arXiv subjects

T. Klein

Publications and source records attributed to T. Klein.

50 records · Page 3Linked to original sources

arXiv2009

Low temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by Gas Immersion Laser Doping. The doping concentration cB has been varied up to approx. 10 at.% by increasing the number of laser shots to 500. No superconductivity could be observed down to 40mK for doping level below 2.5 at.%. The critical temperature Tc then increased steeply to reach 0.6K for cB = 8 at%. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field Hc2(0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. in Nature (London) 444, 465 (2006).

cond-mat.supr-con↗

arXiv2008

Upper and lower critical fields in NdFeAs(O,F) single crystals : a study by Hall probe magnetization and specific heat

The upper and lower critical fields have been deduced from specific heat and Hall probe magnetization measurements in non-optimally doped NdFeAs(O,F) single crystals ($T_c \sim 32-35$K). The anisoptropy of the penetration depth ($Γ_λ$) is temperature independent and on the order of $4.0 \pm 1.5$. Similarly specific heat data lead an anisotropy of the coherence lenght $Γ_ξ\sim 5.5 \pm 1.5$ close to $T_c$. Our results suggest the presence of rather large thermal fluctuations and to the existence of a vortex liquid phase over a broad temperature range ($\sim 5$K large at 2T).

cond-mat.supr-con↗

arXiv2006

Influence of Al doping on the critical fields and gap values in magnesium diboride single crystals

The lower ($H_{c1}$) and upper ($H_{c2}$) critical fields of Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, 0.1 and $\gtrsim 0.2$) have been deduced from specific heat and local magnetization measurements, respectively. We show that $H_{c1}$ and $H_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter $Γ_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \gtrsim 0.2$ whereas $Γ_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.

cond-mat.supr-con↗

arXiv2006

Tunneling Spectroscopy and Vortex Imaging in Boron-Doped Diamond

We present the first scanning tunneling spectroscopy study of single-crystalline boron doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak coupling BCS law with $Δ(0)/k_B T_c \simeq 1.74$. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.

cond-mat.supr-con↗

arXiv2005

Concentration around the mean for maxima of empirical processes

In this paper we give optimal constants in Talagrand's concentration inequalities for maxima of empirical processes associated to independent and eventually nonidentically distributed random variables. Our approach is based on the entropy method introduced by Ledoux.

math.PR↗

arXiv2003

Two-band Effects in the Angular Dependence of Hc2 of MgB2 Single Crystals

The angular dependence of the upper critical field Hc2 of MgB2 single crystals is studied at various temperatures by means of specific heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular- and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a new pathway to the study of disorder and doping effects in MgB2.

cond-mat.supr-con↗

arXiv2003

Anisotropies of the lower and upper critical fields in MgB$_2$ single crystals

The temperature dependence of the London penetration depth ($λ$) and coherence length ($ξ$) has been deduced from Hall probe magnetization measurements in high quality MgB$_2$ single crystals in the two main crystallographic directions. We show that, in contrast to conventional superconductors, MgB$_2$ is characterized by two different anisotropy parameters ($Γ_λ= λ_c/λ_{ab}$ and $Γ_ξ= ξ_{ab}/ξ_c$) which strongly differ at low temperature and merge at $T_c $. These results are in very good agreement with recent calculations in weakly coupled two bands suprerconductors (Phys. Rev. B, 66, 020509(R) (2002).

cond-mat.supr-con↗

arXiv2002

Point-contact spectroscopy of MgB2

Point-contact spectroscopy measurements on magnesium diboride reveal the existence of two superconducting energy gaps closing at the same transition temperature in line with the multiband model of superconductivity. The sizes of the two gaps ($Δ_π$ = 2.8 meV and $Δ_σ$ = 6.5-7 meV) are respectively smaller and larger than the expected weak coupling value of the one-gap superconductor. The smaller gap is rapidly filled by a small magnetic field of about 1-2 Tesla much lower than the real upper critical field $H_{c2}$. The larger gap is closed at the anisotropic $H_{c2}$. Above the gap energies reproducible non-linearities are observed at the characteristic phonon energies of MgB$_2$.

cond-mat.supr-con↗

arXiv2002

Anisotropy of the Upper Critical Field and Critical Current in Single Crystal MgB$_2$

We report on specific heat, high magnetic field transport and $ac-$susceptibility measurements on magnesium diboride single crystals. The upper critical field $H_{c2}$ for magnetic fields perpendicular and parallel to the Mg and B planes is presented for the first time in the entire temperature range. A very different temperature dependence has been observed in the two directions which yields to a temperature dependent anisotropy with $Γ\sim$ 5 at low temperatures and about 2 near $T_c$. A peak effect is observed in susceptibility measurements for $H \sim $2 T parallel to the $c-$axis and the critical current density presnts a sharp maximum for $H$ parallel to the ab-plane.

cond-mat.supr-con↗

arXiv2002

Superconducting Transition and Phase Diagram of Single Crystal MgB2

The superconducting phase diagram of MgB2 was determined from magnetization, magneto-transport and the first single-crystal specific heat measurements. A zero-temperature in-plane coherence length of 8 nm is determined. The superconducting anisotropy increases from a value around 2 near Tc to above 4.5 at 22 K. For H||c a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for H||c which might account for the wide spread in reported values of the anisotropy.

cond-mat.supr-con↗

arXiv2002

Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)Bi03

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K,Ba)BiO$_3$ single crystals. The temperature dependence of the magnetic field $H_{Cp}$ corresponding to the onset of the specific heat anomaly presents a clear positive curvature. $H_{Cp}$ is significantly smaller than the field $H_Δ$ for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization present a strong deviation from the Ginzburg--Landau theory emphasazing the peculiar nature of the superconducting transition in this material.

cond-mat.supr-con↗

arXiv2001

A Bragg glass phase in the vortex lattice of a type II superconductor

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of the crystalline order. The resulting state of matter has been a longstanding puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into crystallites. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The lattice of vortices that can contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron diffraction data of the vortex lattice in type II superconductors provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transition and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and conductivity of electronic crystals.

cond-mat.supr-con↗

arXiv2001

Magnetotransport and the upper critical magnetic field in MgB2

Magnetotransport measurements are presented on polycrystalline MgB2 samples. The resistive upper critical magnetic field reveals a temperature dependence with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper critical field is much higher than what is known so far on polycrystals of MgB2 but it is in agreement with recent data obtained on epitaxial MgB2 films. The deviation of Bc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.

cond-mat.supr-con↗

arXiv2001

Upper critical field in {Ba$_{1-x}$K$_x$BiO$_3$}: magnetotransport versus magnetotunneling

Elastic tunneling is used as a powerful direct tool to determine the upper critical field $H_{c2}(T)$ in the high-$T_c$ oxide Ba$_{1-x}$K$_x$BiO$_3$. The temperature dependence of $H_{c2}$ inferred from the tunneling follows the Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A comparison will be made with resistively determined critical field data.

cond-mat.supr-con↗