arXiv subjects
Publications and source records attributed to Y. Kopelevich.
We review our recent work on magnetic properties of graphite and related carbon materials. The results demonstrate that a structural disorder, topological defects, as well as adsorbed foreign atoms can be responsible for the occurrence of both ferromagnetic and superconducting patches in graphitic structures.
Applied magnetic field induces metal - insulator and re-entrant insulator-metal transitions in both graphite and rhombohedral bismuth. The corresponding transition boundaries plotted on the magnetic field - temperature (B - T) plane nearly coincide for these semimetals and can be best described by power laws T ~ (B - B_c)^k, where B_c is a critical field at T = 0 and k = 0.45 +/- 0.05. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the IMT in quasi-3D graphite transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in quasi-2D graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as the signatures of quantum phase transitions associated with the M-I and I-M transformations.
The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(alpha) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters.
Measurements of non-local in-plane resistance originating from transverse vortex-vortex correlations have been performed on a Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} high-T_c superconductor in a magnetic field up to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid vortex lattice does exist over a broad portion of the magnetic field -- temperature (H-T) phase diagram, well above the first-order transition boundary H_{FOT}(T). The results also provide evidence for the vortex lattice melting and vortex liquid decoupling phase transitions, occurring above the H_{FOT}(T).
Magnetization M(H,T) measurements have been performed on microporous carbon (MC) with a three-dimensional nano-array structure corresponding to that of a zeolite Y supercage. The obtained results unambiguously demonstrate the occurrence of high-temperature ferromagnetism in MC, probably originating from a topological disorder associated with curved graphene sheets. The results provide evidence that the ferromagnetic behavior of MC is governed by isolated clusters in a broad temperature range, and suggest the occurrence of percolative-type transition with the temperature lowering. A comparative analysis of the results obtained on MC and related materials is given.
We report on the magnetic field (0T$ \le B \le 9$T) dependence of the longitudinal thermal conductivity $κ(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $κ(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.
Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metallic-like state(s) in graphite in the quantum limit.
Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition (MIT), shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.
This article reviews recent results of magnetotransport and magnetization measurements performed on highly oriented pyrolitic graphite (HOPG) and single crystalline Kish graphite samples. Both metal-insulator and insulator-metal transitions driven by magnetic field applied perpendicular to the basal planes of graphite were found and discussed in the light of relevant theories. The results provide evidence for the existence of localized superconducting domains in HOPG even at room temperature, as well as an interplay between superconducting and ferromagnetic correlations. We also present experimental evidence for the superconductivity occurrence in graphite-sulfur composites.
Measurements of the basal-plane resistivity rho_a(T,H) performed on highly oriented pyrolitic graphite, with magnetic field H parallel to the c-axis in the temperature interval 2 - 300 K and fields up to 8 T, provide evidence for the occurrence of both field - induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations.
We show that the resistivity perpendicular $ρ_c$ and parallel $ρ_a$ to the basal planes of different graphite samples show similar magnetic-field-driven metal-insulator-transitions at a field $B_c \sim 0.1 T$ applied parallel to the c-axis. Our results demonstrate the universality of the recently found scaling in $ρ_a$ of graphite and indicate that the metallic-like temperature dependence of $ρ_c$ is directly correlated to that of $ρ_a$. The similar magnetoresistance found for both resistivities, the violation of Kohler's rule and the field-induced transition indicate that the semiclassical transport theory is inadecuate to understand the transport properties of graphite.
We report the first observation of the internal magnetic-field distribution and flux-line depinning in the vortex-state of a type-II superconductor probed by conduction electrons spin resonance (CESR) technique. The CESR measurements were performed in the recently discovered MgB2 type-II superconductor compound with transition temperature Tc ~ 39 K using microwave sources at 4.1 (S-band) and 9.5 GHz (X-band) corresponding to resonance fields of Hr ~ 1455 and 3390 Oe for g ~ 2.00 in the normal state, respectively. From the distortion of the CESR line in the superconducting state, the field distribution function, n(H), in the vortex-state was inferred, and from the broadening of the line a direct estimate of the standard deviation, sigma ~ 14 Oe, was obtained at ~ 28 K and 7 K for S-band and X-band, respectively. Furthermore, our experiments allowed the determination of the flux-line lattice depinning temperature for both employed microwave frequencies.
We have studied the magnetization of various, well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite to investigate the recently reported ferromagnetic-like signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers - to minimize the diamagnetic background - show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.
We report magnetization measurements performed on graphite--sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T$_{c0}$ = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.
In this work we report a systematic study of electrical current effects on superconducting properties of granular Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ samples with x close to the critical Pr concentration above which the superconductivity vanishes. The results indicate the occurrence of superconductor-insulator quantum phase transition (SIT) driven by the applied electrical current, and suggest that the current-induced SIT can be considered as the dynamical counterpart of the magnetic-field- tuned SIT.
In a recent Letter Mola et al. \cite{mola} reported magnetization measurements $M(H,θ)$ performed on the organic superconductor $κ-$(ET)$_2$Cu(NCS)$_2 (T_c = 9.1$ K) as a function of the magnetic field $H$ applied at different angles $θ$ with respect to the $a$-axis direction. The results \cite{mola} demonstrate: (a) the occurrence of pronounced irreversible magnetization $M_{\rm irr}(H)$ jumps and (b) their sudden cessation for $H \ge H_m(T,θ)$. The boundary line $H_m(T)$ has been interpreted by Mola et al. as the Q2D vortex lattice (VL) quantum melting phase transition line \cite{mola}. The purpose of this comment is to show that the results can be understood in a simple way without invoking "quantum melting phase transition".
The Letter of Wang et al. \cite{wan} is related to the recently discovered vanishing of the second magnetization peak (SMP) in high-$T_c$ superconductors reducing the sample size \cite{kope1,kope2,gal}. This phenomenon has been interpreted \cite{kope1,kope2,gal} in terms of thermomagnetic instability (TMI) effects. The TMI approach has also been adopted by Wang et al. in their previous work \cite{wan2} on the critical sample size temperature dependence $R_{cr}(T)$ measured in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) single crystals. In contrast, in the recently published Letter \cite{wan} the authors claimed that the $R_{cr}(T)$ obtained for Bi2212 crystals is at odds with the TMI model and proposed an alternative explanation of the results assuming that the SMP vanishes as the sample width (or diameter) becomes smaller than the in-plane vortex correlation length $R_c^{2D}(T) \sim J_c(T)^{-1/2}$, where $J_c(T)$ is the critical current density. We demonstrate below that the authors conclusion is misleading.
We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 K and 300 K using S (f = 4 GHz), X (f = 9.4 GHz), and Q (f = 34.4 GHz) microwave bands for the external dc-magnetic field applied parallel (H || c) and perpendicular (H perp c) to the sample hexagonal c-axis. The results obtained in the H || c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field Heff-int(T,H) that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the Heff-int(T,H) with the field-induced metal-insulator transition in graphite and discuss its origin in the light of relevant theoretical models.