arXiv subjects

R. Prozorov

Publications and source records attributed to R. Prozorov.

At least 91 records · Page 5Linked to original sources

arXiv2014

Evolution of the superconducting energy gap structure concomitant with Fermi surface reconstruction in the heavy-fermion superconductor CeCoIn5

The London penetration depth, $λ(T)$ was measured in single crystals of Ce$_{1-x}R_x$CoIn$_5$, $R$=La, Nd and Yb down to 50~mK ($T_c/T \sim$50) using a tunnel-diode resonator. In the cleanest samples $Δλ(T)$ is best described by the power law, $Δλ(T) \propto T^{n}$, with $n \sim 1$, consistent with line nodes. Substitutions of Ce with La, Nd and Yb lead to similar monotonic suppressions of $T_c$, however the effects on $Δλ(T)$ differ. While La and Nd doping results in an increase of the exponent to $n \sim 2$, as expected for a dirty nodal superconductor, Yb doping leads to $n > 3$, inconsistent with nodes, suggesting a change from nodal to nodeless superconductivity where Fermi surface topology changes were reported, implying that the nodal structure and Fermi surface topology are closely linked.

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arXiv2014

Superconductivity and Physical Properties of CaPd2Ge2 Single Crystals

We present the superconducting and normal state properties of CaPd2Ge2 single crystal investigated by magnetic susceptibility χ, isothermal magnetization M, heat capacity C_p, in-plane electrical resistivity ρand London penetration depth λversus temperature T and magnetic field H measurements. Bulk superconductivity is inferred from the ρ(T) and C_p(T) data. The ρ(T) data exhibit metallic behavior and undergoes a superconducting transition with T_c onset = 1.98 K and zero resistivity state at T_c 0 = 1.67 K. The χ(T) reveal the onset of superconductivity at 2.0 K. For T>2.0 K, the χ(T) and M(H) are weakly anisotropic paramagnetic with χ_ab > χ_c. The C_p(T) confirm the bulk superconductivity below T_c = 1.69(3) K. The superconducting state electronic heat capacity is analyzed within the framework of a single-band α-model of BCS superconductivity and various normal and superconducting state parameters are estimated. Within the α-model, the C_p(T) data and the ab plane λ(T) data consistently indicate a moderately anisotropic s-wave gap with Δ(0)/k_B T_c ~ 1.6, somewhat smaller than the BCS value of 1.764. The relationship of the heat capacity jump at T_c and the penetration depth measurement to the anisotropy in the s-wave gap is discussed.

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arXiv2014

A comprehensive scenario of the single crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ ($0.22 \leq x \leq 1$)

Large high-quality single crystals of hole-doped iron-based superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ were grown over a broad composition range $0.22 \leq x \leq 1$ by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and adjusted temperature window of the growth are necessary to obtain single crystals of heavily K doped crystals (0.65$\leq x \leq$ 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the $c$-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity, $ρ(T)$ found doping-independent, constant within error bars, resistivity at room temperature, $ρ(300K)$, in sharp contrast with significant doping dependence in electron and isovalent substituted BaFe$_2$As$_2$ based compositions. The shape of the temperature dependent resistivity, $ρ(T)$, shows systematic doping-evolution, being close to $T^2$ in overdoped and revealing significant contribution of the $T$-linear component at optimum doping. The slope of the upper critical field, $d H_{c2}/dT$, scales linearly with $T_c$ for both $H\parallel c$, $ H_{c2,c}$, and $H \parallel ab$, $H_{c2,ab}$. The anisotropy of the upper critical field, $γ\equiv H_{c2,ab} / H_{c2,c}$ determined near zero-field $T_c$ increases from $\sim$2 to 4-5 with increasing K doping level from optimal $x \sim$0.4 to strongly overdoped $x$=1.

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arXiv2013

London Penetration Depth and Pair Breaking

The London penetration depth is evaluated for isotropic materials for any transport and pair-breaking Born scattering rates. Besides known results, a number of new features are found. The slope $|dρ/dθ|$ of the normalized superfluid density $ρ=λ^2(0)/λ^2(θ)$ at the transition $θ=T/T_c=1$ has a minimum near the value of the pair-breaking parameter separating gapped and gapless states. The low-$T$ exponentially flat part of $ρ$ for the s-wave materials is suppressed by increasing pair breaking. For strong $T_c $ suppression by magnetic impurities the "Homes scaling" $λ^{-2}(0) \propto σT_c$ with $σ$ being the normal conductivity gives way to $λ^{-2}(0) \propto σT_c^2$. For the d-wave order parameter, the transport and spin-flip Born scattering rates enter the theory only as a sum, in particular, they affect the $T_c$ depression in the same manner. We confirm that the linear low temperature behavior of $ρ$ in a broad range of the combined scattering parameter turns to the $T^2$ behavior only when the critical temperature is suppressed at least by a factor of 3 relative to the clean limit $T_{c0}$. Moreover, in this range, $ρ(θ)$ is only weakly dependent on the scattering parameter, i.e. it is nearly universal.

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arXiv2013

On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance

In the approximation of validity of the Drude expression for quasiparticle conductivity and the assumption that all electrons at T = 0 transform into a superfluide condensate, the expression for the quasiparticle scattering rate tau -1 in terms of the real and imaginary parts of the microwave surface impedance has been obtained. The resulting expression is a generalization of the well-known expression for tau^-1, valid for omega x tau is much less 1, to an arbitrary value of omega x tau, where omega is the frequency of the microwave field. From experimental Ka-band impedance measurements, temperature dependence of tau^-1 in superconducting single crystal pnictides Ba(Fe1-xCox)2As2 is obtained using the generalized expression. It is shown that under condition of the given work, the approximation omega x tau is much less 1 gives the considerable error in determination of tau^-1.

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arXiv2013

Effect of heavy-ion irradiation on London penetration depth in over-doped Ba(Fe,Co)2As2

Irradiation with 1.4 GeV $^{208}$Pb ions was used to induce artificial disorder in single crystals of iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and to study its effect on the temperature-dependent London penetration depth and transport properties. Study was undertaken on overdoped single crystals with $x$=0.108 and $x$=0.127 characterized by notable modulation of the superconducting gap. Irradiation with doses 2.22$\times10^{11}$$\textit{d}$/cm$^2$ and 2.4$\times10^{11}$$\textit{d}$/cm$^2$, corresponding to the matching fields of $B_ϕ = $6 T and 6.5 T, respectively, suppresses the superconducting $T_c$ by approximately 0.3 to 1 K. The variation of the low-temperature penetration depth in both pristine and irradiated samples is well described by the power-law, $Δλ(T)=AT^n$. Irradiation increases the magnitude of the pre-factor $A$ and decreases the exponent $n$, similar to the effect of irradiation in optimally doped samples. This finding supports universal $s_{\pm}$ pairing in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ compounds for the whole Co doping range.

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arXiv2013

Superconducting and Normal State Properties of APd2As2 (A = Ca, Sr, Ba) Single Crystals

The synthesis and crystallography, magnetic susceptibility chi, magnetization M, specific heat Cp, in-plane electrical resistivity rho and in-plane magnetic penetration depth measurements are reported for single crystals of APd2As2 (A = Ca, Sr, Ba) versus temperature T and magnetic field H. The crystals were grown using PdAs self-flux. CaPd2As2 and SrPd2As2 crystallize in a collapsed body-centered tetragonal ThCr2Si2-type structure (I4/mmm), whereas BaPd2As2 crystallizes in the primitive tetragonal CeMg2Si2-type structure (P4/mmm), in agreement with literature data. The rho(T) data exhibit metallic behavior for all three compounds. Bulk superconductivity is reported for CaPd2As2 and SrPd2As2 below Tc = 1.27 and 0.92 K, respectively, whereas only a trace of superconductivity is found in BaPd2As2. No other phase transitions were observed. The chi(T) and M(H) data reveal anisotropic diamagnetism in the normal state, with chi_c > chi_ab for CaPd2As2 and BaPd2As2, and chi_c < chi_ab for SrPd2As2. The normal and superconducting state data indicate that CaPd2As2 and SrPd2As2 are conventional type-II nodeless s-wave electron-phonon superconductors. The electronic superconducting-state heat capacity data for CaPd2As2, which has an extremely sharp heat capacity jump at Tc, are analyzed using our recent elaboration of the alpha-model of the BCS theory of superconductivity, which indicates that the s-wave gap in this compound is anisotropic in momentum space.

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arXiv2013

Rutgers relation for the analysis of superfluid density in superconductors

It is shown that the thermodynamic Rutgers relation for the second order phase transitions can be used for the analysis of the superfluid density data irrespective of complexities of the Fermi surface, structure of the superconducting gap, pairing strength, or scattering. The only limitation is that critical fluctuations should be weak, so that the mean field theory of the second order phase transitions is applicable. By using the Rutgers relation, the zero temperature value of the London penetration depth, $λ(0)$, is related to the specific heat jump $ΔC$ and the slope of upper critical field $dH_{c2}/dT$ at the transition temperature $T_c$, provided the data on $Δλ=λ(T)-λ(0)$ are available in a broad temperature domain. We then provide a new way of determination of $λ(0)$, the quantity difficult to determine within many techniques.

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arXiv2013

Orbital upper critical field of type-II superconductors with pair breaking

The orbital upper critical field $H_{c2} $ is evaluated for isotropic materials with arbitrary transport and pair-breaking scattering rates. It is shown that unlike transport scattering which enhances $H_{c2} $, the pair breaking suppresses the upper critical field and reduces the dimensionless ratio $h^*(0)=H_{c2}(0)/T_c(dH_{c2}/dT)_{T_c} $ from the Helfand-Werthamer value of $\approx 0.7$ to 0.5 for a strong pair-breaking. $h^*(T)$ is evaluated for arbitrary transport and pair-breaking scattering. A phenomenological model for the pair-breaking suppression by magnetic fields is introduced. It shows qualitative features such as a positive curvature of $H_{c2}(T) $ and the low temperature upturn usually associated with multi-band superconductivity.

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arXiv2013

Sign-reversal of the in-plane resistivity anisotropy in hole-doped iron pnictides

The in-plane anisotropy of the electrical resistivity across the coupled orthorhombic and magnetic transitions of the iron pnictides has been extensively studied in the parent and electron-doped compounds. All these studies universally show that the resistivity $ρ_{a}$ across the long orthorhombic axis $a_{O}$ - along which the spins couple antiferromagnetically below the magnetic transition temperature - is smaller than the resistivity $ρ_{b}$ of the short orthorhombic axis $b_{O}$, i. e. $ρ_{a}<ρ_{b}$. Here we report that in the hole-doped compounds Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, as the doping level increases, the resistivity anisotropy initially becomes vanishingly small, and eventually changes sign for sufficiently large doping, i. e. $ρ_{b}<ρ_{a}$. This observation is in agreement with a recent theoretical prediction that considers the anisotropic scattering of electrons by spin-fluctuations in the orthorhombic/nematic state.

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arXiv2013

Exchange randomness and spin dynamics in the frustrated magnetic Keplerate {W72V30}

The magnetic properties and spin dynamics of the spin frustrated polyoxometalate {W72V30}, where 30 V^{4+} ions (s = 1/2) occupy the sites of an icosidodecahedron, have been investigated by low temperature magnetization, magnetic susceptibility, proton and vanadium nuclear magnetic resonance, and theoretical studies. The field-dependent magnetization at 0.5 K increases monotonically up to 50 T without any sign of staircase behavior. This low-temperature behavior cannot be explained by a Heisenberg model based on a single value of the nearest-neighbor exchange coupling. We analyze this behavior upon assuming a rather broad distribution of nearest-neighbor exchange interactions. Slow spin dynamics of {W72V30} at low temperatures is observed from the magnetic field and temperature dependence of nuclear spin-lattice relaxation rate 1/T_1 measurements.

cond-mat.str-el↗

arXiv2013

Millimeter-wave surface impedance of optimally-doped Ba(Fe1-xCox)2As2 single crystals

Precision measurements of active and reactive components of in-plane microwave surface impedance were performed in single crystals of optimally-doped Fe-based superconductor Ba(Fe1-xCox)2As2 (x = 0.074, Tc = 22.8 K). Measurements in a millimeter wavelength range (Ka band, 35-40 GHz) were performed using whispering gallery mode excitations in the ultrahigh quality factor quasioptical sapphire disk resonator with YBa2Cu2O7 superconducting (Tc = 90 K) end plates. The temperature variation of the London penetration depth is best described by a power-law function, delta λ(T) is proportional to T with the exponent n, n = 2.8, in reasonable agreement with radio-frequency measurements on crystals of the same batch. This power-law dependence is characteristic of a nodeless superconducting gap in the extended s-wave pairing scenario with a strong pair-breaking scattering. The quasiparticle conductivity of the samples, σ1(T), gradually increases with the decrease of temperature, showing no peak below or at Tc, in notable contrast with the behavior found in the cuprates. The temperature-dependent quasiparticle scattering rate was analyzed in a two-fluid model, assuming the validity of the Drude description of conductivity and generalized expression for the scattering rate. This analysis allows us to estimate the range of the values of a residual surface resistance from 3 to 6 mOhm.

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arXiv2013

Effect of Heavy-Ion Irradiation on Superconductivity in Single Crystals of Ba0.6K0.4Fe2As2 Pnictide Superconductor

The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 nm and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T^2 power law was observed below Tc/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provides evidence for an s+/- pairing state.

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arXiv2013

Inter-plane resistivity of isovalent doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$

Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was measured for the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a broad isoelectron phosphorus substitution range from $x$=0 to $x$=0.60, from non-superconducting parent compound to heavily overdoped superconducting composition with $T_c\approx 10 K$. The features due to structural and magnetic transitions are clearly resolved in $ρ_c(T)$ of the underdoped crystals. A characteristic maximum in $ρ_c(T)$, found in the parent BaFe$_2$As$_2$ at around 200 K, moves rapidly with phosphorus substitution to high temperatures. At the optimal doping, the inter-plane resistivity shows $T$-linear temperature dependence without any cross-over anomalies, similar to the previously reported in-plane resistivity. This observation is in stark contrast with dissimilar temperature dependences found at optimal doping in electron-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Our finding suggests that despite similar values of the resistivity and its anisotropy, the temperature dependent transport in the normal state is very different in electron and isoelectron doped compounds. Similar temperature dependence of both in-plane and inter-plane resistivities, in which the dominant contributions are coming from different parts of the Fermi surface, suggests that scattering is the same on the whole Fermi surface. Since magnetic fluctuations are expected to be much stronger on the quasi-nested sheets, this observation may point to the importance of the inter-orbital scattering between different sheets.

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arXiv2013

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi$_3$

Single crystals of MgCNi$_3$, with areas sized up to 1 mm$^{2}$, were grown by the self flux method using a cubic anvil high pressure technique. The first critical field \textit{H$_{c1}$}, determined from a zero temperature extrapolation, is around 18 mT. Using the tunnel - diode resonator technique, the London penetration depth was measured with no applied \textit{dc} field and the Campbell penetration depth was measured with the external \textit{dc} fields up to 9T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, $λ(0) = 245 \pm 10$ nm was determined from the thermodynamic Rutgers formula. The superfluid density, $ρ_s=(λ(0)/λ(T))^2$ was found to follow the clean isotropic \textit{s}-wave behavior predicted by the weak - coupling BCS theory in the whole temperature range. The low - temperature behavior of the London penetration depth fits the BCS analytic form as well and produces close to the weak - coupling value of $Δ(0)/k_BT_c = 1.71$. The temperature dependence of the upper critical field, \textit{$H_{c2}$}, was found to be isotropic with a slope at \textit{T$_c$} of -2.63 T/K and \textit{H$_{c2}$}(0) $\approx$ 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi$_3$, an irreversible feature has been observed in the TDR response when the sample is field-cooled and warmed versus zero-field-cooled and warmed. This feature possesses a non-monotonic field dependence and has commonly been referred to as the peak effect and is most likely related to a field - dependent non - parabolic pinning potential.

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arXiv2013

Magnetic field tuned quantum criticality of heavy fermion system YbPtBi

In this paper, we present the systematic measurements of the temperature and magnetic field dependences of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by applied magnetic field of $H_{c}$ $\sim$ 4 kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of its low temperature states emerges: (I) AFM state, characterized by spin density wave (SDW) like feature, which can be suppressed to $T$ = 0 by the relatively small magnetic field of $H_{c}$ $\sim$ 4\,kOe, (II) field induced anomalous state in which the electrical resistivity follows $Δρ(T) \propto T^{1.5}$ between $H_{c}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL) state in which $Δρ(T) \propto T^{2}$ for $H \geq$ 8 kOe. Regions I and II are separated at $T$ = 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state.

cond-mat.str-el↗

arXiv2013

Magnetic penetration depth in single crystals of SrPd$_2$Ge$_2$ superconductor

The in-plane magnetic penetration depth, $λ_m(T)$, was measured in single crystals of SrPd$_2$Ge$_2$ superconductor in a dilution refrigerator down to T=60 mK and in magnetic fields up to $H_{dc} = 1$ T by using a tunnel diode resonator. The London penetration depth, $λ$, saturates exponentially approaching $T\rightarrow 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $λ(0) = 426$ nm which was used to calculate the superfluid density, $ρ_s(T)=λ^2(0)/λ^2(T)$. Analysis of $ρ_s(T)$ in the full temperature range shows that it is best described by a single - gap behavior, perhaps with somewhat stronger coupling. In a magnetic field, the measured penetration depth is given by the Campbell penetration depth which was used to calculate the theoretical critical current density $j_c$. For $H \le 0.45$ T, the strongest pinning is achieved not at the lowest, but at some intermediate temperature, probably due to matching effect between temperature - dependent coherence length and relevant pinning lengthscale. Finally, we find a compelling evidence for surface superconductivity. Combining all measurements, the entire $H$-$T$ phase diagram of SrPd$_2$Ge$_2$ is constructed with an estimated $H_{c2}(0)=0.4817$ T.

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arXiv2013

Angular-dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

In-plane resistivity measurements as a function of temperature, magnetic field and its orientation with respect to the crystallographic $ab-$plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and $x$=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The dependence of the $H_{c2}$ on angle $θ$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}(θ)$, clearly deviates from Ginzburg-Landau functional form.

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