Upper critical fields of the superconducting layered compounds Nb1?x Ta x Se2

Dalrymple, B.J.; Prober, D. E.

doi:10.1007/bf00681811

Cited by 47 publications

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“…Previous work on this material system [8] shows that substitution of 3% of Ta suppresses the CDW transition found in pure NbSe2 and further substitution decreases T^ so that for jc=0.2, T^ =5 K. Since Nb and Ta are isoelectronic and have similar atomic radii, large changes in the electronic properties are not expected with alloying; however, the mean free path is found to systematically decrease with increased Ta substitution [9]. Thus upon substitution one can continuously vary the properties from those of a clean superconductor to one approaching the dirty limit.…”

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confidence: 67%

“…Table I lists the material parameters for the samples used in this study. The Ginzburg-Landau coherence length parallel to the planes, ^ii(r=0), has been determined from the upper-critical-field measurements of Dalrymple and Prober [9]. The mean free path was calculated from the low-temperature resistivity PLT (determined just above Tc) measured using a four-point method on the same crystals used in the STM studies.…”

Section: N(er)^rc (E-id^-a'mentioning

confidence: 99%

“…Single crystals are grown by the usual iodineassisted chemical transport reaction at 660-720 °C. The samples are cleaved in air just prior to installation in our low-temperature STM [9] and topographic images of the surface routinely show atomic resolution with large flat plateaus even up to Ixl-^m^-size scales. The STM is situated in the bore of an 8.5-T superconducting magnet and is placed in 1.5 mTorr of He exchange gas in a vacuum can which separates it from the helium bath.…”

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Scanning tunneling spectroscopy of a vortex core from the clean to the dirty limit

et al. 1991

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The local density of states of a superconducting vortex core has been measured as a function of disorder in the alloy system Nbi -vTavSei using a low-temperature scanning tunneling microscope. The peak observed in the zero-bias conductance at a vortex center is found to be very sensitive to disorder. As the mean free path is decreased by substitutional alloying the peak gradually disappears and for x=0.2 the density of states in the vortex center is found to be equal to that in the normal state. The vortex-core spectra hence may provide a sensitive measure of the quasiparticle scattering time. PACS numbers: 74.50.+r, 61.16.Di Recent low-temperature scanning tunneling microscope (STM) studies of the vortex lattice in 2//-NbSe2 [1] have revived theoretical [2-5] and experimental [6] interest in the electronic structure of flux lines in type-II superconductors. The STM permits a direct measurement of the local density of states with atomic resolution. Prior to these first STM measurements, the spatial variation of the density of states in the mixed state of a superconductor was inaccessible to experimental techniques. This quantity is obtained by measuring the local tunneling conductance versus bias voltage as a function of position. It is in fact this spectroscopic capability of the STM which allows one to diflferentiate between core regions and superconducting regions in the mixed state and, hence, to obtain an image of the vortex lattice in a superconductor. The first observation of the Abrikosov vortex lattice using an STM by Hess et al.[1] relied on the change in differential conductance at the energy gap of the superconductor to provide the contrast. These first measurements also revealed an unexpected peak in the differential conductance at zero bias at the center of a vortex. This peak has since been associated with lowlying quasiparticle bound states in the vortex core [2][3][4] and qualitative agreement between the experimental conductance curves and theory has been obtained. In addition, new radial structure in the vortex-core density of states has been predicted [2] and subsequently confirmed by experiment [7].Although low-temperature STM holds promise as a powerful local probe of superconductivity, only pure 2H-NbSe2 has been investigated by this technique at present. NbSe2 is a clean type-II superconductor with a transition temperature (Tc) of 7.2 K and a charge-density-wave (CDW) transition at 33 K which creates a gap in a fraction of the Fermi surface at low temperature. In this work we consider the effect of changing the superconductor's material properties in a systematic manner on the vortex-core spectra. In particular, we have investigated the evolution of the zero-bias conductance peak (ZBCP) observed in the center of a vortex core as a function of disorder.We have studied a range of compositions of the layered compound 2//-Nbi-vTa.vSe2 from x=0 to x=0.2. Previous work on this material system [8] shows that substitution of 3% of Ta suppresses the CDW transition found in pure NbSe2 and further su...

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Section: N(er)^rc (E-id^-a'mentioning

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Scanning tunneling spectroscopy of a vortex core from the clean to the dirty limit

et al. 1991

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“…It was also taken into account by Takanaka 18 9,15,19,20,22,23 For example, a detailed theory is still absent for the large positive (upward) curvature observed in H c2 ͑T Շ T c ͒ of layered superconductors, 55,56 except a qualitative description by Takanaka 18 and Dalrymple and Prober. 57 Based on these observations, we here derive an H c2 equation that is numerically tractable for arbitrary crystal structures and field directions by using Fermi surfaces from ab initio electronic-structure calculations. This kind of calculations has been performed only for Nb by Butler so far.…”

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Ab initiocalculations ofHc2in type-II superconductors: Basic formalism and model calculations

Kita

Arai

2004

Phys. Rev. B

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Detailed Fermi-surface structures are essential to describe the upper critical field H c2 in type-II superconductors, as first noticed by and Hohenberg and Werthamer [Phys. Rev. 153, 493 (1967)] and shown explicitly by Butler for high-purity cubic Niobium [Phys. Rev. Lett. 44, 1516Lett. 44, (1980]. We derive an H c2 equation for classic type-II superconductors that is applicable to systems with anisotropic Fermi surfaces and/or energy gaps under arbitrary field directions. It can be solved efficiently by using Fermi surfaces from ab initio electronicstructure calculations. Thus, it is expected to enhance our quantitative understanding on H c2 . Based on the formalism, we calculate H c2 curves for Fermi surfaces of a three-dimensional tight-binding model with cubic symmetry, an isotropic gap, and no impurity scatterings. It is found that, as the Fermi surface approaches to the Brillouin zone boundary, the reduced critical field h * ͑T / T c ͒, which is normalized by the initial slope at T c , is enhanced significantly over the curve for the spherical Fermi surface with a marked upward curvature. Thus, the Fermi-surface anisotropy can be a main source of the upward curvature in H c2 near T c .

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“…Since cold working introduces defects such as dislocations into the atomic crystal lattice, one may attempt to attribute the upward shift in the onset ® eld of the peak e ect to an increase of H c2 (T). It was known (Dalrymple and Prober 1984) that by replacing 2 at.% of Nb with Ta in 2H± NbSe 2 one can raise T c by 0.2 K. Such T c enhancement was believed to be due to the suppression of charge density waves by random Ta impurities (Dalrymple and Prober 1984). To determine whether a similar e ect can be induced by cold working, we measured (using a commercial Quantum Design SQUID magnetometer) the dc magnetization of a faceted 2H± NbSe 2 crystal before and after cold working (bending and¯attening), as shown in ® gure 3.…”

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Equilibrium and driven vortex phases in the anomalous peak effect

Ling¹,

Smullin²,

Berger³

et al. 1999

Philosophical Magazine Letters

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We report experimental tests of the present models of the peak e ect in weakly disordered type-II superconductors. Our results favour the scenario in which the peak e ect arises from a crossover between the Larkin pinning length and a rapidly falling elastic length in a vortex phase populated with thermally excited topological defects. A thickness dependence study of the onset of the peak e ect at varying driving currents suggests that both screw and edge dislocations are involved in the vortex lattice disordering. The driven dynamics in threedimensional samples are drastically di erent from those in two-dimensional samples. We suggest that this may be a consequence of the absence of a Peierls potential for screw dislocations in a three-dimensional vortex line lattice.

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Upper critical fields of the superconducting layered compounds Nb1?x Ta x Se2

Cited by 47 publications

References 44 publications

Scanning tunneling spectroscopy of a vortex core from the clean to the dirty limit

Scanning tunneling spectroscopy of a vortex core from the clean to the dirty limit

Ab initiocalculations ofHc2in type-II superconductors: Basic formalism and model calculations

Equilibrium and driven vortex phases in the anomalous peak effect

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