arXiv subjects

G. Deutscher

Publications and source records attributed to G. Deutscher.

25 records · Page 2Linked to original sources

arXiv2002

Study of the Order Parameter symmetry in overdoped Y1-xCaxBa2Cu3O7-delta by measurement of Andreev reflection, evidence for enhancement of the sub dominant order parameter amplitude in high transparency contacts

We have measured the differential Conductance of Au/Y1-xCaxBa2Cu3O7-delta point contacts in the regime where it is dominated by Andreev reflection, which enhances its value at low bias. We find that the characteristics can not be fitted by a pure d-wave Order Parameter (OP). Using the formalism developed by Kashiwaya and Tanaka[1], the best fits are obtained by adding a sub-dominant imaginary OP, whose amplitude appears to depend on the transparency of the contact. At high transparencies it can reach up to 60% of the total amplitude of the OP. for lower transparencies it is substantially smaller. We attribute this enhancement, at high transparencies to a proximity effect between the Au tip and the superconducting electrode

cond-mat.supr-con↗

arXiv2002

Origin of hysteretic field splitting of the Zero Bias Conductance Peak in (100) and (110) oriented Y1Ba2Cu3O7-x films

We have studied the evolution of the Zero-Bias Conductance Peak (ZBCP) splitting under applied magnetic fields in tunneling experiments on Y1Ba2Cu3O7-x(YBCO), and particular its hysteresis. We have been able to distinguish between two possible contributions to the splitting. One of them is connected to Meissner screening currents whose variation in increasing fields is governed by the Bean-Livingston barrier that delays flux entry well above the lower thermodynamical critical field Hc1, up to the fields of the order of the thermodynamical critical field Hc. The other contribution, dominant in (110) oriented films, is seen in decreasing fields where there are no Meissner screening currents, since there is no barrier to flux exit and it may be connected to the magnetic induction in the sample as proposed by Laughlin.

cond-mat.supr-con↗

arXiv2002

Using a hybrid superconducting-ferromagnetic tip as a magnetic scanning tunneling microscope

Approaching a two-component tip made of a superconductor (S) and a ferromagnet (F) from a magnetic sample allows for two distinct tunneling processes between the ferromagnets, through S: i) Charge and spin are conserved; ii) Charge and spin are reversed, e.g. a Cooper pair flows from S, one electron going into F, the other into the sample. At subgap voltages, this allows two currents to flow from the tip : one is insensitive to the spin polarizations and allows for surface topography, the other directly tracks the relative spin polarizations of F and the sample. The whole device acts as a STM sensitive to the spin polarization at the Fermi level (MSTM). Its sensitivity is studied and optimized with respect to the tip geometry.

cond-mat.mes-hall↗

arXiv2001

Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter

Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy order parameter. The subdominant (is or idxy) component manifests itself in a splitting of the zero bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy (STM), and in macroscopic planar junctions, for films prepared by either RF sputtering or laser ablation. The STM measurements exhibit fairly uniform splitting size in [110] oriented areas on the order of 10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter.

cond-mat.supr-con↗

arXiv2001

Thickness independence of field induced time reversal symmetry breaking in Y/sub1 Ba/sub2 Cu/sub3 O/sub7-delta thin films

We have measured the field dependence of the Zero Bias Conductance Peak (ZBCP) of tunnel junctions prepared on (110) oriented Y1Ba2Cu3O7-d films of various thickness. We have found that the field splitting of the ZBCP, believed to be due to the Time Reversal Symmetry Breaking (TRSB) of Andreev surface bound states, is independent of film thickness. This rules out that Meissner screening currents are at the origin of TRSB. We conclude that TRSB must, instead, be due to a field-induced modification of the Order parameter. Different possibilities involving an imaginary component are discussed.

cond-mat.supr-con↗

arXiv2001

Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.

cond-mat.supr-con↗

arXiv2000

Effect of doping on the linear temperature dependence of the magnetic penetration depth in cuprate superconductors

We report on measurements of the low temperature dependence of the magnetic penetration depth lambda_ab in several single crystals of La_2-xSr_xCuO_4, Bi_2Sr_2CaCu_2O_8 and YBa_2Cu_3O_6+x at various doping levels ranging from the under- to the overdoped regimes by using a novel single-coil technique achieving 10 pm resolution. We have found a linear dependence of lambda_ab in all samples with a rapidly increasing slope dlambda_ab/dT as doping decreases. Our analysis of the data indicates that a superconducting gap with d-wave symmetry is sufficient to quantitatively account for the above slope values in the optimally or over- doped samples. In the underdoped samples, the d-wave model predicts much smaller values than those measured by assuming realistic values for the zero-temperature lambda_ab and gap delta. The experimental data are compatible with a model of thermodynamic phase fluctuations of the order parameter. Therefore, we put forward the hypothesis that the gapless properties observed in cuprates may have qualitatively different physical origins depending on the doping level.

cond-mat.supr-con↗