arXiv subjects

T. Schneider

Publications and source records attributed to T. Schneider.

At least 73 records · Page 4Linked to original sources

arXiv2006

On the occurrence of Kosterlitz-Thouless behavior in cuprate superconductors

The observation of the characteristic Kosterlitz-Thouless behavior requires the attaintment of the two dimensional limit where the correlation-length anisotropy, gamma=gsiab/gsic, diverges. Our findings strongly suggest that the failure of several experiments on films and single crystals to observe any trace of KT-behavior is attributable either to inhomogeneities or doping by means of chemical substitution.

cond-mat.supr-con↗

arXiv2006

Electric field effect modulation of transition temperature, mobile carrier density and in-plane penetration depth in NdBa2Cu3O(7-delta) thin films

We explore the relationship between the critical temperature, T_c, the mobile areal carrier density, n_2D, and the zero temperature magnetic in-plane penetration depth, lambda_ab(0), in very thin underdoped NdBa2Cu3O{7-delta} films near the superconductor to insulator transition using the electric field effect technique. We observe that T_c depends linearly on both, n_2D and lambda_ab(0), the signature of a quantum superconductor to insulator (QSI) transition in two dimensions with znu-bar where z is the dynamic and nu-bar the critical exponent of the in-plane correlation length.

cond-mat.supr-con↗

arXiv2005

Magnetic field induced finite size effect in type-II superconductors

We explore the occurrence of a magnetic field induced finite size effect on the specific heat and correlation lengths of anisotropic type-II superconductors near the zero field transition temperature Tc. Since near the zero field transition thermal fluctuations are expected to dominate and with increasing field strength these fluctuations become one dimensional, whereupon the effect of fluctuations increases, it appears unavoidable to account for thermal fluctuations. Invoking the scaling theory of critical phenomena it is shown that the specific heat data of nearly optimally doped YBa2Cu3O7-x are inconsistent with the traditional mean-field and lowest Landau level predictions of a continuous superconductor to normal state transition along an upper critical field Hc2(T). On the contrary, we observe agreement with a magnetic field induced finite size effect, whereupon even the correlation length longitudinal to the applied field H cannot grow beyond the limiting magnetic length L(H). It arises because with increasing magnetic field the density of vortex lines becomes greater, but this cannot continue indefinitely. L(H) is then roughly set on the proximity of vortex lines by the overlapping of their cores. Thus, the shift and the rounding of the specific heat peak in an applied field is traced back to a magnetic field induced finite size effect in the correlation length longitudinal to the applied field.

cond-mat.supr-con↗

arXiv2005

Comment to: Correlation Between Superfluid Density and T_{c} of Underdoped YBa2Cu3O6+x Near the Superconductor-Insulator Transition

Zuev et al. reported measurements of the in-plane magnetic penetration depth of supposedly underdoped films of YBa2Cu3O6+x, with Tc from 6 to 50 K. They argue that their data is inconsistent with a linear relationship between Tc and the inversw of the T=0 in-plane penetration depth squared, as expected for a two dimensional quantum superconductor to insulator (2D-QSI) transition where Tc vanishes and the anisotropy diverges. It is shown that the data of Zuev et al. do not fall onto the regime where the linear relationship applies, because the observed reduction of Tc is not associated with an increase of the anisotropy.

cond-mat.supr-con↗

arXiv2005

Frequency comparisons and absolute frequency measurements of 171Yb+ single-ion optical frequency standards

We describe experiments with an optical frequency standard based on a laser cooled $^{171}$Yb$^+$ ion confined in a radiofrequency Paul trap. The electric-quadrupole transition from the $^2S_{1/2}(F=0)$ ground state to the $^2D_{3/2}(F=2)$ state at the wavelength of 436 nm is used as the reference transition. In order to compare two $^{171}$Yb$^+$ standards, separate frequency servo systems are employed to stabilize two probe laser frequencies to the reference transition line centers of two independently stored ions. The experimental results indicate a relative instability (Allan standard deviation) of the optical frequency difference between the two systems of $σ_y(1000 {\rm s})=5\cdot 10^{-16}$ only, so that shifts in the sub-hertz range can be resolved. Shifts of several hertz are observed if a stationary electric field gradient is superimposed on the radiofrequency trap field. The absolute optical transition frequency of Yb$^+$ at 688 THz was measured with a cesium atomic clock at two times separated by 2.8 years. A temporal variation of this frequency can be excluded within a $1σ$ relative uncertainty of $4.4\cdot 10^{-15}$ yr$^{-1}$. Combined with recently published values for the constancy of other transition frequencies this measurement provides a limit on the present variability of the fine structure constant $α$ at the level of $2.0\cdot 10^{-15}$ yr$^{-1}$.

physics.atom-ph↗

arXiv2005

Evidence for a smooth superconductor to normal state transition for nonzero applied magnetic field in Sr0.9La0.1CuO2

We present and analyze field cooled magnetization data, revealing for 0<H<5 T remarkable consistency with a magnetic field induced finite size effect. It is traced back to the fact that the correlation length gsi cannot grow beyond the limiting length scale LH set by the magnetic field, where at temperature Tp(H), gsi(T) = LH. Thus, in sufficiently homogeneous samples and nonzero H the transition from the superconducting to the normal state turns out to be smooth and the appropriately scaled magnetization data fall near Tp(H) on a universal curve. Consistent with the generic behavior of optimally doped cuprates we also show that the pressure effect on Tc is negligibly small, while the negative value of the relative volume change mirrors that of the ansisotropy.

cond-mat.supr-con↗

arXiv2005

Pressure effects on the superconducting properties of YBa_2Cu_4O_8

Measurements of the magnetization under high hydrostatic pressure (up to 10.2 kbar) in YBa_2Cu_4O_8 were carried out. From the scaling analysis of the magnetization data the pressure induced shifts of the transition temperature T_c, the volume V and the anisotropy γhave been obtained. It was shown that the pressure induced relative shift of T_c mirrors essentially that of the anisotropy. This observation uncovers a novel generic property of anisotropic type II superconductors, that inexistent in the isotropic case.

cond-mat.supr-con↗

arXiv2005

Evidence for a smooth superconductor to normal state transition for nonzero applied magnetic field in RbOs2O6

The effect of the magnetic field on the critical behavior of RbOs2O6 is investigated. We present and analyze magnetization data, revealing remarkable consistency with a magnetic field induced finye size effect. It is traced back that at temperatures Tp(H) smaller than Tc the correlation length cannot grow beyond the limiting length scale set by the magnetic field. Thus, for nonzero H the transition from the superconducting to the normal state turns out to be smooth and the apropriately scaled magnetization data fall on a single curve.

cond-mat.supr-con↗

arXiv2005

Pressure and isotope effect on the anisotropy of MgB$_{2}$

We analyze the data for the pressure and boron isotope effect on the temperature dependence of the magnetization near $T_{c}$. Invoking the universal scaling relation for the magnetization at fixed magnetic field it is shown that the relative shift of $T_{c}$, induced by pressure or boron isotope exchange, mirrors essentially that of the anisotropy. This uncovers a novel generic property of anisotropic type II superconductors, inexistent in the isotropic case. For MgB$_{2}$ it implies that the renormalization of the Fermi surface topology due to pressure or isotope exchange is dominated by a mechanism controlling the anisotropy.

cond-mat.supr-con↗

arXiv2004

New limit on the present temporal variation of the fine structure constant

The comparison of different atomic transition frequencies over time can be used to determine the present value of the temporal derivative of the fine structure constant alpha in a model-independent way without assumptions on constancy or variability of other parameters. We have measured an optical transition frequency at 688 THz in ^{171}Yb+ with a cesium atomic clock at two times separated by 2.8 years and find a value for the fractional variation of the frequency ratio $f_{\rm Yb}/f_{\rm Cs}$ of $(-1.2\pm 4.4)\cdot 10^{-15}$ yr$^{-1}$, consistent with zero. Combined with recently published values for the constancy of other transition frequencies this measurement sets an upper limit on the present variability of alpha at the level of $2.0\cdot 10^{-15}$ yr$^{-1}$, corresponding so far to the most stringent limit from laboratory experiments.

physics.atom-ph↗

arXiv2004

Implications of the isotope effects on the magnetization, magnetic torque and susceptibility

We analyze the magnetization, magnetic torque and susceptibility data of La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal 3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that on the anisotropy. Invoking the generic behavior of the anisotropy the doping dependence of the isotope effects on the critical properties, including Tc, correlation lengths and magnetic penetration depths are traced back to a change of the mobile carrier concentration.

cond-mat.supr-con↗

arXiv2004

Evidence for charged critical behavior in the pyrochlore superconductor RbOs2O6

We analyze magnetic penetration depth data of the recently discovered superconducting pyrochlore oxide RbOs2O6. Our results strongly suggest that in RbOs2O6 charged critical fuctuations dominate the temperature dependence of the magnetic penetration depth near Tc. This is in contrast to the mean-field behavior observed in conventional superconductors and the uncharged critical behavior found in nearly optimally doped cuprate superconductors. However, this finding agrees with the theoretical predictions for charged criticality and the charged criticality observed in underdoped YBa2Cu3O6.59.

cond-mat.supr-con↗

arXiv2004

Evidence for charged critical fluctuations in underdoped Y-123

We report and analyze in-plane penetration depth measurements in YBa2Cu3O7-x taken close to the critical temperature Tc. In underdoped YBa2Cu3O6:59 we find consistent evidence for charged critical behavior. Noting that the effective dimensionless charge scales as 1/sqrt(Tc), this new critical behavior should be generically observable in suitably underdoped cuprates.

cond-mat.supr-con↗

arXiv2004

Relationship between and implications of the isotope and pressure effects on transition temperature, penetration depths and conductivities

It is shown that the empirical relations between transition temperature, normal state conductivity linearly extrapolated to the value at the transition temperature, zero temperature penetration depths, etc., as observed in a rich variety of cuprate superconductors, are remarkably consistent with the universal critical properties of anisotropic systems which fall into the 3D-XY universality class and undergo a crossover to a quantum critical point in 2D. The variety includes n- and p-type cuprates, comprises the underdoped and overdoped regimes and the consistency extends up to six decades in the scaling variables. The resulting scaling relations for the oxygen isotope and hydrostatic pressure effects agree with the experimental data and reveal that these effects originate from local lattice distortions preserving the volume of the unit cell. These observations single out 3D and anisotropic microscopic models which incorporate local lattice distortions, fall in the experimentally accessible regime into the 3D-XY universality class, and incorporate the crossover to 2D quantum criticality where superconductivity disappears.

cond-mat.supr-con↗

arXiv2004

Implications evinced by the phase diagram, anisotropy, magnetic penetration depths, isotope effects and conductivities of cuprate superconductors

Anisotropy, thermal and quantum fluctuations and their dependence on dopant concentration appear to be present in all cuprate superconductors, interwoven with the microscopic mechanisms responsible for superconductivity. Here we review anisotropy, in-plane and c-axis penetration depths, isotope effect and conductivity measurements to reassess the universal behavior of cuprates as revealed by the doping dependence of these phenomena and of the transition temperature.

cond-mat.supr-con↗

arXiv2004

Comparison of two single-ion optical frequency standards at the sub-Hertz level

We describe experimental investigations on an optical frequency standard based on a laser cooled 171Yb+ ion confined in a radiofrequency Paul trap. The electric-quadrupole transition from the 2S1/2(F=0) ground state to the 2D3/2(F=2) state at the wavelength of 436 nm is used as the reference transition. The reference transition is probed by a frequency-doubled, frequency-stabilized diode laser and is resolved with a Fourier-limited full halfwidth of approximately 30 Hz. In order to compare two 171Yb+ standards, separate frequency shift and servo systems are employed to stabilise the probe frequency to the reference transition line centers of two independently stored 171Yb+ ions. The present experimental results indicate a relative instability (Allan standard deviation)of the optical frequency difference between the two systems of sigma_y(1000 s)=1.0E-15 and a mean frequency difference of 0.2 Hz. Shifts in the range of several Hertz are observed in the frequency difference if a stationary electric field gradient is superimposed on the radiofrequency trap field. This measurement permits a first experimental estimate of the electric quadrupole moment of the 2D3/2 state of Yb+.

physics.atom-ph↗

arXiv2004

Finite-size and pressure effects in YBa_2Cu_4O_8 probed by magnetic field penetration depth measurements

We explore the combined pressure and finite-size effects on the in-plane penetration depth λ_{ab} in YBa_2Cu_4O_8. Even though this cuprate is stoichiometric the finite-size scaling analysis of λ_{ab}^{-2}(T) uncovers the granular nature and reveals domains with nanoscale size L_{c} along the c-axis. L_{c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence of a phase coherent macroscopic superconducting state in cuprate superconductors.

cond-mat.supr-con↗

arXiv2004

Evidence for granularity, anisotropy and lattice distortions in cuprate superconductors and their implications

Granularity, anisotropy, local lattice distortions and their dependence on dopant concentration appear to be present in all cuprate superconductors, interwoven with the microscopic mechanisms responsible for superconductivity. Here we review anisotropy and penetration depth measurements to reassess the evidence for granularity, as revealed by the notorious rounded phase transition, the evidence for the three dimensional nature of superconductivity, uncovered by the doping dependence of transition temperature and anisotropy, and to reassess the relevance of the electron-lattice coupling, emerging from the oxygen isotope effects.

cond-mat.supr-con↗