arXiv subjects

I. L. Landau

Publications and source records attributed to I. L. Landau.

At least 19 recordsLinked to original sources

arXiv2010

Determination of an onset of superconducting diamagnetism by scaling of the normal-state magnetization

We propose a simple scaling procedure for the normal-state magnetization Mn data collected as functions of temperature T in different magnetic fields H. As a result, the Mn(T) curves collected in different fields collapse on to a single Msc(T) line. In this representation, the onset of superconducting diamagnetism manifests itself by a sharp divergence of the Msc(T) curves for different H values. As will be demonstrated, this allows for a reliable determination of temperature Tonset, at which superconducting diamagnetism become observable.

cond-mat.supr-con↗

arXiv2008

Superconducting phase formation in random neck syntheses: a study of the Y-Ba-Cu-O system by magneto-optics and magnetometry

Magneto-optical imaging and magnetization measurements were applied to investigate local formation of superconducting phase effected by a random neck synthesis in Y-Ba-Cu-O system. Polished pellets of strongly inhomogeneous ceramic samples show clearly the appearance of superconducting material in the intergrain zones of binary primary particles reacted under different conditions. Susceptibility measurements allows evaluation of superconducting critical temperature, which turned out to be close to that of optimally doped YBCO.

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arXiv2008

Comparison of the scaling analysis of the mixed-state magnetization data with direct measurements of the upper critical field in Y-123

By comparison of recent direct measurements of the temperature dependence of the upper critical field $H_{c2}$ in an Y-123 high temperature superconductor with the scaling analysis of magnetization data, collected in fields H << H_c2, we demonstrate that that the temperature dependence of the Ginzburg-Landau parameter kappa is negligible. Another conclusion is that the normalized temperature dependence of H_c2 is independent of the orientation of the magnetic field in respect to crystallographic axes of the sample. We also discuss that isotropy of the temperature dependence of H_c2 straightforwardly follows from the Ginzburg-Landau theory if kappa does not depend on temperature.

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arXiv2007

Detailed magnetization study of superconducting properties of YBCO ceramic spheres

We present a magnetization study of low density YBCO ceramics carried out in magnetic fields 0.5 Oe < H < 50 kOe. It was demonstrated that superconducting links between grains may be completely suppressed either by a magnetic field of the order of 100 Oe (at low temperatures) or by an increase of temperature above 70 K. This property of present samples allowed to evaluate the ratio between an average grain size and the magnetic field penetration depth lambda. Furthermore, at temperatures T > 85 K, using low-field magnetization measurements, we could evaluate the temperature dependence of lambda, which turned out to be very close to predictions of the conventional Ginzburg-Landau theory. Although present samples consisted of randomly oriented grains, specifics of magnetization measurements allowed for evaluation of lambda_ab(T). Good agreement between our estimation of the grain size with the real sample structure provides evidence for the validity of this analysis of magnetization data. Measurements of equilibrium magnetization in high magnetic fields were used for evaluation of Hc2(T). At temperatures close to T_c, the Hc2(T) dependence turned out to be linear in agreement with the Ginzburg-Landau theory. The value of temperature, at which Hc2 vanishes, coincides with the superconducting critical temperature evaluated from low-field measurements.

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arXiv2007

On the interpretation of muon-spin-rotation experiments in the mixed state of type-II superconductors

We argue that claims about magnetic field dependence of the magnetic field penetration depth lambda, which were made on the basis of moun-spin-rotation studies of some superconductors, originate from insufficient accuracy of theoretical models employed for the data analysis. We also reanalyze some of already published experimental data and demonstrate that numerical calculations of Brandt [E.H. Brandt, Phys. Rev. B 68, 54506 (2003)] may serve as a reliable and powerful tool for the analysis of the data collected in experiments with conventional superconductors. Furthermore, one can use this approach in order to distinguish between conventional and unconventional superconductors. It is unfortunate that these calculations have practically never been employed for such analyses.

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arXiv2006

Two types of Hc2(T) dependences in Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+delta) with different Yttrium content

We reanalyze the magnetization data collected on Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+y) samples (Kim at al, Phys. Rev. B 72, 64525 (2005)) and argue that the method, which was used for the analysis of equilibrium magnetization data, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda (T), obtained in this work, are misinterpreted. Using a different approach to analysis, we demonstrate that the normalizedHc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory. Another important observation is that the Hc2(T) dependencies for two samples with different levels of doping are qualitatively different.

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arXiv2006

Temperature dependences of the upper critical field and the Ginzburg-Landau parameter of Li2Pd3B from magnetization measurements

We present temperature dependences of the upper critical magnetic field and the Ginzburg-Landau parameter for a ternary boride superconductor Li_2Pd_3B obtained from magnetization measurements. A specially developed scaling approach was used for the data analysis. The resulting H_c2(T) curve turns out to be surprisingly close to predictions of the BCS theory. The magnetic field penetration depth, evaluated in this work, is in excellent agreement with recent muon-spin-rotation experiments. We consider this agreement as an important proof of the validity of our approach.

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arXiv2006

Muon-spin-rotation measurements of the penetration depth in Li_2Pd_3B

Measurements of the magnetic field penetration depth $λ$ in the ternary boride superconductor Li$_2$Pd$_3$B ($T_c\simeq7.3$ K) have been carried out by means of muon-spin rotation ($μ$SR). The absolute values of $λ$, the Ginzburg-Landau parameter $κ$, and the first $H_{c1}$ and the second $H_{c2}$ critical fields at T=0 obtained from $μ$SR were found to be $λ(0)=252(2)$ nm, $κ(0)=27(1)$, $μ_0H_{c1}(0)=9.5(1)$ mT, and $μ_0H_{c2}(0)=3.66(8)$ T, respectively. The zero-temperature value of the superconducting gap $Δ_0=$1.31(3) meV was found, corresponding to the ratio $2Δ_0/k_BT_c=4.0(1)$. At low temperatures $λ(T)$ saturates and becomes constant below $T\simeq 0.2T_c$, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Li$_2$Pd$_3$B is a s-wave BCS superconductor with the only one isotropic energy gap.

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arXiv2006

Comment on "Hole concentration dependence of penetration depth and upper critical field in Bi2Sr2(Ca,Y)Cu2O8 extracted from reversible magnetization"

We argue that the method, which was used for the analysis of equilibrium magnetization data in the original publication, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda(T), obtained in this work, are incorrect. Using a different approach, we reanalyze the presented experimental data and demonstrate that the normalized Hc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory. Another interesting observation is that the Hc2(T) dependencies for two samples with close levels of doping are rather different.

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arXiv2005

Comment on "Enhanced two-dimensional properties of the four-layered cuprate high-Tc superconductor TlBa-2Ca-3Cu-4O-y"

We reanalyze published magnetization data and demonstrate that the conclusion of the original authors, claiming enhanced two-dimensional properties of the cuprate superconductor Tl-1234, is not supported by the experimental results. Our analysis shows that the magnetic field dependence of the mixed-state magnetization for this particular sample is amazingly close to the results of numerical calculations by E. H. Brandt for an ideal vortex lattice without fluctuations. This good agreement between experiment and theory allows for the evaluation of the absolute values of the upper critical field.

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arXiv2004

Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter kappa does not depend on temperature, the magnetic susceptibility is a universal function of H/H_c2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical fieldH_c2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of H_c2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high temperature superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.

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arXiv2004

On the interpretation of the equilibrium magnetization in the mixed state of high-Tc superconductors

We apply a recently developed scaling procedure to the analysis of equilibrium magnetization M(H) data that were obtained for T-2212 and Bi-2212single crystals and were reported in the literature. The temperature dependencies of the upper critical field and the magnetic field penetration depth resulting from our analysis are distinctly different from those obtained in the original publications. We argue that theoretical models, which are usually employed for analyses of the equilibrium magnetization in the mixed state of type-II superconductors are not adequate for a quantitative description of high-Tc superconductors. In addition, we demonstrate that the scaled equilibrium magnetization M(H) curve for a Tl-2212 sample reveals a pronounced kink, suggesting a phase transition in the mixed state.

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arXiv2004

Equilibrium magnetization in the vicinity of the first order phase transition in the mixed state of high-Tc superconductors

We present the results of a scaling analysis of isothermal magnetization M(H) curves measured in the mixed state of high-Tc superconductors in the vicinity of the established first order phase transition. The most surprising result of our analysis is that the difference between the magnetization above and below the transition may have either sign, depending on the particular chosen sample. We argue that this observation, based on M(H) data available in the literature, is inconsistent with the interpretation that the well known first order phase transition in the mixed state of high-Tc superconductors always represents the melting transition in the vortex system.

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arXiv2003

Ambivalence of the anisotropy of the vortex lattice in an anisotropic type-II superconductor

We present a geometry-based discussion of possible vortex configurations in the mixed state of anisotropic type-II superconductors. It is shown that, if energy considerations assign six nearest neighbors to each vortex, two distinct modifications of the vortex lattice are possible. It is expected that certain conditions lead to a first order phase transition from one modification of the vortex lattice to the other upon varying the external magnetic field.

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arXiv2003

Unusual magnetic field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity κin the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c-axis, at several temperatures below T_c. With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H_c2, an unexpected hysteretic behavior of κ(H) with all the generic features of a first order phase transition. The transition is not manifest in the κ(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.

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arXiv2002

Reinterpretation of the equilibrium magnetization of a Tl-based single crystal. Another phase transition in the mixed state of high-Tc superconductors?

We apply a recently developed scaling procedure for the analysis of the equilibrium magnetization M that was measured on a Tl-based single crystal and was recently reported in the literature. The results of our analysis are distinctly different from those obtained in the original publication where the Hao-Clem model served to analyze the magnetization data. We argue that the Hao-Clem model is not adequate for a quantitative description of the mixed state in high-Tc superconductors especially in high magnetic fields. The scaled equilibrium magnetization data reveal a pronounced kink in the M(H) dependence that might be indicative of a phase transition in the mixed state.

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arXiv2002

On the equilibrium magnetization of high-Tc superconductors below the irreversibility line

By scaling isothermal magnetization data measured at different temperatures in the mixed state of high-Tc superconductors, we show that in some cases the sample magnetization, measured in increasing magnetic field below the irreversibility line, is identical with the equilibrium magnetization even in magnetic fields well within the irreversible regime. This surprising behavior can hardly be explained in terms of traditional models of vortex pinning in the bulk of the sample.

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arXiv2002

Temperature dependence of the upper critical field of high-Tc superconductors from isothermal magnetization data. Application to polycrystalline samples and ceramics

Using a recently developed scaling procedure that allows to establish the temperature dependence of the normalized upper critical field from measurements of the equilibrium isothermal magnetization, we analyze experimental data obtained on polycrystalline and ceramic samples. We show that the scaling procedure works quite well in all cases that we have considered. This provides a very strong evidence that the temperature dependencies of the normalized upper critical field are independent of the orientation of an applied magnetic field with respect to the crystallographic axes of these materials.

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