recently proposed scenario in which dynamical short-range antiferromagnetic/charge order correlations play a preeminent role in the transport properties of colossal magnetoresistive manganites [R. Yu \textit{et al}., Phys. Rev. B \textbf{77}, 214434 (2008)].

cond-mat.str-el↗

arXiv2009

The Fulde-Ferrell-Larkin-Ovchinnikov State in the Organic Superconductor k-(BEDT-TTF)2Cu(NCS)2 as Observed in Magnetic Torque Experiments

We present magnetic-torque experiments on the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21 T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pro-nounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde-Ferrell-Larkin-Ovchinnikov state.

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arXiv2009

Real-space vortex glass imaging and the vortex phase diagram of SnMo6S8

Using scanning tunnelling microscopy at 400mK, we have obtained maps of around 100 vortices in SnMo6S8 from 2 - 9T. The orientational and positional disorder at 5 and 9T show that these are the first large-scale images of a vortex glass. At higher temperature a magnetisation peak effect is observed, whose upper boundary coincides with a lambda anomaly in the specific heat. Our data favour a kinetic glass description of the vortex melting transition, indicating that vortex topological disorder persists at fields and temperatures far below the peak effect in low-Tc superconductors.

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arXiv2008

On the importance of the electron-phonon coupling function on the superconducting transition temperature in dodecaboride superconductors: A comparison of LuB12 with ZrB12

We report a detailed study of specific heat, electrical resistivity and optical spectroscopy in the superconducting boride LuB12 (Tc = 0.4 K) and compare it to the higher Tc compound ZrB12 (Tc = 6 K). Both compounds have the same structure based on enclosed metallic Lu or Zr ions in oversized boron cages. The infrared reflectivity and ellipsometry in the visible range allow us to extract the optical conductivity from 6 meV to 4 eV in the normal state from 20 to 280 K. By extracting the superconducting properties, phonon density of states and electron-phonon coupling function from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds. The phonon density of states seems to be insignificantly modified by substitution of Zr with Lu. However, the soft vibrations of the metal ions in boron cages, responsible for the relatively high Tc in ZrB12, have almost no contribution to the electron-phonon coupling in LuB12.

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arXiv2007

Calorimetric Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in the Layered Organic Superconductor κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$

The specific heat of the layered organic superconductor $κ$-% (BEDT-TTF)$_2$Cu(NCS)$_2$, where BEDT-TTF is bisethylenedithio-% tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper critical field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.

cond-mat.supr-con↗

arXiv2006

Thermodynamic origin of the peak effect in the superconductor Nb3Sn

We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb3Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can however be restored by a small AC field. In this case a first-order vortex melting transition with a clear hysteresis is found. In the absence of an AC field the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.

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arXiv2006

Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta

In- and out-of-plane optical spectra and specific heat measurements for the single layer cuprate superconductor Hg-1201 at optimal doping (Tc = 97 K) are presented. Both the in-plane and out-of-plane superfluid density agree well with a recently proposed scaling relation rho_{s}=sigma_{dc}T_{c}. It is shown that there is a superconductivity induced increase of the in-plane low frequency spectral weight which follows the trend found in underdoped and optimally doped Bi-2212 and optimally doped Bi-2223. We observe an increase of optical spectral weight which corresponds to a change in kinetic energy of approximately 0.5 meV/Cu which is more than enough to explain the condensation energy. The specific heat anomaly is 10 times smaller than in YBCO and 3 times smaller than in Bi-2212. The shape of the anomaly is similar to the one observed in YBCO showing that the superconducting transition is governed by thermal fluctuations.

cond-mat.supr-con↗

arXiv2006

Thermal fluctuations and vortex melting in the classical superconductor Nb3Sn from high-resolution specific-heat measurements

The range of critical thermal fluctuations in classical bulk superconductors is extremely small and especially in low fields hardly experimentally inaccessible. With a new type of calorimeter we have been able to resolve a small lambda anomaly within a narrow temperature range around the Hc2 line. We show that the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical fluctuations. The lower onset of the fluctuation regime shows many characteristics of a continuous solid-to-liquid transition in the vortex matter. It can be driven into a first-order vortex melting transition by a small AC field which helps the vortex matter to reach equilibrium.

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arXiv2005

Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6

The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.

cond-mat.supr-con↗

arXiv2005

On the Origin of the Double Superconducting Transition in Overdoped YBa2Cu3Ox

The superconducting transition in a single overdoped, detwinned YBa2Cu3Ox (YBCO) crystal is studied using four different probes. Whereas the AC and DC magnetic susceptibilities find a dominant transition at 88 K with a smaller effect near 92 K, the specific heat and electrical resistivity reveal only a single transition at 88 K and 92 K, respectively. Under hydrostatic pressures to 0.60 GPa these two transitions shift in opposite directions, their separation increasing. The present experiments clearly show that the bulk transition lies at 88 K and originates from fully oxygenated YBCO; the 92 K transition likely arises from filamentary superconductivity in a minority optimally doped phase (< 1 %) of YBCO located at or near the crystal surface.

cond-mat.supr-con↗

arXiv2005

Specific heat, magnetic susceptibility, resistivity and thermal expansion of the superconductor ZrB12

In an attempt to clarify conflicting published data, we report new measurements of specific heat, resistivity, magnetic susceptibility, and thermal expansivity up to room temperature for the 6 K superconductor ZrB12, using well-characterized single crystals with a residual resistivity ratio >9. The specific heat gives the bulk result 2*delta(0)/(k_B*Tc)=3.7 for the superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for the Cooper pairs. The Sommerfeld constant gamma_n = 0.34 mJ K-2 gat-1 and the magnetic susceptibility xsi = -2.1 x 10^-5 indicate a low density of states at the Fermi level. The Debye temperature theta_D is in the range 1000-1200 K near zero and room temperature, but decreases by a factor of ~2 at ~35 K. The specific heat and resistivity curves are inverted to yield approximations of the phonon density of states F(omega) and the spectral electron-phonon scattering function alpha_tr^2 F(omega), respectively. Both unveil a 15 meV mode, attributed to Zr vibrations in oversized B cages, which gives rise to electron-phonon coupling. The thermal expansivity further shows that this mode is anharmonic, while the vanishingly small discontinuity at Tc establishes that the cell volume is nearly optimal with respect to Tc.

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arXiv2005

Reversible and irreversible magnetocaloric effect in the NdBa_2Cu_3O_7 superconductor in relation with specific heat and magnetization

A recently developed technique for measuring the isothermal magnetocaloric coefficient ($M_T$) is applied to the study of a superconducting NdBa$_2$Cu$_3$O$_7$ single crystal. Results are compared with magnetization ($M$) and specific heat ($C$). In the reversible region both $C$ and $M_T$ follow the scaling law of the 3D-xy universality class. The anomalies connected with flux-line lattice melting are visible on $M_T(B)$ curves as peaks and steps, similar to $C(T)$ curves yet with much smaller background. At lower temperature, in the irreversible region the $M_T(B)$ behaviour resembles more that of $M(B)$, exhibiting the "fishtail" effect. Our results confirm that the peculiarities of the phase diagram known from the high temperature superconductor YBa$_2$Cu$_3$O$_7$, e.g. vortex melting, dominance of critical fluctuations and absence of a $B_{c2}$ critical field line, are a common property of RE-123 systems.

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arXiv2004

Specific heat of Nb_3Sn: The case for a second energy gap

We present new specific heat data for Nb3Sn, a well-known technically applied superconductor with a critical temperature T_c ~ 18 K, in the temperature range from 1.2 to 200 K in zero magnetic field, and from 1.5 to 22 K in fields H <= 16 T. The particularly dense and homogeneous polycrystalline sample used for this study is characterized in detail. We determine the bulk upper critical field H_c2(T) from specific heat data, and the Sommerfeld constant Gamma from the entropy S(T). We investigate in detail a low-temperature anomaly already noticed in previous investigations in zero field, and find that this feature can be quantitatively ascribed to the presence of a second superconducting gap 2 Delta_S(0) ~ 0.8 k_B T_c, in addition to the main one 2 Delta_L(0) ~ 4.9 k_B T_c. The signature of this minor gap, which affects 7.5% of the electronic density-of-states, vanishes in fields above ~ 7 T.

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arXiv2003

High-Resolution Thermal Expansion of MgB2

The thermal expansion of polycrystalline MgB2 from 5-300 K is studied using high-resolution capacitance dilatometry. The thermal expansivity exhibits a small jump of -5.8x10-8 K-1 at Tc (in accord with expectations from the Ehrenfest relationship and published specific heat and pressure data) and a negative peak-like feature close to 5 K. No indications of any structural instabilities are observed.

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arXiv2003

Crystal-lattice coupling to the vortex-melting transition in YBa2Cu3O7-d

Distinct discontinuities in the thermal expansion of the crystal lattice are observed at the melting-transition of the vortex lattice in a naturally untwinned reversible YBa2Cu3O7-d-single crystal using high-resolution dilatometry. This coupling between the vortex transition and the crystal lattice demonstrates that the crystal lattice is more than a mere host for the vortices, and it is attributed to a strong pressure dependence of the superconducting transition temperature and thus to the condensation energy at the vortex melting temperature.

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arXiv2003

Magnetic-field-induced finite-size effect in the high-temperature superconductor YBa2Cu3O7-a comparison to rotating superfluid 4 He

The effect of strong magnetic fields (11 T) on superconductivity in YBa2Cu3O7-d is investigated using high-resolution thermal expansion. We show that the field-induced broadening of the superconducting transition is due to a finite-size effect resulting from the field-induced vortex-vortex length scale. The physics of this broadening has recently been elucidated for the closely related case of rotating superfluid 4 He [R. Haussmann, Phys. Rev. B60, 12373 (1999)]. Our results imply that the primary effect of magnetic fields of the order of 10 T is to destroy the phase coherence; the pairing, on the other hand, appears to be quite insensitive to these fields.

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