arXiv subjects

C. C. Almasan

Publications and source records attributed to C. C. Almasan.

32 records · Page 2Linked to original sources

arXiv2006

High Temperature Mixed State $c-$Axis Dissipation in Low Carrier Density $Y_{0.54}Pr_{0.46}Ba_{2}Cu_{3}O_{7-δ}$

The nature of the out-of-plane dissipation was investigated in underdoped $Y_{0.54}Pr_{0.46}Ba_{2}Cu_{3}O_{7-δ}$ single crystals at temperatures close to the critical temperature. For this goal, temperature and angle dependent out-of-plane resistivity measurements were carried out both below and above the critical temperature. We found that the Ambegaokar-Halperin relationship [V. Ambegaokar, and B. I. Halperin, Phys. Rev. Lett. \textbf{22}, 1364 (1969)] depicts very well the angular magnetoresistivity in the investigated range of field and temperature. The main finding is that the in-plane phase fluctuations decouple the layers above the critical temperature and the charge transport is governed only by the quasiparticles. We also have calculated the interlayer Josephson critical current density, which was found to be much smaller than the one predicted by the theory of layered superconductors. This discrepancy could be a result of the d-wave symmetry of the order parameter and/or of the non BCS temperature dependence of the c-axis penetration length.

cond-mat.supr-con↗

arXiv2003

Detecting percolative metal-insulator transition in manganites by resistive relaxation

We report an experimental study of the time dependence of resistivity of a La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ ultrathin film in order to elucidate the underlying mechanism for metal-insulator transition and colossal magnetoresistance CMR effect. There is a clear change of sign in the resistive relaxation rate across the metal-insulator transition driven by temperature or magnetic field. When measuring in increasing temperature or decreasing magnetic field, the resistivity increases with time in the metallic state but decreases with time in the insulating state. These relaxation processes indicate that the metal-insulator transition and the associated CMR are a direct result of phase separation and of percolation of the metallic phase.

cond-mat.str-el↗

arXiv2003

Variable-range-hopping conductivity of half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$

We report measurements of in-plane $ρ_{ab}$ and out-of-plane $ρ_{c}$ resistivities on a single crystal of the half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. In the temperature $T$ range 220 to 300 K, the resistive anisotropy $ρ_{c}/ρ_{ab}=A+B/T$ ($A$ and $B$ constants), which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field $H$ and $\sin^{2}ϕ$ dependences of the negative magnetoresistivity $\ln [ρ_{i}(T,H,ϕ)/ρ_{i}(T,H=0)]$ ($i=ab,c$), where $ϕ$ is the in-plane angle between the magnetic field and the current.

cond-mat.str-el↗

arXiv2003

Spin-wave scattering at low temperatures in manganite films

The temperature $T$ and magnetic field $H$ dependence of the resistivity $ρ$ has been measured for La$_{0.8-y}$Sr$_{0.2}$MnO$_{3}$ (y=0 and 0.128) films grown on (100) SrTiO$_{3}$ substrates. The low-temperature $ρ$ in the ferromagnetic metallic region follows well $ρ(H,T)=ρ_{0}(H)+A(H)ω_{s}/\sinh (\hbar ω_{s}/2k_{B}T)+B(H)T^{7/2}$ with $ρ_{0}$ being the residual resistivity. We attribute the second and third term to small-polaron and spin-wave scattering, respectively. Our analysis based on these scattering mechanisms also gives the observed difference between the metal-insulator transition temperatures of the films studied. Transport measurements in applied magnetic field further indicate that spin-wave scattering is a key transport mechanism at low temperatures.

cond-mat.str-el↗

arXiv2003

Small-polaron hopping conductivity in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

We report anisotropic resistivity measurements on a La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal over a temperature $T$ range from 2 to 400 K and in magnetic fields $H$ up to 14 T. For $T\geq 218$ K, the temperature dependence of the zero-field in-plane $ρ_{ab}(T)$ resistivity obeys the adiabatic small polaron hopping mechanism, while the out-of-plane $ρ_{c}(T)$ resistivity can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between the resistivity and magnetization, we developed a model which allows the determination of $ρ_{ab,c}(H,T)$. The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.

cond-mat.str-el↗

arXiv2002

Low-temperature electrical transport in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

The temperature $T$ and magnetic field $H$ dependence of anisotropic in-plane $ρ_{ab}$ and out-of-plane $ρ_{c}$ resistivities have been investigated in single crystals of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. Below the Curie transition temperature $T_c=$ 125 K, $ρ_{ab}$ and $ρ_{c}$ display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K $\leq T \leq$ 110 K), both $ρ_{ab}(T)$ and $ρ_{c}(T)$ follow a $T^{9/2}$ dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient $B_{ab}^{fit}$ and the ratio of the exchange interaction $J_{ab}/J_c$ obtained by fitting the data are in excellent agreement with the calculated $B_{ab}$ based on the two-magnon model and $J_{ab}/J_c$ deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower $T$, in the non-metallic regime ($T<$ 50 K), {\it both} the in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities obey a $T^{1/2}$ dependence, consistent with weak localization effects. Hence, this demonstrates the three-dimensional metallic nature of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ at $T<T_c$.

cond-mat.mtrl-sci↗

arXiv2001

Magnetotransport Mechanisms in Strongly Underdoped YBa_2Cu_3O_x Single Crystals

We report magnetoresistivity measurements on strongly underdoped YBa_2Cu_3O_x (x=6.25, 6.36) single crystals in applied magnetic fields H || c-axis. We identify two different contributions to both in-plane and out-of-plane magnetoresistivities. The first contribution has the same sign as the temperature coefficient of the resistivity \partial ln(ρ_i)/\partial T (i={c,ab}). This contribution reflects the incoherent nature of the out-of-plane transport. The second contribution is positive, quadratic in field, with an onset temperature that correlates to the antiferromagnetic ordering.

cond-mat.supr-con↗

arXiv2001

Magnetotransport Properties of Antiferromagnetic YBa_2Cu_3O_6.25 Single Crystals

In-plane and out-of-plane magnetoresistivities (MR) of antiferromagnetic YBa_2Cu_3O_6.25 single crystals were measured in magnetic fields H applied along the (ab) plane. In-plane MR is a superposition of two components: The first component is strongly in-plane anisotropic, changing sign from negative when H is parallel to the electrical current I to positive when H is perpendicular to I. The second component is positive, quadratic in H, and isotropic in the (ab)-plane. The out-of-plane MR displays a fourfold symmetry upon in-plane rotation of the magnetic field, with maxima along the easy axes of antiferromagnetic spin ordering and minima along unfavorable directions of spin orientation (45 degrees from the Cu-O-Cu bonds).

cond-mat.supr-con↗

arXiv2001

Magnetoconductivity due to Quantum Interference in Strongly Underdoped YBa_2Cu_3O_x

We report magnetoconductivity measurements on YBa_2Cu_3O_x (x=6.25 and 6.36) single crystals. Our main result is that both the in-plane Delta_sigma_ab and out-of-plane Delta_sigma_c magnetoconductivities exhibit the field dependence characteristic of "two-dimensional" quantum interference in applied magnetic fields H parallel to the c-axis. Namely, Delta_sigma_c,ab are proportional to ln(H/H_0), with Delta_sigma_c/sigma_c substantially greater than Delta_sigma_ab/sigma_ab. We interpret this result as evidence of interlayer incoherence in these crystals, so that the phase-coherent trajectories are mostly confined to one bilayer.

cond-mat.supr-con↗

arXiv1999

Dynamics of Flux Creep in Underdoped Single Crystals of Y_1-xPr_xBa_2Cu_3O_7-d

Transport as well as magnetic relaxation properties of the mixed state were studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two correlated phenomena - a coupling transition and a transition to quantum creep. The distribution of transport current below the coupling transition is highly nonuniform, which facilitates quantum creep. We speculate that in the mixed state below the coupling transition, where dissipation is nonohmic, the current distribution may be unstable with respect to self-channeling resulting in the formation of very thin current-carrying layers.

cond-mat.supr-con↗

arXiv1999

Relationship Between Conductivity and Phase Coherence Length in Cuprates

The large ($10^2 - 10^5$) and strongly temperature dependent resistive anisotropy $η= (σ_{ab}/σ_c)^{1/2}$ of cuprates perhaps holds the key to understanding their normal state in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities. It can be shown that $η$ is determined by the ratio of the phase coherence lengths $\ell_i$ in the respective directions: $σ_{ab}/σ_c = \ell_{ab}^2/\ell_{c}^2$. In layered crystals in which the out-of-plane transport is incoherent, $\ell_{c}$ is fixed, equal to the interlayer spacing. As a result, the T-dependence of $η$ is determined by that of $\ell_{ab}$, and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ ($0 < x \leq 0.55$) and show that $σ_{ab}$ of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped $Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $σ_{ab}(η)$ indicates a crossover from incoherent to coherent transport in the c-direction.

cond-mat.supr-con↗

arXiv1999

Is There a Unified Description of Conductivity of Layered Cuprates ?

We present a novel approach to the analysis of the normal state in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities of anisotropic layered crystals such as oxygen deficient $YBa_{2}Cu_{3}O_{x}$. It can be shown that the resistive anisotropy is determined by the ratio of the phase coherence lengths in the respective directions; i.e., $σ_{ab}/σ_c=\ell_{ab}^2/\ell_c^2$. From the idea that at all doping levels and temperatures $T$ the out-of-plane transport in these crystals is incoherent, follows that $\ell_c$ is T-independent, equal to the spacing $\ell_0$ between the neighboring bilayers. Thus, the T-dependence of $\ell_{ab}$ is given by the measured anisotropy, and $σ_{ab}(\ell_{ab})$ dependence is obtained by plotting $σ_{ab}$ vs $\ell=(σ_{ab}/σ_c)^{1/2}\ell_0$. The analysis of several single crystals of $YBa_{2}Cu_{3}O_{x}$ ($6.35<x<6.93$) shows that for all of them $σ_{ab}(\ell)$ is described by a universal dependence $σ_{ab}/\barσ=f(\ell/\bar\ell)$ with doping dependent parameters $\barσ$ and $\bar\ell$.

cond-mat.str-el↗

arXiv1999

Conductivity of Layered Crystals

We show that the resistive anisotropy of an anisotropic medium is determined by the ratio of the phase coherence lengths. In layered crystals in which the interlayer transport is incoherent, the out-of-plane phase coherence length is fixed and temperature independent. This leads to a temperature dependent resistive anisotropy and to the coexistence of metallic in-plane and non-metallic out-of-plane conductivities. Our approach provides a description of the c-axis conductivity in the highly nonclassical regime, characteristic of layered cuprates.

cond-mat.supr-con↗

arXiv1999

Quantum Creep in Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7}$ Crystals: Magnetic Relaxation and Transport

We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7} crystals. A transition from thermally activated flux creep to temperature independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures T * 5 K. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Based on these observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts.

cond-mat.supr-con↗