Showing papers on "Relaxation (NMR) published in 2022"

Momentum relaxation effects in 2D-Xene field effect device structures

Abstract: We analyze the electric field driven topological field effect transition on 2D-xene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh non-equilibrium Green's function technique with the inclusion of momentum and phase relaxation, within the self-consistent Born approximation. Details of the transition with applied electric field are elucidated for the ON-OFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries band-tails which potentially activates parasitic OFF currents, thereby degrading the ON-OFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance.

Electron-phonon superconductivity in C-doped topological nodal-line semimetal Zr5Pt3: a muon spin rotation and relaxation (μSR) study.

Abstract: In the present work, we demonstrate that C-doped Zr5Pt3is an electron-phonon superconductor (with critical temperatureTC= 3.8 K) with a nonsymmorphic topological Dirac nodal-line semimetal state, which we report here for the first time. The superconducting properties of Zr5Pt3C0.5have been investigated by means of magnetization, resistivity, specific heat, and muon spin rotation and relaxation (μSR) measurements. We find that at low temperatures, the depolarization rate is almost constant and it can be well described by a single-bands-wave model with a superconducting gap of 2Δ(0)/kBTC= 3.84, somewhat higher than the value of BCS theory. From the transverse field μSR analysis, we estimate the London penetration depthλL= 469 nm, superconducting carrier densityns= 1.83 × 1026 m-3, and effective massm* = 1.428me. The zero field μSR confirms the absence of any spontaneous magnetic field in the superconducting ground state. In order to gain additional insights into the electronic ground state of C-doped Zr5Pt3, we also performed first-principles calculations within the framework of density functional theory (DFT). The observed homogenous electronic character of the Fermi surface as well as the mutual decrease ofTCand density of states at the Fermi level are consistent with the experimental findings of this study. However, the band structure reveals the presence of robust, gapless fourfold-degenerate nodal lines protected by 63screw rotations and glide mirror planes. Therefore, Zr5Pt3represents a novel, unprecedented condensed matter system to investigate the intricate interplay between superconductivity and topology.

Revisiting the factors influencing the magnetic resonance contrast of Gd2O3 nanoparticles

Abstract: Gadolinium oxide nanoparticles (GONs) have the potential to be one of the best candidates for the contrast agents of magnetic resonance imaging. Even though the influence of parameters on the relaxation has been substantially demonstrated, the variation of the r1 of GONs with a similar structure and surface chemistry implied our limited understanding. We herein synthesized GONs with adjustable size, shape, and crystallinity, modified them with a series of molecules with different acidities, and recorded their r1 values and imaging contrast. Our results showed that the isoelectric point could be regarded as an indicator of the relaxation covering the influence of both surface modification and size, which highlighted the impact of protons dissociated from the contrast agents. We further showed that the nanoparticles with lower crystallinity possess higher relaxivity, and this phenomenon manifested significantly under a low field. Our work clarified that the longitudinal relaxivity of Gd2O3 nanoparticles is sensitively dependent on the numbers of H+ generated from the surface and in the environment, which may shed light on developing high-performance nanoparticulate T1 contrast agents.

Competing magnetic interactions and magnetocaloric effect in Ho5Sn3.

Abstract: The rare-earth intermetallic compound Ho5Sn3demonstrates fascinating magnetic properties, which include temperature-driven multiple magnetic transitions and field-driven metamagnetism. We address the magnetic character of this exciting compound through a combined experimental and theoretical studies. Ho5Sn3orders antiferromagnetically below 28 K, and shows further spin reorientation transitions at 16 K and 12 K. We observe a sizable amount of low-temperature magnetocaloric effect (MEC) in Ho5Sn3with a maximum value of entropy change ΔS= -9.5 J Kg-1 K-1for an applied field ofH= 50 kOe at around 30 K. The field hysteresis is almost zero above 15 K where the MEC is important. Interestingly, ΔSis found to change its sign from positive to negative as the temperature is increased above about 8 K, which can be linked to the multiple spin reorientation transitions. The signature of the metamagnetism is visible in the ΔSversusHplot. The magnetic ground state, obtained from the density functional theory based calculation, is susceptible to the effective Coulomb interaction (Ueff) between electrons. Depending upon the value ofUeff, the ground state can be ferromagnetic or antiferromagnetic. The compound shows large relaxation (14% change in magnetisation in 60 min) in the field cooled state with a logarithmic time variation, which may be connected to the competing magnetic correlations observed in our theoretical calculations. The competing magnetic ground states are equally evident from the small value of the paramagnetic Curie-Weiss temperature.

Spatiotemporal characteristics of spin transport in compensated metals with electron-hole exchange interaction.

Abstract: We develop a theory describing spatiotemporal behavior of spin transport in two-band metals by postulating a spin-exchange interaction between electrons and holes. Starting with the semiclassical Boltzmann equation, we derive a system of coupled diffusion equations and solve them analytically under steady-state conditions. The solutions reveal two types of electron-hole coupled-spin transport modes: a dissipative mode and a nondissipative mode with an infinite spin diffusion length. The two modes are the manifestations of two types of spin coupling channels. Besides the exchange interaction, we incorporate into our derivation the relaxation caused by the spin-orbit interaction to show how it affects the spin transport characteristics of the two modes.

Coupled electronic and magnetic relaxation in Fe1+y Te: direct evidence for the interaction between itinerant carriers and local moments

Abstract: Iron chalcogenides are of particular interests among iron-based superconductors due to their distinct properties such as high-Tcon FeSe monolayer and competing magnetic correlations in Fe1+yTe. Here we report unusual transport properties observed near the critical composition of Fe1+yTe (y∼ 0.09) where competing magnetic correlations exist. The resistivity exhibits surprising temperature-dependent relaxation behavior belowTN, resulting in the increase of resistivity with time for 35 K

Molecular Relaxation in a Liquid Crystal After Switching Off the Acoustic Action

Abstract: Relaxation processes in thin (up to 125 μm) homeotropic layers of nematic liquid crystals (n-methoxybenzylidene-n-butylaniline), which were observed after switching off periodic shear oscillations with a low frequency (about 200 Hz), were experimentally studied. The foner method was used to excite vibrations in a liquid crystal sample. The amplitude of the oscillations was determined by inductive and optical methods. The cell with the sample was made in the form of a multi-layer structure. A liquid crystal was placed between the three glass plates. The middle plate floated in the liquid crystal and transmitted oscillatory movements to it. The dependences of the angle of deviation of the director from the normal to the cell on the time and relaxation times, the relaxation time on the shear amplitude and exposure time, as well as on the thickness of the liquid crystal layer; the time dependences of the optical signal at different exposure times and oscillation amplitudes; the dependences of the maximum values of the angle of rotation of the director in the XY plane on the shear amplitude and exposure time. It is found that after switching off the external perturbation, the relaxation processes of the liquid crystal director field depend on the exposure time, the thickness of the liquid crystal layer, and the oscillation amplitude. The relaxation dependences of the angle of inclination of the director after switching off the shift are described by an exponential dependence. Relaxation times are tens of seconds. The value of the relaxation time coincides with the relaxation time for the classical Fredericks effect and is about 30 s.

Finite size effect on the magnetic glass.

Abstract: The nature of glass formation and crystallization in structural glass is yet to be understood despite the intense studies of many decades. Analogous to the structural glasses, hindered first order magnetic transitions produce magnetic glasses, where the volume fraction of two phases having long range structural and magnetic order are frozen in time. Here, we have prepared Pr0.5Ca0.5Mn0.975Al0.025O3nanoparticles of different size as a case study and investigated the formation and stability of the magnetic glass state at the length scale of a few nanometers. We have observed a profound interplay between the glass state and sample size: stability of the glass state highly increases and scales linearly with decrease in the sample size. Smaller the particle size, slower is the crystallization rate. The crystallization occurs through both homogeneous and heterogeneous nucleation and is controlled by the surface to volume ratio of the particles. Our results emphasize on an important fact that glass transition is not a phase transition in actual sense, rather it is a kinetic phenomena. The length scale associated with different nucleation processes is an important length scale and it controls the glass dynamics. Besides, apart from the intrinsic metastability due to magnetic glass, we also distinguish a secondary source of relaxation, which is dominant at low magnetic fields, predominantly arising due to surface spin disorder.