arXiv subjects

K. Shrestha

Publications and source records attributed to K. Shrestha.

8 recordsLinked to original sources

arXiv2021

Spectroscopic Evidence for the Direct Involvement of Local Moments in the Pairing Process of the Heavy-Fermion Superconductor CeCoIn$_5$

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, 'composite pairing' based on the cooperative two-channel Kondo effect directly involving the f-electron moments has also been proposed for some heavy fermion compounds including CeCoIn$_5$. The origin of the spin resonance peak observed in neutron scattering measurements on CeCoIn$_5$ is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn5 over wide ranges of temperature and magnetic field. The pairing gap opens at T$_p$ ~ 5 K, well above the bulk T$_c$ = 2.3 K, and its directional dependence is consistent with d$_{x^2-y^2}$ symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T$_p$. The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f-electron local moments are directly involved in the pairing process in CeCoIn$_5$.

cond-mat.supr-con↗

arXiv2019

The grain-size effect on thermal conductivity of uranium dioxide

We have investigated the grain boundary scattering effect on the thermal transport behavior of uranium dioxide (UO$_2$). The polycrystalline samples having different grain-sizes (0.125, 1.8, and 7.2 $μ$m) have been prepared by spark plasma sintering technique and characterized by x-ray powder diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy. The thermal transport properties (the thermal conductivity and thermoelectric power) have been measured in the temperature range 2-300~K and the results were analyzed in terms of various physical parameters contributing to the thermal conductivity in these materials in relation to grain-size. We show that thermal conductivity decreases systematically with lowering grain-size in the temperatures below 30 K, where the boundary scattering dominates the thermal transport. At higher temperatures more scattering processes are involved in the heat transport in these materials, making the analysis difficult. We determined the grain boundary Kapitza resistance that would result in the observed increase in thermal conductivity with grain size, and compared the value with Kapitza resistances calculated for UO$_2$ using molecular dynamics from the literature.

cond-mat.mtrl-sci↗

arXiv2018

On the magnetic and electronic properties of NpPdSn

We have studied NpPdSn by means of the heat capacity, electrical resistivity, Seebeck and Hall effect, $^{237}$Np Mössbauer spectroscopy, and neutron diffraction measurements in the temperature range 2-300 K and under magnetic fields up to 14 T. NpPdSn orders antiferromagnetically below the Néel temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a a doubly degenerate ground state. In the magnetic state the electrical resistivity and heat capacity are characterized by electron-magnon scattering with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanced Sommerfeld coefficient and typical behavior of magnetorestistivity, Seebeck and Hall coefficients are all characteristic of systems with strong electronic correlations. The low temperature antiferromagnetic state of NpPdSn is verified by neutron diffraction and $^{237}$Np Mössbauer spectroscopy and possible magnetic structures are discussed.

cond-mat.str-el↗

arXiv2018

Combination of thermal and electric properties measurement techniques in a single setup suitable for radioactive materials in controlled environments and based on the 3-omega approach

We have designed and developed a new experimental setup, based on the 3-omega method, to measure thermal conductivity, heat capacity and electrical resistivity of a variety of samples in a broad temperature range (2-550 K) and under magnetic fields up to 9 T. The validity of this method is tested by measuring various types of metallic (copper, platinum, and constantan) and insulating (SiO_2) materials, which have a wide range of thermal conductivity values (1-400 Wm-1K-1). We have successfully employed this technique for measuring the thermal conductivity of two actinide single crystals, uranium dioxide, and uranium nitride. This new experimental approach for studying nuclear materials will help to advance reactor fuel development and understanding. We have also shown that this experimental setup can be adapted to the Physical Property Measurement System (Quantum Design) environment and/or other cryocooler systems.

physics.ins-det↗

arXiv2017

Weak antilocalization effect due to topological surface states in Bi$_2$Se$_{2.1}$Te$_{0.9}$

We have investigated the weak antilocalization (WAL) effect in the p-type Bi$_2$Se$_{2.1}$Te$_{0.9}$ topological system. The magnetoconductance shows a cusp-like feature at low magnetic fields, indicating the presence of the WAL effect. The WAL curves measured at different tilt angles merge together when they are plotted as a function of the normal field components, showing that surface states dominate the magnetoconductance in the Bi$_2$Se$_{2.1}$Te$_{0.9}$ crystal. We have calculated magnetoconductance per conduction channel and applied the Hikami-Larkin-Nagaoka formula to determine the physical parameters that characterize the WAL effect. The number of conduction channels and the phase coherence length do not change with temperature up to T=5 K. In addition, the sample shows a large positive magnetoresistance that reaches 1900% under a magnetic field of 35 T at T=0.33K with no sign of saturation. The magnetoresistance value decreases with both increasing temperature and tilt angle of the sample surface with respect to the magnetic field. The large magnetoresistance of topological insulators can be utilized in future technology such as sensors and memory devices.

cond-mat.str-el↗

arXiv2017

Large magnetoresistance and Fermi surface study of Sb$_2$Se$_2$Te single crystal

We have studied the magnetotransport properties of a Sb$_2$Se$_2$Te single crystal. Magnetoresistance (MR) is maximum when the magnetic field is perpendicular to the sample surface and reaches to a value of 1100\% at $B$=31 T with no sign of saturation. MR shows Shubnikov de Haas (SdH) oscillations above $B$=15 T. The frequency spectrum of SdH oscillations consists of three distinct peaks at $α$=32 T, $β$=80 T and $γ$=117 T indicating the presence of three Fermi surface pockets. Among these frequencies, $β$ is the prominent peak in the frequency spectrum of SdH oscillations measured at different tilt angles of the sample with respect to the magnetic field. From the angle dependence $β$ and Berry phase calculations, we have confirmed the trivial topology of the $β$-pocket. The cyclotron masses of charge carriers, obtained by using the Lifshitz-Kosevich formula, are found to be $m^{*}_β=0.16m_o$ and $m^{*}_γ=0.63m_o$ for the $β$ and $γ$ bands respectively. Large MR of Sb$_2$Se$_2$Te is suitable for utilization in electronic instruments such as a computer hard disc, high field magnetic sensors, and memory devices.

cond-mat.mes-hall↗

arXiv2017

Extremely large non-saturating magnetoresistance and ultrahigh mobility due to topological surface states in metallic Bi2Te3 topological insulator

Weak antilocalization (WAL) effects in Bi2Te3 single crystals have been investigated at high and low bulk charge carrier concentrations. At low charge carrier density the WAL curves scale with the normal component of the magnetic field, demonstrating the dominance of topological surface states in magnetoconductivity. At high charge carrier density the WAL curves scale with neither the applied field nor its normal component, implying a mixture of bulk and surface conduction. WAL due to topological surface states shows no dependence on the nature (electrons or holes) of the bulk charge carriers. The observations of an extremely large, non-saturating magnetoresistance, and ultrahigh mobility in the samples with lower carrier density further support the presence of surface states. The physical parameters characterizing the WAL effects are calculated using the Hikami-Larkin-Nagaoka formula. At high charge carrier concentrations, there is a greater number of conduction channels and a decrease in the phase coherence length compared to low charge carrier concentrations. The extremely large magnetoresistance and high mobility of topological insulators have great technological value and can be exploited in magneto-electric sensors and memory devices.

cond-mat.str-el↗

arXiv2017

Quantum oscillations in metallic Sb2Te2Se topological insulator

We have studied the magnetotransport properties of the metallic, p-type Sb2Te2Se which is a topological insulator. Magnetoresistance shows Shubnikov de Haas oscillations in fields above B=15 T. The maxima/minima positions of oscillations measured at different tilt angles with respect to the B direction align with the normal component of field Bcosine, implying the existence of a 2D Fermi surface in Sb2Te2Se. The value of the Berry phase determined from a Landau level fan diagram is very close to 0.5, further suggesting that the oscillations result from topological surface states. From Lifshitz-Kosevich analyses, the position of the Fermi level is found to be EF =250 meV, above the Dirac point. This value of EF is almost 3 times as large as that in our previous study on the Bi2Se2:1Te0:9 topological insulator; however, it still touches the tip of the bulk valence band. This explains the metallic behavior and hole-like bulk charge carriers in the Sb2Te2Se compound.

cond-mat.mes-hall↗