arXiv subjects

Jean J. Heremans

Publications and source records attributed to Jean J. Heremans.

14 recordsLinked to original sources

arXiv2026

Electron Temperature and Electron-Electron Scattering Length in GaAs/AlGaAs Using Mesoscopic Multiparallel Aperture Geometries

Quantifying electron temperatures and electron-electron scattering lengths is essential for understanding electron transport regimes in two-dimensional systems. To that end, this work experimentally investigates low-temperature ballistic transport using semiclassical cyclotron orbits in a multiparallel aperture geometry. The mesoscopic geometry is fabricated on a GaAs/AlGaAs heterostructure of very high electron mobility. The amplitudes of maxima in magnetoresistance due to ballistic transport are observed to decay exponentially with temperature, allowing determination of the electron-electron scattering length from the data. A quantitative relation between electron temperature and an applied dc bias heating current is established by comparing magnetoresistance peak amplitudes measured as a function of temperature and dc bias, reflecting bias-induced Joule heating. In addition, the resistance measured near zero magnetic field decreases with increasing temperature, providing signatures of superballistic conductance.

cond-mat.mes-hall↗

arXiv2026

Comparative study of room temperature and quench condensed bismuth films: morphology and electronic characteristics

A comparison between properties of bismuth thin films deposited at substrate temperatures of 296 K (room temperature) and 77 K (quench condensed) is studied across epitaxial, amorphous, and van der Waals substrates. The experiments demonstrate changes in crystallinity, morphology, and electrical transport arising from the influence of substrate temperature. Moreover, the work highlights changes in grain size, roughness, X-ray diffraction peak intensities, and preferred orientation between the two deposition temperatures. The orientation of the films deposited at 77 K is preferentially (110), compared to (111) for films deposited at room temperature. Films grown at 77 K differ from those deposited at room temperature, exhibiting lower surface roughness but smaller grain size, which leads to increased electrical resistivity in quench condensed films. The decrease of substrate temperature during the deposition appears to induce slightly more strain in depositions on the amorphous and van der Waals substrates than on the epitaxial substrates. Lastly, quench condensed films exhibit lower carrier mobility and lower carrier density compared to room temperature films. This study elucidates previously incompletely understood processes in bismuth deposition and raises new questions regarding growth on van der Waals surfaces.

cond-mat.mtrl-sci↗

arXiv2023

Suppression of Spin Pumping at Metal Interfaces

An electrically conductive metal typically transmits or absorbs a spin current. Here, we report on evidence that interfacing two metal thin films can suppress spin transmission and absorption. We examine spin pumping in ferromagnet/spacer/ferromagnet heterostructures, in which the spacer -- consisting of metallic Cu and Cr thin films -- separates the ferromagnetic spin-source and spin-sink layers. The Cu/Cr spacer largely suppresses spin pumping -- i.e., neither transmitting nor absorbing a significant amount of spin current -- even though Cu or Cr alone transmits a sizable spin current. The antiferromagnetism of Cr is not essential for the suppression of spin pumping, as we observe similar suppression with Cu/V spacers where V is a nonmagnetic analogue of Cr. We speculate that diverse combinations of spin-transparent metals may form interfaces that suppress spin pumping, although the underlying mechanism remains unclear. Our work may stimulate a new perspective on understanding and engineering spin transport in metallic multilayers.

cond-mat.mtrl-sci↗

arXiv2023

Fractional focusing peaks and collective dynamics in two-dimensional Fermi liquids

Carrier transport in materials is often diffusive due to momentum-relaxing scattering with phonons and defects. Suppression of momentum-relaxing scattering can lead to the ballistic and hydrodynamic transport regimes, wherein complex non-Ohmic current flow patterns, including current vortices, can emerge. In the ballistic regime addressed here, transverse magnetic focusing is habitually understood in a familiar single-particle picture of carriers injected from a source, following ballistic cyclotron orbits and reaching a detector. We report on a distinctive nonlocal magnetoresistance phenomenon exclusive to fermions, in an enclosed mesoscopic geometry wherein transverse focusing magnetoresistance peaks also occur at values of the cyclotron diameter that are incommensurate with the distance between the source and detector. In low-temperature experiments and simulations using GaAs/AlGaAs heterostructures with high electron mobility, we show that the peaks occur independently of the location of the detector, and only depend on the source-drain separation. We reproduce the experimental findings using simulations of ballistic transport in both semiclassical and quantum-coherent transport models. The periodicity of magnetic field at which the peaks occur is matched to the lithographically defined device scale. It is found that, unlike in transverse magnetic focusing, the magnetoresistance structure cannot be attributed to any set of ordered single-particle trajectories but instead requires accounting for the collective dynamics of the fermion distribution and of all particle trajectories. The magnetoresistance is further associated with current flow vorticity, a collective phenomenon.

cond-mat.mes-hall↗

arXiv2022

Precision measurement of electron-electron scattering in GaAs/AlGaAs using transverse magnetic focusing

Electron-electron (e-e) interactions assume a cardinal role in solid-state physics. Quantifying the e-e scattering length is hence critical. In this paper we show that the mesoscopic phenomenon of transverse magnetic focusing (TMF) in two-dimensional electron systems forms a precise and sensitive technique to measure this length scale. Conversely we quantitatively demonstrate that e-e scattering is the predominant effect limiting TMF amplitudes in high-mobility materials. Using high-resolution kinetic simulations, we show that the TMF amplitude at a maximum decays exponentially as a function of the e-e scattering length, which leads to a ready approach to extract this length from the measured TMF amplitudes. The approach is applied to measure the temperature-dependent e-e scattering length in high-mobility GaAs/AlGaAs heterostructures. The simulations further reveal current vortices that accompany the cyclotron orbits - a collective phenomenon counterintuitive to the ballistic transport underlying a TMF setting.

cond-mat.mes-hall↗

arXiv2020

Magnetic Damping in Epitaxial Fe Alloyed with Vanadium and Aluminum

To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced by nearly a factor of 2, whereas damping in FeAl alloys increases with Al content. Our experimental and computational results indicate that reducing the density of states at the Fermi level, rather than the average atomic number, has a more significant impact in lowering damping in Fe alloyed with light elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic metals.

cond-mat.mtrl-sci↗

arXiv2020

Dynamic nuclear spin polarization induced by Edelstein effect at Bi(111) surfaces

Nuclear spin polarization induced by hyperfine interaction and the Edelstein effect due to strong spin-orbit interaction is investigated by quantum transport in Bi(111) thin film samples. The Bi(111) films are deposited on mica by van der Waals epitaxial growth. The Bi(111) films show micrometer-sized triangular islands with 0.39 nm step height, corresponding to the Bi(111) bilayer height. At low temperatures a high current density is applied to generate a non-equilibrium carrier spin polarization by the Edelstein effect at the Bi(111) surface, which then induces dynamic nuclear polarization by hyperfine interaction. Comparative quantum magnetotransport antilocalization measurements indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization and allow a quantification of the Overhauser field. Hence nuclear polarization was both achieved and quantified by a purely electronic transport-based approach.

cond-mat.mes-hall↗

arXiv2020

Sub-Nanosecond Spin-Transfer Torque in an Ensemble of Superparamagnetic-Like Nanomagnets

Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as is the case for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a material with weak, short-range magnetic order -- namely, a macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect sub-ns dynamics of the Co nanomagnets, excited into precession with cone angle $\geq$0.003$^{\circ}$ by an oscillating spin current. XFMR measurements reveal that as the net moment of the ensemble decreases, the strength of the spin-transfer torque increases relative to those of magnetic field torques. Our findings point to spin-transfer torque as an effective way to manipulate the state of nanomagnet ensembles at sub-ns timescales.

cond-mat.mes-hall↗

arXiv2020

Conductivity-Like Gilbert Damping due to Intraband Scattering in Epitaxial Iron

Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for simple ferromagnetic metals. In this Letter, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivity-like damping, which cannot be accounted for by classical eddy current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. Our results resolve the longstanding question about a fundamental damping mechanism and offer hints for engineering low-loss magnetic metals for cryogenic spintronics and quantum devices.

cond-mat.mtrl-sci↗

arXiv2019

Current-induced spin-orbit field in permalloy interfaced with ultrathin Ti and Cu

How spin-orbit torques emerge from materials with weak spin-orbit coupling (e.g., light metals) is an open question in spintronics. Here, we report on a field-like spin-orbit torque (i.e., in-plane spin-orbit field transverse to the current axis) in SiO$_2$-sandwiched permalloy (Py), with the top Py-SiO$_2$ interface incorporating ultrathin Ti or Cu. In both SiO$_2$/Py/Ti/SiO$_2$ and SiO$_2$/Py/Cu/SiO$_2$, this spin-orbit field opposes the classical Oersted field. While the magnitude of the spin-orbit field is at least a factor of 3 greater than the Oersted field, we do not observe evidence for a significant damping-like torque in SiO$_2$/Py/Ti/SiO$_2$ or SiO$_2$/Py/Cu/SiO$_2$. Our findings point to contributions from a Rashba-Edelstein effect or spin-orbit precession at the (Ti, Cu)-inserted interface.

cond-mat.mtrl-sci↗

arXiv2017

Epitaxial thin films of pyrochlore iridate Bi_{2+x}Ir_{2-y}O_{7-delta}: structure, defects and transport properties

While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. Here we report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi2Ir2O7. The films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of Bi_Ir antisite defects, qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2010

Towards a spin dual of the fractional quantum Hall effect

Electromagnetic duality between the Aharonov-Bohm and the Aharonov-Casher quantum mechanical phases predicts the existence of a new collective state of matter which can be regarded as a spin dual to the fractional quantum Hall effect. The state, induced by electric fields, is driven by effective spin-spin interactions. We derive experimental and materials conditions of spin-spin interactions and electric fields under which the new state may be observed.

cond-mat.mes-hall↗

arXiv2008

High Temperature Superconductivity and Effective Gravity

We argue that an approach involving effective gravity could play a crucial role in elucidating the properties of the high temperature superconducting materials. In particular we propose that the high critical temperature might be naturally explained in a framework constructed as a direct condensed matter analog of the Randall-Sundrum approach to a geometrization of the hierarchy problem in high-energy physics.

hep-th↗

arXiv2003

Hydrodynamic effects in interacting Fermi electron jets

We theoretically study hydrodynamic phenomena originating from electron-electron collisions in a two-dimensional Fermi system. We demonstrate that an electron beam sweeping past an aperture creates a pumping effect, attracting carriers from this aperture. This pumping effect originates from the specific electric potential distribution induced by the injected electrons. In the regions nearby the main stream of injected electrons, a positive potential is induced by the injected electrons. Thus, the normally repulsive Coulomb interaction leads to an attractive force in the Fermi system. This quantum pumping mechanism in a Fermi system differs qualitatively from the Bernoulli pumping effect in classical liquids. We also discuss possible experimental realizations.

cond-mat.mes-hall↗