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V. N. Zavaritsky

Publications and source records attributed to V. N. Zavaritsky.

18 recordsLinked to original sources
arXiv2007

Comment on `Intrinsic tunnelling spectroscopy of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$: The junction-size dependence of self-heating'[Phys.Rev.B 73, 224501 (2006)]

The recent PRB 73, 224501 (2006) henceforth referred as Ref.\cite{0} asserts that self-heating decreases with sample area reduction and claims to identify the intrinsic cause of ITS in submicrometre `mesa'. I will show that this assertion lacks substantiation. I will further demonstrate that one and the same $R(T)$ and the parameter-free Newton's Law of Cooling describe quantitatively a rich variety of ITS behaviours taken by Ref.\cite{0} above and below $T_c$ at bath temperatures spanned over 150K. Thus this finding presents strong evidence in favour of heating as the cause of the `intrinsic tunnelling spectra' (ITS) promoted by Ref.\cite{0}.

cond-mat.supr-con↗
arXiv2007

Reply to comment on "Essence of intrinsic tunnelling: Distinguishing intrinsic features from artefacts

The recent PRB, henceforth referred as Ref.[1], experimentally resolves the intrinsic shape of the c-axis current-voltage characteristics (IVC) of HTSC and demonstrates that at sufficiently high heat loads the heating-induced IVC nonlinearities exceed the intrinsic ones so radically that the latter might be safely ignored. The author of the comment fails to take account of the experimental findings by Ref.[1] and seeks to cast doubt on all its conclusions through reference to a brush-like IVC, which is claimed to be free of heating. I will show that this claim lacks substantiation; indeed it can be stated with certainty that the IVC is not free from heating. I will further show that the data selected for this comment make it possible to explore for the first time the effect of temperature on a range of loads where the genuine response is not hidden by heating and to demonstrate for the first time that $R(T)$ of the same sample is responsible for a rich variety of IVC behaviours taken above and below $T_c$ at bath temperatures spanned over 180K. Thus these data in fact provide strong novel evidence in favour of the major conclusions by Ref.[1], in particular the extrinsic cause of the key findings by intrinsic tunnelling spectroscopy.

cond-mat.supr-con↗
arXiv2006

Comment on "Single intrinsic Josephson junction with double-sided fabrication technique" by You et al [Appl. Phys. Lett. 88, 222501 (2006)]

In a recent letter, henceforth referred to as Ref.[1], You et al postulate that Bi2212 factually represents a series array of SIS junctions and claim that the nonlinear current-voltage characteristics (IVC) of their bridge are free of heating. Earlier experiments cast serious doubt upon the accuracy of the principal postulate (of this letter), see Ref.[2] and references therein. In what follows I will demonstrate that the major claim by the authors of Ref.[1] is at odds with their own data which suggest an extrinsic cause for the IVC nonlinearities.

cond-mat.supr-con↗
arXiv2006

Reply to cond-mat/0601101

The author of the comment promotes a brush-like IVC, which is claimed to be free of self-heating. I will show that this claim lacks substantiation as the IVC is definitely not free from heating and that the self-heating cause of IVC-2 is indirectly admitted by the author of the comment. I will further show that the data selected for this comment in fact provide additional experimental evidence in favour of the major conclusions by the commented article in particular of the extrinsic cause of the key findings by intrinsic tunnelling spectroscopy.

cond-mat.supr-con↗
arXiv2004

What does intrinsic tunnelling spectroscopy really examine?

The out-of-plane current-voltage (I-V) characteristics of Bi2212 are studied in experimental environments of different heat transfer efficiency, allowing practical separation of intrinsic and extrinsic phenomena. {\it Intrinsic} (heating-free) response is Ohmic in the normal state of Bi2212, while its resistance, R=V/I, is found to be a good practical measure of the mean temperature of the sample in the overheated case. A self-heating model proposed for the latter case provides a qualitative and quantitative description of key findings of intrinsic tunnelling spectroscopy including (pseudo)gaps, quasiparticle and normal state resistances. The model also naturally explains the `superconducting' gap closure well below $T_c$ of the material as well as its survival at a magnetic field significantly exceeding $H_{c2}$. The generic shape of the individual branches of the brush-like part of I-V established under conditions of negligible overheating suggests a phase-slip origin of the so-called `intrinsic Josephson effect' (IJE).

cond-mat.supr-con↗
arXiv2004

The "normal" state of superconducting cuprates might really be normal after all

High magnetic field studies of cuprate superconductors revealed a non-BCS temperature dependence of the upper critical field $H_{c2}(T)$ determined resistively by several groups. These determinations caused some doubts on the grounds of both the contrasting effect of the magnetic field on the in-plane and out-of-plane resistances reported for large Bi2212 sample and the large Nernst signal \emph{well above} $T_{c}$. Here we present both $ρ_{ab}(B)$ and $ρ_{c}(B)$ of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None of our measurements revealed a situation when on the field increase $ρ_c$ reaches its maximum while $ρ_{ab}$ remains very small if not zero. The resistive %upper critical fields estimated from the in-plane and out-of-plane $H_{c2}(T)$ estimated from $ρ_{ab}(B)$ and $ρ_{c}(B)$ are approximately the same. Our results support any theory of cuprates that describes the state above the resistive phase transition as perfectly normal with a zero off-diagonal order parameter. In particular, the anomalous Nernst effect above the resistive phase transition in high-$T_{c}$ cuprates can be described quantitatively as a normal state phenomenon in a model with itinerant and localised fermions and/or charged bosons.

cond-mat.supr-con↗
arXiv2004

How to map a pseudogap?

A pseudogap (PG) is believed to be responsible for the non Fermi-liquid normal state of cuprate superconductors. In particular, field induced PG collapse causes negative longitudinal magnetoresistance (MR), for details, see V.N. Zavaritsky, M. Springford, A.S. Alexandrov, cond-mat/0006089; cond-mat/0011192. The PG collapses because of spin-splitting of the polaron band while the orbital effects are irrelevant. Recently these conclusions, including the Zeeman relation, $k_BT^*=gB_{pg}$, which couples the PG temperature, $T^*$, and the PG closing field, $B_{pg}$, were reaffirmed by T. Shibauchi {\it et al.}, Phys. Rev. Lett. {\bf 86}, 5763 (2001). It will demonstrate that the article by T. Shibauchi {\it et al.} lacks consistency and its conclusions are based on fallacious propositions and unsupported by the authors' own experimental results.

cond-mat.supr-con↗
arXiv2004

How normal is the "normal" state of superconducting cuprates?

High magnetic field studies of the cuprate superconductors revealed a non-BCS temperature dependence of the upper critical field $H_{c2}(T)$ determined resistively by several groups. These determinations caused some doubts on the grounds of the contrasting effect of the magnetic field on the in-plane, $ρ_{ab}$, and out-of-plane, $ρ_{c}$ resistances reported for large sample of Bi2212. Here we present careful measurements of both $ρ_{ab}(B)$ and $ρ_{c}(B)$ of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None of our measurements revealed a situation when on field increase $ρ_c$ reaches its maximum while $ρ_{ab}$ remains very small if not zero. The resistive $H_{c2}(T)$ estimated from $ρ_{ab}(B)$ and $ρ_{c}(B)$ are approximately the same. We also present a simple explanation of the unusual Nernst signal in superconducting cuprates as a normal state phenomenon. Our results support any theory of cuprates, which describes the state above the resistive phase transition as perfectly 'normal' with a zero off-diagonal order parameter.

cond-mat.supr-con↗
arXiv2004

Nernst effect in poor conductors and the cuprate superconductors

We calculate the Nernst signal in disordered conductors with the chemical potential near the mobility edge. The Nernst effect originates from interference of itinerant and localised-carrier contributions to the thermomagnetic transport. It reveals a strong temperature and magnetic field dependence, which describes quantitatively the anomalous Nernst signal in high-Tc cuprates.

cond-mat.supr-con↗
arXiv2003

'Giant' normal state magnetoresistances of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons.

cond-mat.supr-con↗
arXiv2003

Hall effect and resistivity in underdoped cuprates

The behaviour of the Hall ratio $R_{H}(T)$ as a function of temperature is one of the most intriguing normal state properties of cuprate superconductors. One feature of all the data is a maximum of $R_{H}(T)$ in the normal state that broadens and shifts to temperatures well above $T_c$ with decreasing doping. We show that a model of preformed pairs-bipolarons provides a selfconsistent quantitative description of $R_{H}(T)$ together with in-plane resistivity and uniform magnetic susceptibility for a wide range of doping.

cond-mat.supr-con↗
arXiv2003

Intrinsic tunneling or Joule heating?

It is shown that the `tunnelling spectra' reported by Yurgens et al. could be reproduced qualitatively and quantitatively using the experimental out-of-plane normal state resistance R(T) and assuming that the heating of the mesa, caused by the Joule dissipation, is the only reason for effects observed at high bias.

cond-mat.supr-con↗
arXiv2002

Universal upper critical field of unconventional superconductors

The resistive upper critical field, Hc2(T) of cuprates, superconducting spin-ladders, and organic (TMTSF)2X systems is shown to follow a universal nonlinear temperature dependence in a wide range near Tc, while its low-temperature behaviour depends on the chemical formula and sample quality. Hc2(T) is ascribed to the Bose-Einstein condensation field of preformed pairs. The universality originates from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) from the kinetic and thermodynamic measurements is resolved in the framework of the charged Bose-gas model with impurity scattering.

cond-mat.supr-con↗
arXiv2000

C-axis negative magnetoresistance and upper critical field of Bi2Sr2CaCu2O8

The out-of-plane resistance and the resistive upper critical field of BSCCO-2212 single crystals with Tc=91-93 K have been measured in magnetic fields up to 50 T over a wide temperature range. The results are characterised by a positive linear magnetoresistance in the superconducting state and a negative linear magnetoresistance in the normal state. The zero field normal state c-axis resistance, the negative linear normal state magnetoresistance, and the divergent upper critical field Hc2(T)are explained in the framework of the bipolaron theory of superconductivity.

cond-mat.supr-con↗
arXiv1998

Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals

The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined from magnetization curves for different field sweep rates dH/dt and temperatures. The obtained results are consistent with theoretical reports in the literature about vortex creep over surface and geometrical barriers. The frequently observed low-temperature upturn of H_p is shown to be related to metastable configurations due to barriers for vortex entry. Data of the true lower critical field H_c1 are presented. The low-temperature dependence of H_c1 is consistent with a superconducting state with nodes in the gap function. [PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs]

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