arXiv subjects

Aakash Pushp

Publications and source records attributed to Aakash Pushp.

11 recordsLinked to original sources

arXiv2025

Floating zone growth of high-purity MgO substrate single crystals

MgO single crystals with diameters between 3.5 and 5 mm and lengths up to 40 mm were grown by the optical floating zone technique (OFZ). Despite challenging material properties such as the high melting point of 2825 {\deg}C, very high evaporation rate and perfect {100} cleaving characteristics, crack-free crystals were grown at high growth rates exceeding 40 mm/h and at high thermal gradients. Chemical investigations revealed that the OFZ technique is suitable for the preparation of substrate crystals with a purity of 5N to facilitate the development of novel demonstrator devices based on epitaxially grown thin films. The achieved purity level is improved by more than one order of magnitude if compared to commercial MgO substrate single crystals graded as high purity.

cond-mat.mtrl-sci↗

arXiv2025

Half-Metallic Fe/MgO Superlattice: An Ideal Candidate for Magnetic Tunnel Junction Electrodes

Magnetic Tunnel Junction (MTJ) based Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM) is poised to replace embedded Flash for advanced applications such as automotive microcontroller units. To achieve deeper technological adoption, MTJ needs to exhibit three key features: low magnetization (Ms), high perpendicular magnetic anisotropy (PMA) and high tunnel magnetoresistance (TMR). Here, we theoretically show that when Fe/MgO multilayers are inserted into the fixed and free layers of the MTJ, these three conditions are simultaneously met. As the number of Fe/MgO multilayers in MTJ electrodes is increased, we find that the electron transport evolves from direct barrier tunneling of majority spin states to the resonant tunneling of minority spin states. Remarkably, the projected density of states (PDOS) of Fe/MgO superlattice at the MgO tunnel barrier exhibits half-metallicity near the Fermi Energy, where the minority states exist while the majority states are gapped out, resulting in astronomically high TMR.

cond-mat.mes-hall↗

arXiv2019

Majorana Braiding in Realistic Nanowire Y-Junctions and Tuning Forks

Majorana fermions are predicted to arise at the ends of nanowire devices which combine superconductivity, strong spin-orbit coupling and an external magnetic field. By manipulating networks of these devices with suitable gating, it has been suggested that braiding operations may be performed which act as logic operations, suitable for quantum computation. However, the unavoidable misalignment of the magnetic field in any realistic device geometry has raised questions about the feasibility of such braiding. In this paper, we numerically simulate braiding operations in devices with Y-junction and tuning fork geometries using an experimentally motivated nanowire model. We study how the static and dynamical features vary with geometric parameters and identify parameter choices that optimise the probability of a successful braid. Notably, we find that there is an optimal Y-junction half-angle (about 20 degrees for our parameter values), which balances two competing mechanisms that reduce the energy gap to excitations. In addition, we find that a tuning fork geometry has significant advantages over a Y-junction geometry, as it substantially reduces the effect of dynamical phase oscillations that complicate the braiding process. Our results suggest that performing a successful braid is in principle possible with such devices, and lies within experimental reach.

cond-mat.mes-hall↗

arXiv2015

Giant thermal spin torque assisted magnetic tunnel junction switching

Spin-polarized charge-currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer-torque (STT). Recently, by taking advantage of the spin dependent thermoelectric properties of magnetic materials, novel means of generating spin-currents from temperature gradients, and their associated thermal spin torques (TSTs) have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge-currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe.

cond-mat.mes-hall↗

arXiv2013

Domain wall trajectory determined by its fractional topological edge defects

A domain wall (DW) in a ferromagnetic nanowire is composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively, whose relative spatial arrangement determines the chirality of the DW. Here we show how we can understand and control the trajectory of DWs in magnetic branched networks, composed of connected nanowires, by considering their fractional elementary topological defects and how they interact with those innate to the network. We first develop a highly reliable mechanism for the injection of a DW of a given chirality into a nanowire and show that its chirality determines which branch the DW follows at a symmetric Y-shaped magnetic junction - the fundamental building block of the network. Using these concepts, we unravel the origin of the one-dimensional nature of magnetization reversal of connected artificial spin ice systems that have been observed in the form of Dirac strings.

cond-mat.mes-hall↗

arXiv2011

Scattering from incipient stripe order in the high-temperature superconductor Bi2Sr2CaCu2O8+d

Recently we have used spectroscopic mapping with the scanning tunneling microscope to probe modulations of the electronic density of states in single crystals of the high temperature superconductor Bi2Sr2CaCu2O8+d (Bi-2212) as a function of temperature [C. V. Parker et al., Nature (London) 468, 677 (2010)]. These measurements showed Cu-O bond-oriented modulations that form below the pseudogap temperature with a temperature-dependent energy dispersion displaying different behaviors in the superconducting and pseudogap states. Here we demonstrate that quasiparticle scattering off impurities does not capture the experimentally observed energy- and temperature-dependence of these modulations. Instead, a model of scattering of quasiparticles from short-range stripe order, with periodicity near four lattice constants (4a), reproduces the experimentally observed energy dispersion of the bond-oriented modulations and its temperature dependence across the superconducting critical temperature, Tc. The present study confirms the existence of short-range stripe order in Bi-2212.

cond-mat.str-el↗

arXiv2010

Appearance of fluctuating stripes at the onset of the pseudogap in the high-Tc Superconductor Bi2Sr2CaCu2O8+x

Doped Mott insulators have been shown to have a strong propensity to form patterns of holes and spins often referred to as stripes. In copper-oxides, doping also gives rise to the pseudogap state, which transforms into a high temperature superconductor with sufficient doping or by reducing the temperature. A long standing question has been the interplay between pseudogap, which is generic to all hole-doped cuprates, and stripes, whose static form occurs in only one family of cuprates over a narrow range of the phase diagram. Here we examine the spatial reorganization of electronic states with the onset of the pseudogap state at T* in the high-temperature superconductor Bi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling microscope (STM). We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. As expected, the stripe patterns are strongest when the hole concentration in the CuO2 planes is close to 1/8 (per Cu). While demonstrating that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the cuprate phase diagram, our experiments indicate that they are a consequence of pseudogap behavior rather than its cause.

cond-mat.supr-con↗

arXiv2010

Nanoscale Proximity Effect in the High Temperature Superconductor Bi-2212

High temperature cuprate superconductors exhibit extremely local nanoscale phenomena and strong sensitivity to doping. While other experiments have looked at nanoscale interfaces between layers of different dopings, we focus on the interplay between naturally inhomogeneous nanoscale regions. Using scanning tunneling microscopy to carefully track the same region of the sample as a function of temperature, we show that regions with weak superconductivity can persist to elevated temperatures if bordered by regions of strong superconductivity. This suggests that it may be possible to increase the maximum possible transition temperature by controlling the distribution of dopants.

cond-mat.supr-con↗

arXiv2009

Extending Universal Nodal Excitations Optimizes Superconductivity in Bi2Sr2CaCu2O8+d

Understanding the mechanism by which d-wave superconductivity in the cuprates emerges and is optimized by doping the Mott insulator is one of the major outstanding problems in condensed matter physics. Our high-resolution scanning tunneling microscopy measurements of the high transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+d show that samples with different Tcs in the low doping regime follow a remarkably universal d-wave low energy excitation spectrum, indicating a doping independent nodal gap. We demonstrate that Tc instead correlates with the fraction of the Fermi surface over which the samples exhibit the universal spectrum. Optimal Tc is achieved when all parts of the Fermi surface follow this universal behavior. Increasing temperature above Tc turns the universal spectrum into an arc of gapless excitations, while overdoping breaks down the universal nodal behavior.

cond-mat.supr-con↗

arXiv2008

Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2CaCu2O8+d

Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+d using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.

cond-mat.supr-con↗

arXiv2007

Visualizing pair formation on the atomic scale in the high-Tc superconductor Bi2Sr2CaCu2O8+d

Pairing of electrons in conventional superconductors occurs at the superconducting transition temperature Tc, creating an energy gap D in the electronic density of states (DOS). In the high-Tc superconductors, a partial gap in the DOS exists for a range of temperatures above Tc. A key question is whether the gap in the DOS above Tc is associated with pairing, and what determines the temperature at which incoherent pairs form. Here we report the first spatially resolved measurements of gap formation in a high-Tc superconductor, measured on Bi2Sr2CaCu2O8+d samples with different Tc values (hole concentration of 0.12 to 0.22) using scanning tunnelling microscopy. Over a wide range of doping from 0.16 to 0.22 we find that pairing gaps nucleate in nanoscale regions above Tc. These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state. Despite the inhomogeneity, we find that every pairing gap develops locally at a temperature Tp, following the relation 2D/kBTp = 8. At very low doping (<0.14), systematic changes in the DOS indicate the presence of another phenomenon, which is unrelated and perhaps competes with electron pairing. Our observation of nanometre-sized pairing regions provides the missing microscopic basis for understanding recent reports of fluctuating superconducting response above Tc in hole-doped high-Tc copper oxide superconductors.

cond-mat.supr-con↗