arXiv subjects

R. Movshovich

Publications and source records attributed to R. Movshovich.

At least 37 records · Page 2Linked to original sources

arXiv2013

Anomalous doping effect on the superconducting state in CeCoIn$_5$ at high magnetic fields

We investigated the effect of electron and hole doping on the high-field low-temperature superconducting state in CeCoIn$_5$ by measuring specific heat of CeCo(In$_{\rm 1-x}$M$_{\rm x}$)$_5$ with M=Sn, Cd and Hg and $x$ up to 0.33% at temperatures down to 0.1\,K and fields up to 14\,T. Although both Cd- and Hg-doping (hole-doping) suppresses the zero-field $T_c$ monotonically, $H_{c2}$ increases with small amounts of doping and has a maximum around $x$=0.2% (M=Cd). On the other hand, with Sn-doping (electron-doping) both zero-field $T_c$ and $H_{c2}$ decrease monotonically. The critical temperature for the high-field low-temperature superconducting state (so called {\it Q}-state) correlates with $H_{c2}$ and $T_c$, which we interpret in support of the superconducting origin of this state.

cond-mat.str-el↗

arXiv2013

Two-band effect on the vortex dynamics and critical current density in an anisotropic MgB$_2$ thin film

We report the influence of intrinsic superconducting parameters on the vortex dynamics and the critical current densities of a MgB$_2$ thin film. The small magnetic penetration depth of λ= 50 nm at T=4 K is related to a clean π-band, and transport and magnetization data show an upper critical field similar to those reported in clean single crystals. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S(T) at low magnetic field can be explained by a simple Anderson-Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field, which is consistent with a two-band contribution.

cond-mat.supr-con↗

arXiv2013

Intrinsic superconducting properties and vortex dynamics in heavily overdoped Ba(Fe$_{0.86}$Co$_{0.14}$)$_2$As$_2$ single crystal

In this work we report the influence of intrinsic superconducting parameters on the vortex dynamics in an overdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (x=0.14) single crystal. We find a superconducting critical temperature of 13.5 K, magnetic penetration depth $λ_{ab}$(0) = 660 $\pm$ 50 nm, coherence length $ξ_{ab}$(0) = 5 nm, and the upper critical field anisotropy $γ_{T\rightarrow Tc}$ $\approx$ 3.7. In fact, the Ginzburg-Landau model may explain the angular dependent $H_{c2}$ for this anisotropic three-dimensional superconductor. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. In addition, we found no sign of correlated pinning along the c axis. Our results show that vortex core to defect size ratio and $λ$ play an important role in the resulting vortex dynamics in materials with similar intrinsic thermal fluctuations.

cond-mat.supr-con↗

arXiv2013

Ferromagnetic bubble clusters in Y$_{0.67}$Ca$_{0.33}$MnO$_3$ thin films

We studied the ferromagnetic topology in a Y$_{0.67}$Ca$_{0.33}$MnO$_3$ thin film with a combination of magnetic force microscopy and magnetization measurements. Our results show that the spin-glass like behavior, reported previously for this system, could be attributed to frustrated interfaces of the ferromagnetic clusters embedded in a non-ferromagnetic matrix. We found temperature dependent changes of the magnetic topology at low temperatures, which suggests a non-static Mn$^{3+}$/Mn$^{4+}$ ratio.

cond-mat.str-el↗

arXiv2012

Magnetic penetration-depth measurements of a suppressed superfluid density of superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals by proton irradiation

We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density $ρ_{s}$ in superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth $λ(T)$ using magnetic force microscopy. Our findings indicate that a radiation dose of 2$\times10^{16}$cm$^{-2}$ produced by 3 MeV protons results in a reduction of the superconducting critical temperature $T_{c}$ by approximately 10%. % with no appreciable change in the slope of the upper critical fields. In contrast, $ρ_{s}(0)$ is suppressed by approximately 60%. This break-down of the Abrikosov-Gorkov theory may be explained by the so-called "Swiss cheese model", which accounts for the spatial suppression of the order parameter near point defects similar to holes in Swiss cheese. Both the slope of the upper critical field and the penetration depth $λ(T/T_{c})/λ(0)$ exhibit similar temperature dependences before and after irradiation. This may be due to a combination of the highly disordered nature of Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ with large intraband and simultaneous interband scattering as well as the $s^\pm$-wave nature of short coherence length superconductivity.

cond-mat.supr-con↗

arXiv2012

Controllable chirality-induced geometrical Hall effect in a frustrated highly-correlated metal

A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase which can lead to a Hall voltage independent of the spin-orbit coupling within the material--a geometrical Hall effect. We show that the highly-correlated metal UCu5 possesses an unusually large controllable geometrical Hall effect at T<1.2K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20% of the ν=1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu5 opens a new field of inquiry into the importance of the role of frustration in highly-correlated electron materials.

cond-mat.str-el↗

arXiv2012

Direct measurement of the magnetic penetration depth by magnetic force microscopy

We present an experimental approach using magnetic force microscopy for measurements of the absolute value of the magnetic penetration depth $λ$ in superconductors. $λ$ is obtained in a simple and robust way without introducing any tip modeling procedure via direct comparison of the Meissner response curves for a material of interest to those measured on a reference sample. Using a well-characterized Nb film as a reference, we determine the absolute value of $λ$ in a Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ single crystal and a MgB$_2$ thin film through a comparative experiment. Our apparatus features simultaneous loading of multiple samples, and allows straightforward measurement of the absolute value of $λ$ in superconducting thin film or single crystal samples.

cond-mat.supr-con↗

arXiv2012

Measurement of the magnetic penetration depth of a superconducting MgB$_{2}$ thin film with a large intraband diffusivity

We report the temperature dependent magnetic penetration depth $λ(T)$ and the superconducting critical field $H_{c2}(T)$ in a 500-nm MgB$_{2}$ film. Our analysis of the experimental results takes into account the two gap nature of the superconducting state and indicates larger intraband diffusivity in the three-dimensional (3D) $π$ band compared to that in the two-dimensional (2D) $σ$ band. Direct comparison of our results with those reported previously for single crystals indicates that larger intraband scattering in the 3D $π$ band leads to an increase of $λ$. We calculated $λ$ and the thermodynamic critical field $H_{c}\approx$2000 Oe employing the gap equations for two-band superconductors. Good agreement between the measured and calculated $λ$ value indicates the two independent measurements, such as magnetic force microscopy and transport, provide a venue for investigating superconducting properties in multi-band superconductors.

cond-mat.supr-con↗

arXiv2012

Large magnetic penetration depth and thermal fluctuations in a Ca$_{10}$(Pt$_{3}$As$_{8}$)[(Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$]$_{5}$ (x=0.097) single crystal

We have measured the temperature dependence of the absolute value of the magnetic penetration depth $λ(T)$ in a Ca$_{10}$(Pt$_{3}$As$_{8}$)[(Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$]$_{5}$ (x=0.097) single crystal using a low-temperature magnetic force microscope (MFM). We obtain $λ_{ab}$(0)$\approx$1000 nm via extrapolating the data to $T = 0$. This large $λ$ and pronounced anisotropy in this system are responsible for large thermal fluctuations and the presence of a liquid vortex phase in this low-temperature superconductor with critical temperature of 11 K, consistent with the interpretation of the electrical transport data. The superconducting parameters obtained from $λ$ and coherence length $ξ$ place this compound in the extreme type \MakeUppercase{\romannumeral 2} regime. Meissner responses (via MFM) at different locations across the sample are similar to each other, indicating good homogeneity of the superconducting state on a sub-micron scale.

cond-mat.supr-con↗

arXiv2011

Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Γ7 doublet ground state and a Γ8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ6 excited state resides at 32 meV; however, the Γ8/Γ6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch- Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ7 ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

cond-mat.str-el↗

arXiv2011

Electronic inhomogeneity in a Kondo lattice

Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials, including the high-Tc copper-oxide superconductors, manganites, and most recently the iron-pnictide superconductors. The complexity generated by competing phases in these materials constitutes a considerable theoretical challenge-one that still defies a complete description. Here, we report a new manifestation of electronic inhomogeneity in a strongly correlated f-electron system, using CeCoIn5 as an example. A thermodynamic analysis of its superconductivity, combined with nuclear quadrupole resonance measurements, shows that nonmagnetic impurities (Y, La, Yb, Th, Hg and Sn) locally suppress unconventional superconductivity, generating an inhomogeneous electronic "Swiss cheese" due to disrupted periodicity of the Kondo lattice. Our analysis may be generalized to include related systems, suggesting that electronic inhomogeneity should be considered broadly in Kondo lattice materials.

cond-mat.str-el↗

arXiv2011

Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2

We have performed low-temperature specific heat $C$ and thermal conductivity $κ$ measurements on the Ni-pnictide superconductors BaNi$_2$As$_2$ ($T_\mathrm{c}$=0.7 K and SrNi$_2$P$_2$ ($T_\mathrm{c}$=1.4 K). The temperature dependences $C(T)$ and $κ(T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $κ$ for BaNi$_2$As$_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba$_{0.6}$La$_{0.4}$Ni$_2$As$_2$ shows a mild suppression of $T_\mathrm{c}$ and $H_\mathrm{c2}$ relative to BaNi$_2$As$_2$.

cond-mat.supr-con↗

arXiv2010

Unusual metamagnetism in CeIrIn$_5$

We report a high field investigation (up to 45 T) of the metamagnetic transition in CeIrIn$_5$ with resistivity and de-Haas-van-Alphen (dHvA) effect measurements in the temperature range 0.03-1 K. As the magnetic field is increased the resistivity increases, reaches a maximum at the metamagnetic critical field, and falls precipitously for fields just above the transition, while the amplitude of all measurable dHvA frequencies are significantly attenuated near the metamagnetic critical field. However, the dHvA frequencies and cyclotron masses are not substantially altered by the transition. In the low field state, the resistivity is observed to increase toward low temperatures in a singular fashion, a behavior that is rapidly suppressed above the transition. Instead, in the high field state, the resistivity monotonically increases with temperature with a dependence that is more singular than the iconic Fermi-liquid, temperature-squared, behavior. Both the damping of the dHvA amplitudes and the increased resistivity near the metamagnetic critical field indicate an increased scattering rate for charge carriers consistent with critical fluctuation scattering in proximity to a phase transition. The dHvA amplitudes do not uniformly recover above the critical field, with some hole-like orbits being entirely suppressed at high fields. These changes, taken as a whole, suggest that the metamagnetic transition in CeIrIn$_5$ is associated with the polarization and localization of the heaviest of quasiparticles on the hole-like Fermi surface.

cond-mat.str-el↗

arXiv2010

Observations of Pauli Paramagnetic Effects on the Flux Line Lattice in CeCoIn5

From small-angle neutron scattering studies of the flux line lattice (FLL) in CeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain the field- and temperature-dependence of the FLL form factor, which is a measure of the spatial variation of the field in the mixed state. We extend our earlier work [A.D. Bianchi et al. 2008 Science 319, 177] to temperatures up to 1250 mK. Over the entire temperature range, paramagnetism in the flux line cores results in an increase of the form factor with field. Near H_c2 the form factor decreases again, and our results indicate that this fall-off extends outside the proposed FFLO region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing. At higher temperatures, a gradual crossover towards more conventional mixed state behavior is observed.

cond-mat.supr-con↗

arXiv2009

Field-induced quantum critical point in the pressure-induced superconductor CeRhIn5

When subjected to pressure, the prototypical heavy-fermion antiferromagnet CeRhIn5 becomes superconducting, forming a broad dome of superconductivity centered around 2.35 GPa (=P2) with maximal Tc of 2.3 K. Above the superconducting dome, the normal state shows strange metallic behaviours, including a divergence in the specific heat and a sub-T-linear electrical resistivity. The discovery of a field-induced magnetic phase that coexists with superconductivity for a range of pressures P < P2 has been interpreted as evidence for a quantum phase transition, which could explain the non-Fermi liquid behavior observed in the normal state. Here we report electrical resistivity measurements of CeRhIn5 under magnetic field at P2, where the resistivity is sub-T-linear for temperatures above Tc (or T_FL) and a T^2-coefficient A found below T_FL diverges as Hc2 is approached. These results are similar to the field-induced quantum critical compound CeCoIn5 and confirm the presence of a quantum critical point in the pressure-induced superconductor CeRhIn5.

cond-mat.str-el↗

arXiv2009

Direct measurements of the penetration depth in a superconducting film using magnetic force microscopy

We report the local measurements of the magnetic penetration depth $λ$ in a superconducting Nb film using magnetic force microscopy (MFM). We developed a method for quantitative extraction of the penetration depth from single-parameter simultaneous fits to the lateral and height profiles of the MFM signal, and demonstrate that the obtained value is in excellent agreement with that obtained from the bulk magnetization measurements.

cond-mat.mtrl-sci↗

arXiv2009

Localized ferromagnetic resonance force microscopy in permalloy-cobalt films

We report Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a justaposed continuous films of permalloy and cobalt. Our studies demonstrate the capability of FMRFM to perform local spectroscopy of different ferromagnetic materials. Theoretical analysis of the uniform resonance mode near the edge of the film agrees quantitatively with experimental data. Our experiments demonstrate the micron scale lateral resolution in determining local magnetic properties in continuous ferromagnetic samples.

cond-mat.mtrl-sci↗

arXiv2009

Low-temperature thermal conductivity of BaFe2As2: Parent compound of iron-arsenide superconductors

We report low-temperature thermal conductivity down to 40 mK of the antiferromagnet BaFe$_2$As$_{2}$, which is the parent compound of recently discovered iron-based superconductors. In the investigated temperature range below 4 K, the thermal conductivity $κ$ is well described by the expression $κ$ = $aT$ + $bT^{2.22}$. We attribute the ``$aT$''-term to an electronic contribution which is found to satisfy the Wiedemann-Franz law in the $T$ $\to$ 0 K limit, and the remaining thermal conductivity, $\sim$ $T^{2.22}$, is attributed to phonon conductivity. A small influence on thermal conductivity by magnetic fields up to 8 T is well accounted by the observed magnetoresistance. The result is consistent with a fully gapped magnon spectrum, inferred previously from inelastic neutron scattering measurements.

cond-mat.str-el↗