arXiv subjects

R. Prozorov

Publications and source records attributed to R. Prozorov.

At least 181 records · Page 10Linked to original sources

arXiv2007

Precise measurements of radio-frequency magnetic susceptibility in (anti)ferromagnetic materials

Dynamic magnetic susceptibility, $χ$, was studied in several intermetallic materials exhibiting ferromagnetic, antiferromagnetic and metamagnetic transitions. Precise measurements by using a 14 MHz tunnel diode oscillator (TDO) allow detailed insight into the field and temperature dependence of $χ$. In particular, local moment ferromagnets show a sharp peak in $χ(T)$ near the Curie temperature, $T_c$. The peak amplitude decreases and shifts to higher temperatures with very small applied dc fields. Anisotropic measurements of CeVSb$_3$ show that this peak is present provided the magnetic easy axis is aligned with the excitation field. In a striking contrast, small moment, itinerant ferromagnets (i.e., ZrZn$_2$) show a broad maximum in $χ(T)$ that responds differently to applied field. We believe that TDO measurements provide a very sensitive way to distinguish between local and itinerant moment magnetic orders. Local moment antiferromagnets do not show a peak at the Néel temperature, $T_N$, but only a sharp decrease of $χ$ below $T_N$ due to the loss of spin-disorder scattering changing the penetration depth of the ac excitation field. Furthermore, we show that the TDO is capable of detecting changes in spin order as well as metamagnetic transitions. Finally, critical scaling of $χ(T,H)$ in the vicinity of $T_C$ is discussed in CeVSb$_3$ and CeAgSb$_2$.

cond-mat.str-el↗

arXiv2007

Radio-frequency magnetic response of vortex lattices undergoing structural transformations

We report a clear anomaly in the rf response of a tunnel-diode resonator measured in borocarbide crystals in dc fields along the $c-$axis. We associate the anomaly in the dynamic magnetic susceptibility, $χ$, with a structural transition in the vortex lattice. $χ$ is sensitive to the Campbell penetration depth $λ_{C}$ of the rf perturbation into the mixed state. $λ_{C}$ depends on the vortex lattice elastic moduli, which in turn depend on the vortex lattice structure. The high-field transition line is clearly mapped and shown to behave in agreement with theory.

cond-mat.supr-con↗

arXiv2007

Penetration depth study of superconducting gap structure of 2\textit{H}-NbSe$_2$

We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths ($λ_a$ and $λ_c$ respectively) in 2\textit{H}-NbSe$_2$. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast to some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.

cond-mat.supr-con↗

arXiv2006

Effect of field dependent core size on reversible magnetization of high-$κ$ superconductors

The field dependence of the vortex core size $ξ(B)$ is incorporated in the London model, in order to describe reversible magnetization $M(B,T)$ for a number of materials with large Ginzburg-Landau parameter $κ$. The dependence $ξ(B)$ is directly related to deviations in $M(\ln B)$ from linear behavior prescribed by the standard London model. A simple method to extract $ξ(B)$ from the magnetization data is proposed. For most materials examined, $ξ(B)$ so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from $μ$SR and small angle neutron scattering data and with that predicted theoretically.

cond-mat.supr-con↗

arXiv2004

Super-hard Superconductivity

We present a study of the magnetic response of Type-II superconductivity in the extreme pinning limit, where screening currents within an order of magnitude of the Ginzburg-Landau depairing critical current density develop upon the application of a magnetic field. We show that this "super-hard" limit is well approximated in highly disordered, cold drawn, Nb and V wires whose magnetization response is characterized by a cascade of Meissner-like phases, each terminated by a catastrophic collapse of the magnetization. Direct magneto-optic measurements of the flux penetration depth in the virgin magnetization branch are in excellent agreement with the exponential model in which J_c(B)=J_co exp(-B/B_o), where J_co~5x10^6 A/cm^2 for Nb. The implications for the fundamental limiting hardness of a superconductor are discussed.

cond-mat.supr-con↗

arXiv2004

Field-dependent diamagnetic transition in magnetic superconductor $Sm_{1.85} Ce_{0.15} Cu O_{4-y}$

The magnetic penetration depth of single crystal $\rm{Sm_{1.85}Ce_{0.15}CuO_{4-y}}$ was measured down to 0.4 K in dc fields up to 7 kOe. For insulating $\rm{Sm_2CuO_4}$, Sm$^{3+}$ spins order at the Néel temperature, $T_N = 6$ K, independent of the applied field. Superconducting $\rm{Sm_{1.85}Ce_{0.15}CuO_{4-y}}$ ($T_c \approx 23$ K) shows a sharp increase in diamagnetic screening below $T^{\ast}(H)$ which varied from 4.0 K ($H = 0$) to 0.5 K ($H =$ 7 kOe) for a field along the c-axis. If the field was aligned parallel to the conducting planes, $T^{\ast}$ remained unchanged. The unusual field dependence of $T^{\ast}$ indicates a spin freezing transition that dramatically increases the superfluid density.

cond-mat.supr-con↗

arXiv2003

Nodal Order Parameter in Electron-Doped Superconducting Films Pr(2-x)Ce(x)CuO(4-y) (x=0.13, 0.15 and 0.17)

The London penetration depth, lambda{ab}(T), is reported for thin films of the electron-doped superconductor Pr{2-x}Ce{x}CuO{4-y} at three doping levels (x = 0.13, 0.15 and 0.17). Measurements down to 0.35 K were carried out using a tunnel diode oscillator with excitation fields applied both perpendicular and parallel to the conducting planes. For all samples and both field orientations lambda{ab}(T) showed power law behavior implying a superconducting gap with nodes.

cond-mat.supr-con↗

arXiv2003

Evidence for Unconventional Superconductivity in the Non-Oxide Perovskite $\mathrm{MgCNi_3}$ from Penetration Depth Measurements

The London penetration depth, $λ(T)$, was measured in polycrystalline powders of the non-oxide perovskite superconductor $\mathrm{MgCNi_3}$ by using a sensitive tunnel-diode resonator technique. The penetration depth exhibits distinctly non s-wave BCS low-temperature behavior, instead showing quadratic temperature dependence, suggestive of a nodal order parameter.

cond-mat.supr-con↗

arXiv2002

Campbell Penetration Depth of a Superconductor in the Critical State

The magnetic penetration depth $λ(T,H,j)$ was measured in the presence of a slowly relaxing supercurrent, $j$. In single crystal $\mathrm{Bi_2Sr_2CaCu_2O_8}$ below approximately 25 K, $λ(T,H,j)$ is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. The Campbell length depends upon the ratio $j/j_{c}$ where $j_{c}$ is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.

cond-mat.supr-con↗

arXiv2001

Energy gap and proximity effect in $MgB_2$ superconducting wires

Measurements of the penetration depth $λ(T,H)$ in the presence of a DC magnetic field were performed in $MgB_2$ wires. In as-prepared wires $λ(T,H<130 Oe)$ shows a strong diamagnetic downturn below $\approx 10 K$. A DC magnetic field of $130 Oe$ completely suppressed the downturn. The data are consistent with proximity coupling to a surface $Mg$ layer left during synthesis. A theory for the proximity effect in the clean limit, together with an assumed distribution of the $Mg$ layer thickness, qualitatively explains the field and temperature dependence of the data. Removal of the $Mg$ by chemical etching results in an exponential temperature dependence for $λ(T)$ with an energy gap of $2 Δ(0)/T_c\approx 1.54$ ($Δ(0) \approx 2.61 meV$), in close agreement with recent measurements on commercial powders and single crystals. This minimum gap is only 44% of the BCS weak coupling value, implying substantial anisotropy.

cond-mat.supr-con↗

arXiv2000

Fluctuations in $YBa_{2}Cu_{3}O_{6.5}$ single crystal: an evidence for 2D->3D crossover

Magnetization measurements as a function of temperature are reported for $YBa_{2}Cu_{3}O_{6.5}$ crystal (T_{c}=45.2 K) for fields between 0.2 to $3.5\ Tesla$. All isochamps for H>1 Tesla intersect at $T_{2D}^{\star}\simeq 42.8\ K$, implying fluctuations contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ''2D scaling'' in the fluctuation regime. Surprisingly, the low field curves also intersect, at $T_{3D}^{\star}\simeq 43.4 K$, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the $2D\to 3D$ crossover are consistent with the experimental observation.

cond-mat.supr-con↗

arXiv2000

Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements

Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda_ab at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose a [110] interface show a strong upturn in lambda_ab at around 7K, when the field is oriented so that the supercurrents flow around this surface. In YBCO this upturn is completely suppressed by a field of ~0.1 T.

cond-mat.supr-con↗

arXiv2000

Unusual temperature dependence of the London penetration depth in all-organic β``-(ET)_2SF_5CH_2CF_2SO_3 single crystals

The temperature dependence of the in-plane, λ_{\parallel}, and interplane, λ_{\perp}, London penetration depth was measured in the metal-free all-organic superconductor β-ET (see title) ($T_c \approx$ 5.2 K). λ_{\parallel} ~T^3 up to 0.5 Tc, a power law previously observed only in materials thought to be p-wave superconductors. λ_{\perp} is larger than the sample dimensions down to the lowest temperatures (0.35 K), implying an anisotropy of λ_{\perp}/λ_{\parallel} ~ 400-800.

cond-mat.supr-con↗

arXiv2000

Measurements of the absolute value of the penetration depth in high-$ T_c$ superconductors using a tunnel diode resonator

A method is presented to measure the absolute value of the London penetration depth, $λ$, from the frequency shift of a resonator. The technique involves coating a high-$T_c$ superconductor (HTSC) with film of low - Tc material of known thickness and penetration depth. The method is applied to measure London penetration depth in YBa2Cu3O{7-δ} (YBCO) Bi2Sr2CaCu2O{8+δ} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-δ}$ (PCCO). For YBCO and BSCCO, the values of $λ(0)$ are in agreement with the literature values. For PCCO $λ\approx 2790$ Å, reported for the first time.

cond-mat.supr-con↗

arXiv2000

Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, $λ(T)$, was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ (PCCO) and Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ (NCCO). In PCCO, the superfluid density varies as $T^2$ from 0.025 up to roughly 0.3 $T/T_c$ suggestive of a d-wave state with impurities. In NCCO, $λ(T)$ shows a pronounced upturn for $T < 4$ K due to the paramagnetic contribution of Nd$^{3+}$ ions. Fits to an s-wave order parameter over the standard BCS range ($T/T_c$ = 0.32) limit any gap to less than $Δ_{min} (0)/T_c = 0.57$ in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of $Δ_{min} (0)/T_c = 0.2$.

cond-mat.supr-con↗

arXiv2000

Nonequilibrium Campbell length: probing the vortex pinning potential

The $AC$ magnetic penetration depth $λ(T,H,j)$ was measured in presence of a macroscopic $DC$ (Bean) supercurrent, $j$. In single crystal BSCCO below approximately 28 K, $λ(T,H,j)$ exhibits thermal hysteresis. The irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. It is demonstrated that below a new irreversibility temperature, the nonequilibrium Campbell length depends upon the ratio $j/j_c$. $λ(T,H,j)$ {\it increases} with $j/j_c$ as expected for a non-parabolic potential well whose curvature {\it decreases} with the displacement. Qualitatively similar results are observed in other high-$T_{c}$ and conventional superconductors.

cond-mat.supr-con↗

arXiv2000

Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study

We report measurements of the magnetic penetration depth λ_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic λ_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.

cond-mat.supr-con↗

arXiv2000

Meissner - London state in superconductors of rectangular cross-section in perpendicular magnetic field

The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.

cond-mat.supr-con↗