arXiv subjects

Sergey L. Bud'ko

Publications and source records attributed to Sergey L. Bud'ko.

At least 127 records · Page 7Linked to original sources

arXiv2016

Superconducting properties of Rh$_{9}$In$_4$S$_4$ single crystals

The synthesis and crystallographic, thermodynamic and transport properties of single crystalline Rh$_{9}$In$_4$S$_4$ were studied. The resistivity, magnetization and specific heat measurements all clearly indicate bulk superconductivity with a critical temperature, $T_{c}\sim$ 2.25 K. The Sommerfeld coefficient $γ$ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 34 mJ mol$^{-1}$ K$^{-2}$ and 217 K respectively. The observed specific heat jump, $ΔC/γT_{c}$ = 1.66, is larger than the expected BCS weak coupling value of 1.43. Ginzburg-Landau (GL) ratio of the low temperature GL-penetration depth, $λ_\text{GL}$$\approx$ 5750Å, to the GL-coherence length, $ξ_\text{GL}$$\approx$ 94Å, is large: $κ$ $\sim$ 60. Furthermore, we observed a peak effect in the resistivity measurement as a function of both temperature and magnetic field.

cond-mat.supr-con↗

arXiv2016

Robust tunability of magnetorestance in Half-Heusler RPtBi (R = Gd, Dy, Tm, and Lu) compounds

We present the magnetic field dependencies of transport properties for $R$PtBi ($R$ = Gd, Dy, Tm, and Lu) half-Heusler compounds. Temperature and field dependent resistivity measurements of high quality $R$PtBi single crystals reveal an unusually large, non-saturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of $H$ = 140 kOe. At 300 K, the large MR effect decreases as the rare-earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional $H^{2}$ behavior. The Hall coefficient ($R_{H}$) for $R$ = Gd indicates a sign change around 120 K, whereas $R_{H}$ curves for $R$ = Dy, Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as $\sim$100 $μ$V/K at 140 kOe. Analysis of the transport data in this series reveals that the rare-earth-based Half-Husler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of non-trivial MR.

cond-mat.str-el↗

arXiv2016

Transition to collapsed tetragonal phase in CaFe2As2 single crystals as seen by 57Fe Mossbauer spectroscopy

Temperature dependent measurements of 57Fe Mossbauer spectra on CaFe2As2 single crystals in the tetragonal and collapsed tetragonal phases are reported. Clear features in the temperature dependencies of the isomer shift, relative spectra area and quadrupole splitting are observed at the transition from the tetragonal to the collapsed tetragonal phase. From the temperature dependent isomer shift and spectral area data, an average stiffening of the phonon modes in the collapsed tetragonal phase is inferred. The quadrupole splitting increases by ~25% on cooling from room temperature to ~100 K in the tetragonal phase and is only weakly temperature dependent at low temperatures in the collapsed tetragonal phase, in agreement with the anisotropic thermal expansion in this material. In order to gain microscopic insight about these measurements we perform ab initio density functional theory calculations of the electric field gradient and the electron density of CaFe2As2 in both phases. By comparing the experimental data with the calculations we are able to fully characterize the crystal structure of the samples in the collapsed-tetragonal phase through determination of the As z-coordinate. Based on the obtained temperature dependent structural data we are able to propose charge saturation of the Fe - As bond region as the mechanism behind the stabilization of the collapsed-tetragonal phase at ambient pressure.

cond-mat.mtrl-sci↗

arXiv2016

Non-monotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of single crystalline FeSe was studied using measurements of the inter-plane resistivity, $ρ_{\textrm{c}}$ in magnetic fields parallel to tetragonal $c$-axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_\textrm{c}$, of FeSe exhibits a non-monotonic dependence with local maximum at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of the upper critical field at $T_\textrm{c}$, $\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also exhibits a non-monotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p p_2$ the slope is in good quantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.

cond-mat.supr-con↗

arXiv2016

Preserved entropy and fragile magnetism

A large swath of strongly correlated electron systems can be associated with the phenomena of preserved entropy and fragile magnetism. In this overview we present our thoughts and plans for the discovery and development of lanthanide and transition metal based, strongly correlated systems that are revealed by suppressed, fragile magnetism or grow out of preserved entropy. We will present and discuss current examples such as YbBiPt, YbAgGe, YbFe2Zn20, PrAg2In, BaFe2As2, CaFe2As2, LaCrSb3 and LaCrGe3 as part of our motivation and to provide illustrative examples.

cond-mat.str-el↗

arXiv2016

Magnetization and transport properties of single crystalline RPd${_2}$P${_2}$ (R=Y, La-Nd, Sm-Ho, Yb)

Single crystals of RPd${_2}$P${_2}$ (R=Y, La-Nd, Sm-Ho, Yb) were grown out of a high temperature solution rich in Pd and P and characterized by room-temperature powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization and temperature-dependent in-plane resistivity measurements. In this series, YPd${_2}$P${_2}$ and LaPd${_2}$P${_2}$ and YbPd${_2}$P${_2}$ (with Yb$^{+2}$) are non local-moment bearing, whereas CePd${_2}$P${_2}$ and PrPd${_2}$P${_2}$ order at low temperature with a ferromagnetic component along the crystallographic c-axis. The rest of the series manifest low temperature antiferromagnetic ordering. EuPd${_2}$P${_2}$ has Eu$^{+2}$ ions and both EuPd${_2}$P${_2}$ and GdPd${_2}$P${_2}$ have isotropic paramagnetic susceptibilities consistent with $L = 0$ and $J=S=\frac{7}{2}$ and exhibit multiple magnetic transitions. For R=Eu-Dy, there are multiple, $T>1.8$ K transitions in zero applied magnetic field and for R=Nd, Eu, Gd, Tb, and Dy there are clear metamagnetic transitions at T=2.0 K for $H<55$ kOe. Strong anisotropies arising mostly from crystal electric field (CEF) effects were observed for most magnetic rare earths with $L\neq 0$. The experimentally estimated CEF parameters B$^2_0$ were calculated from the anisotropic paramagnetic $θ_{ab}$ and $ θ_{c}$ values and compared to theoretical trends across the rare earth series. The ordering temperatures as well as the polycrystalline averaged paramagnetic Curie-Weiss temperature, $θ_{ave}$, were extracted from magnetization and resistivity measurements, and compared to the de-Gennes factor.

cond-mat.str-el↗

arXiv2015

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi2B2C

We present a local probe study of the magnetic superconductor, ErNi$_2$B$_2$C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi$_2$B$_2$C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth $λ$ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. In addition, we estimate the absolute pinning force of Abrikosov vortices, which shows a position- and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.

cond-mat.supr-con↗

arXiv2015

Mechanically-induced disorder in CaFe2As2: a 57Fe Mossbauer study

57Fe Mossbauer spectroscopy was used to perform a microscopic study on the extremely pressure and strain sensitive compound, CaFe2As2, with different degrees of strain introduced by grinding and annealing. At the base temperature, in the antiferromagnetic/orthorhombic phase, compared to a sharp sextet Mossbauer spectrum of single crystal CaFe2As2, which is taken as an un-strained sample, an obviously broadened sextet and an extra doublet were observed for ground CaFe2As2 powders with different degrees of strain. The Mossbauer results suggest that the magnetic phase transition of CaFe2As2 can be inhomogeneously suppressed by the grinding induced strain to such an extent that the antiferromagnetic order in parts of the grains forming the powdered sample remain absent all the way down to 4.6 K. However, strain has almost no effect on the temperature dependent hyperfine magnetic field in the grains with magnetic order. Additional electronic and asymmetry information was obtained from the isomer shift and quadrupole splitting. Similar isomer shift values in the magnetic phase for samples with different degrees of strain, indicate that the stain does not bring any significant variation of the electronic density at 57Fe nucleus position. The absolute values of quadrupole shift in the magnetic phase decrease and approach zero with increasing degrees of strain, indicating that the strain reduces the average lattice asymmetry at Fe atom position.

cond-mat.mtrl-sci↗

arXiv2015

Temperature induced Lifshitz transition in WTe2

We use ultra-high resolution, tunable, VUV laser-based, angle-resolved photoemission spectroscopy (ARPES) and temperature and field dependent resistivity and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compound that manifests exceptionally large, temperature dependent magnetoresistance. The temperature dependence of the TEP shows a change of slope at T=175 K and the Kohler rule breaks down above 70-140 K range. The Fermi surface consists of two electron pockets and two pairs of hole pockets along the X-Gamma-X direction. Upon increase of temperature from 40K, the hole pockets gradually sink below the chemical potential. Like BaFe2As2, WTe2 has clear and substantial changes in its Fermi surface driven by modest changes in temperature. In WTe2, this leads to a rare example of temperature induced Lifshitz transition, associated with the complete disappearance of the hole pockets. These dramatic changes of the electronic structure naturally explain unusual features of the transport data.

cond-mat.str-el↗

arXiv2015

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

cond-mat.str-el↗

arXiv2015

Study of 57Fe Mossbauer effect in RFe2Zn20 ( R = Lu, Yb, Gd)

We report measurements of 57Fe Mossbauer spectra for RFe2Zn20 ( R = Lu, Yb, Gd) from ~ 4.5 K to room temperature. The obtained isomer shift values are very similar for all three compounds, their temperature dependence was analyzed within the Debye model and resulted in an estimate of the Debye temperatures of 450-500 K. The values of quadrupole splitting at room temperature change with the cubic lattice constant a in a linear fashion. For GdFe2Zn20, ferromagnetic order is seen as an appearance of a sextet in the spectra. The 57Fe site hyperfine field for T = 0 was evaluated to be ~ 2.4 T.

cond-mat.mtrl-sci↗

arXiv2015

Superconductivity versus structural phase transition in the closely related Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ and Bi$_{2} $Rh$ _{3}$S$ _{2}$

Single crystals of Bi$_{2} $Rh$ _{3}$S$ _{2}$ and Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ were synthesized by solution growth and the crystal structures, thermodynamic and transport properties of both compounds were studied. In the case of Bi$_{2} $Rh$ _{3}$S$ _{2}$, a structural first-order transition at around 165 K is identified by single crystal diffraction experiments, with clear signatures visible in resistivity, magnetization and specific heat data. No superconducting transition for Bi$_{2} $Rh$ _{3}$S$ _{2}$ was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$, however bulk superconductivity with a critical temperature, $T_{c}\approx$ 1.7 K was observed. The Sommerfeld coefficient $γ$\ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 9.4 mJ mol$^{-1}$ K$^{-2}$ and 209 K respectively for Bi$_{2} $Rh$ _{3}$S$ _{2}$, and 22 mJ mol$^{-1}$ K$^{-2}$ and 196 K respectively for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$. Study of the specific heat in the superconducting state of Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ suggests that Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ is a weakly coupled, BCS superconductor.

cond-mat.supr-con↗

arXiv2015

Physical properties of V$_{1-x}$Ti$_{x}$O$_{2}$ (0 $<$ x $<$ 0.187) single crystals

Free standing, low strain, single crystals of pure and titanium doped VO$_{2}$ were grown out of an excess of V$_{2}$O$_{5}$ using high temperature solution growth techniques. At $T_{MI} \sim$ 340 K, pure VO$_{2}$ exhibits a clear first-order phase transition from a high-temperature paramagnetic tetragonal phase (R) to a low-temperature non-magnetic monoclinic phase (M1). With Ti doping, another monoclinic phase (M2) emerges between the R and M1 phases. The phase transition temperature between R and M2 increases with increasing Ti doping while the transition temperature between M2 and M1 decreases.

cond-mat.mtrl-sci↗

arXiv2015

Origin of the spin reorientation transitions in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys

Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence $K(x)$ with a maximum near $x=0.3$ and a minimum near $x=0.8$ is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, $K$ depends strongly on the tetragonality and doubles under a modest 3% increase of the $c/a$ ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.

cond-mat.mtrl-sci↗

arXiv2015

57Fe Mossbauer study of Lu2Fe3Si5 iron silicide superconductor

With the advent of Fe-As based superconductivity it has become important to study how superconductivity manifests itself in details of 57Fe Mossbauer spectroscopy of conventional, Fe - bearing superconductors. To this end, the iron-based superconductor Lu2Fe3Si5 has been studied by 57Fe Mossbauer spectroscopy over the temperature range from 4.4 K to room temperature with particular attention to the region close to the superconducting transition temperature (Tc = 6.1 K). Consistent with the two crystallographic sites for Fe in this structure, the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. The value of Debye temperature was estimated from temperature dependence of the isomer shift and the total spectral area and compared with the specific heat capacity data. Neither abnormal behavior of the hyperfine parameters at or near Tc, nor phonon softening were observed.

cond-mat.mtrl-sci↗

arXiv2015

Superconductivity of magnesium diboride

Over the past 14 years MgB2 has gone from a startling discovery to a promising, applied superconductor. In this article we present a brief overview of the synthesis and the basic superconducting properties of this remarkable compound. In particular, the effects of pressure, substitutions and neutron irradiation on superconducting properties are discussed.

cond-mat.supr-con↗

arXiv2015

Anisotropic $H_{c2}$, thermodynamic and transport measurements, and pressure dependence of $T_{c}$ in K$_{2}$Cr$_{3}$As$_{3}$ single crystals

We present a detailed study of single crystalline K$_{2}$Cr$_{3}$As$_{3}$ and analyze its thermodynamic and transport properties, anisotropic $H_{c2}(T)$, and initial pressure-dependence of $T_{c}$. In zero-field, the temperature-dependent resistivity is metallic. Deviation from a linear temperature-dependence is evident below 100 K and a $T^{3}$-dependence is roughly followed from just above $T_{c}$ ($\sim$10 K) to $\sim$ 40 K. Anisotropic $H_{c2}(T)$ data were measured up to 140 kOe with field applied along and perpendicular to the rod-like crystals. For the applied field perpendicular to the rod, $H_{c2}(T)$ is linear with a slope $\sim$-70 kOe/K. For field applied along the rod, the slope is about -120 kOe/K below 70 kOe. Above 70 kOe, the magnitude of the slope decreases to $\sim$-70 kOe/K. The electronic specific heat coefficient, $γ$, just above $T_{c}$, is 73 mJ/mol K$^{2}$; the Debye temperature, $Θ_{D}$, is 220 K. The specific heat jump at the superconducting transition $Δ$C $\sim$ 2.2 $γT_{c}$. Finally, for hydrostatic pressures up to $\sim$7 kbar, $T_{c}$ decreases under pressure linearly at a rate of -0.034 K/kbar.

cond-mat.supr-con↗

arXiv2014

Superconductivity In K- And Na- Doped Bafe2as2: What Can We Learn From Heat Capacity And Pressure Dependence of Tc

A brief overview of changes in the superconducting transition temperature under pressure and evolution of specific heat capacity jump at Tc for two related families of iron - based superconductors, Ba{1-x}KxFe2As2 (0.2 < x < 1.0) and Ba{1-x}NaxFe2As2 (0.2 < x < 0.9) will be given. For Ba{1-x}KxFe2As2 the specific heat capacity jump at Tc measured over the whole extent of the superconducting dome shows clear deviation from the empirical, Delta Cp (Tc) ~ Tc^3, scaling (known as the BNC scaling) for x > 0.7. At the same concentrations range apparent equivalence of effects of pressure and K- substitution on Tc fails. These observations suggests a significant change of the superconducting state for x > 0.7. In contrast, the data for the large portion of Ba{1-x}NaxFe2As2 (0.2 < x < 0.9) series follow the BNC scaling. However, the pressure dependence of Tc (measured up to ~ 12 kbar) have clear non-linearities for Na concentration in 0.2 - 0.25 region, that may be consistent with Tc crossing the phase boundaries of the emergent, narrow, antiferromagnetic/tetragonal phase. Results will be discussed in context other studies of these two and related families of iron-based superconductors.

cond-mat.supr-con↗