arXiv subjects

Malcolm R. Beasley

Publications and source records attributed to Malcolm R. Beasley.

14 recordsLinked to original sources

arXiv2022

Rare-Earth Control of the Superconducting Upper Critical Field in Infinite-Layer Nickelates

The consequences of varying the rare-earth element in the superconducting infinite-layer nickelates have been much debated. Here we show striking differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These 5 distinctions originate from the 4f electron characteristics of the rare-earth ions in the lattice: they are absent for La3+, nonmagnetic for the Pr3+ singlet ground state, and magnetic for the Nd3+ Kramer's doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd3+ 4f moments. In the absence of rare-earth effects, we find that the nickelates broadly violate the Pauli limit. Such robust and tunable superconductivity suggests potential in future high-field applications.

cond-mat.supr-con↗

arXiv2020

Isotropic Pauli-Limited Superconductivity in the Infinite Layer Nickelate Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$

The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.

cond-mat.supr-con↗

arXiv2018

Spatially modulated susceptibility in thin film La$_{2-x}$Ba$_x$CuO$_4$

The high critical temperature superconductor Lanthanum Barium Copper Oxide (La2-xBaxCuO4 or LBCO) exhibits a strong anomaly in critical temperature at 1/8th doping, nematicity, and other interesting properties. We report here Scanning Superconducting Quantum Interference Device (SQUID) imaging of the magnetic fields and susceptibility in a number of thin film LBCO samples with doping in the vicinity of the 1/8th anomaly. Spatially resolved measurements of the critical temperatures of these samples do not show a pronounced depression at 1/8th doping. They do, however, exhibit strong, nearly linear modulations of the susceptibility ("straie") of multiple samples with surprisingly long periods of 1-4 microns. Counterintuitively, vortices trap in positions of largest diamagnetic susceptibility in these striae. Given the rich interplay of different orders in this material system and its known sensitivity to epitaxial strain, we propose phase separation as a possible origin of these features and discuss scenarios in which that might arise.

cond-mat.supr-con↗

arXiv2014

Point-contact tunneling spectroscopy measurement of Cu$_x$TiSe$_2$: disorder-enhanced Coulomb effects

We performed point-contact spectroscopy tunneling measurements on Cu$_x$TiSe$_2$ bulk with $x=0.02$ and $0.06$ at temperatures ranging from $T=4-40$ K and observe a suppression in the density of states around zero-bias that we attribute to enhanced Coulomb interactions due to disorder. We find that the correlation gap associated with this suppression is related to the zero-temperature resistivity. We use our results to estimate the disorder-free transition temperature and find that the clean limit $T_{c0}$ is close to the experimentally observed $T_c$.

cond-mat.supr-con↗

arXiv2013

Local sheet conductivity and sheet current density mapping using a single scanning voltage probe

We demonstrate how a single scanning voltage probe can be used to map the local conductivity and current density in a thin film with no a priori knowledge of the geometry of the electrical contacts. With state-of-the-art scanning voltage probes, under appropriate conditions such mapping should be possible down to nanometer scales. The technique requires two non-colinear voltage scans. When only one voltage map is available, determination of the conductivity is not possible because the solution to the governing equation is not unique. The only restriction on the technique is that the sheet conductivity is a local function of position.

cond-mat.mtrl-sci↗

arXiv2013

Measurement of specific contact resistivity using scanning voltage probes

Specific contact resistivity measurements have conventionally been heavy in both fabrication and simulation/calculation in order to account for complicated geometries and other effects such as parasitic resistance. We propose a simpler geometry to deliver current, and the use of a scanning voltage probe to sense the potential variation along the sample surface, from which the specific contact resistivity can be straightforwardly deduced. We demonstrate an analytical example in the case where both materials are thin films. Experimental data with a scanning Kelvin probe measurement on graphene from the literature corroborates our model calculation.

cond-mat.mtrl-sci↗

arXiv2013

Local transport measurements at mesoscopic length scales using scanning tunneling potentiometry

Under mesoscopic conditions, the transport potential on a thin film with current is theoretically expected to bear spatial variation due to quantum interference. Scanning tunneling potentiometry is the ideal tool to investigate such variation, by virtue of its high spatial resolution. We report in this {\it Letter} the first detailed measurement of transport potential under mesoscopic conditions. Epitaxial graphene at a temperature of 17K was chosen as the initial system for study because the characteristic transport length scales in this material are relatively large. Tip jumping artifacts are a major possible contribution to systematic errors; and we mitigate such problems by using custom-made slender and sharp tips manufactured by focussed ion beam. In our data, we observe residual resistivity dipoles associated with topoographical defects, and local peaks and dips in the potential that are not associated with topographical defects.

cond-mat.mes-hall↗

arXiv2011

Quenching of Impurity Spins at Cu/CuO Interfaces: An Antiferromagnetic Proximity Effect

It is observed that the magnetoconductance of bilayer films of copper (Cu) and copper monoxide (CuO) has distinct features compared of that of Cu films on conventional band insulator substrates. We analyze the data above 2 K by the theory of weak antilocalization in two-dimensional metals and suggest that spin-flip scatterings by magnetic impurities inside Cu are suppressed in Cu/CuO samples. Plausibly the results imply a proximity effect of antiferromagnetism inside the Cu layer, which can be understood in the framework of Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. The data below 1 K, which exhibit slow relaxation reminiscent of spin glass, are consistent with this interpretation.

cond-mat.str-el↗

arXiv2011

Theoretical Description of Scanning Tunneling Potentiometry

A theoretical description of scanning tunneling potentoimetry (STP) measurement is presented to address the increasing need for a basis to interpret experiments on macrscopic samples. Based on a heuristic understanding of STP provided to facilitate theoretical understanding, the total tunneling current related to the density matrix of the sample is derived within the general framework of quantum transport. The measured potentiometric voltage is determined implicitly as the voltage necessary to null the tunneling current. Explicit expressions of measured voltages are presented under certain assumptions, and limiting cases are discussed to connect to previous results. The need to go forward and formulate the theory in terms of a local density matrix is also discussed.

cond-mat.mes-hall↗

arXiv2008

Tetragonal CuO: A new end member of the 3d transition metal monoxides

Monoclinic CuO is anomalous both structurally as well as electronically in the 3$d$ transition metal oxide series. All the others have the cubic rock salt structure. Here we report the synthesis and electronic property determination of a tetragonal (elongated rock salt) form of CuO created using an epitaxial thin film deposition approach. In situ photoelectron spectroscopy suggests an enhanced charge transfer gap $Δ$ with the overall bonding more ionic. As an end member of the 3d transition monoxides, its magnetic properties should be that of a high $T_N$ antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2007

Electronic properties of buried hetero-interfaces of LaAlO3 on SrTiO3

We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.

cond-mat.mtrl-sci↗

arXiv2007

Origin of charge density at LaAlO3-on-SrTiO3 hetero-interfaces; possibility of intrinsic doping

As discovered by Ohtomo et al., a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

cond-mat.mtrl-sci↗

arXiv2007

Dependence of electronic structure of SrRuO3 and the degree of correlation on cation off-stoichiometry

We have grown and studied high quality SrRuO3 films grown by MBE as well as PLD. By changing the oxygen activity during deposition we were able to make SrRuO3 samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). Samples with strontium vacancies were found impossible to produce since the ruthenium would precipitate out as RuO2. The volume of the unit cell of SrRuO3 becomes larger as more ruthenium vacancies are introduced. The residual resistivity ratio (RRR) and room temperature resistivity were found to systematically depend on the volume of the unit cell and therefore on the amount of ruthenium vacancies. The RRR varied from ~30 for stoichiometric samples to less than two for samples that were very ruthenium poor. The room temperature resistivity varied from 190 microOhm cm for stoichoimetric samples to over 300 microOhm cm for very ruthenium poor samples. UPS spectra show a shift of weight from the coherent peak to the incoherent peak around the Fermi level when samples have more ruthenium vacancies. Core level XPS spectra of the ruthenium 3d lines show a strong screened part in the case of stoichiometric samples. This screened part disappears when ruthenium vacancies are introduced. Both the UPS and the XPS results are consistent with the view that correlation increases as the amount of ruthenium vacancies increase.

cond-mat.mtrl-sci↗

arXiv2003

Angular dependence of domain wall resistivity in SrRuO$_{\bf 3}$ films

${\rm SrRuO_3}$ is a 4d itinerant ferromagnet (T$_{c}$ $\sim $150 K) with stripe domain structure. Using high-quality thin films of SrRuO$_{3}$ we study the resistivity induced by its very narrow ($\sim 3$ nm) Bloch domain walls, $ρ_{DW}$ (DWR), at temperatures between 2 K and T$_{c}$ as a function of the angle, $θ$, between the electric current and the ferromagnetic domains walls. We find that $ρ_{DW}(T,θ)=\sin^2θρ_{DW}(T,90)+B(θ)ρ_{DW}(T,0)$ which provides the first experimental indication that the angular dependence of spin accumulation contribution to DWR is $\sin^2θ$. We expect magnetic multilayers to exhibit a similar behavior.

cond-mat.str-el↗