arXiv subjects

Paul M. Haney

Publications and source records attributed to Paul M. Haney.

At least 37 records · Page 2Linked to original sources

arXiv2018

Thermally driven anomalous Hall effect transitions in FeRh

Materials exhibiting controllable magnetic phase transitions are currently in demand for many spintronics applications. Here we investigate from first principles the electronic structure and intrinsic anomalous Hall, spin Hall and anomalous Nernst response properties of the FeRh metallic alloy which undergoes a thermally driven antiferromagnetic-to-ferromagnetic phase transition. We show that the energy band structures and underlying Berry curvatures have important signatures in the various Hall effects. Specifically, the suppression of the anomalous Hall and Nernst effects in the AFM state and a sign change in the spin Hall conductivity across the transition are found. It is suggested that the FeRh can be used a spin current detector capable of differentiating the spin Hall effect from other anomalous transverse effects. The implications of this material and its thermally driven phases as a spin current detection scheme are also discussed.

cond-mat.mtrl-sci↗

arXiv2018

Charged grain boundaries and carrier recombination in polycrystalline thin film solar cells

We present analytical relations for the dark recombination current of a $pn^+$ junction with positively charged columnar grain boundaries in the high defect density regime. We consider two defect state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of acceptor+donor states, or to the previously studied single acceptor+donor state, the grain boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage $V_{\rm oc}$, so that our dark $J(V)$ relations determine $V_{\rm oc}$ for a given short circuit current $J_{\rm sc}$. We finally explicitly show how $V_{\rm oc}$ depends on the grain boundary defect state configurations.

cond-mat.mes-hall↗

arXiv2017

Optical spin transfer and spin-orbit torques in thin film ferromagnets

We study the optically induced torques in thin film ferromagnetic layers under excitation by circularly polarized light. We study cases both with and without Rashba spin-orbit coupling using a 4-band model. In the absence of Rashba spin-orbit coupling, we derive an analytic expression for the optical torques, revealing the conditions under which the torque is mostly derived from optical spin transfer torque (i.e. when the torque is along the direction of optical angular momentum), versus when the torque is derived from the inverse Faraday effect (i.e. when the torque is perpendicular to the optical angular momentum). We find the optical spin transfer torque dominates provided that the excitation energy is far away from band edge transitions, and the magnetic exchange splitting is much greater than the lifetime broadening. For the case with large Rashba spin-orbit coupling and out-of-plane magnetization, we find the torque is generally perpendicular to the photon angular momentum and is ascribed to an optical Edelstein effect.

cond-mat.mes-hall↗

arXiv2016

Charged grain boundaries reduce the open circuit voltage of polycrystalline solar cells--An analytical description

Analytic expressions are presented for the dark current-voltage relation $J(V)$ of a $pn^+$ junction with positively charged columnar grain boundaries with high defect density. These expressions apply to non-depleted grains with sufficiently high bulk hole mobilities. The accuracy of the formulas is verified by direct comparison to numerical simulations. Numerical simulations further show that the dark $J(V)$ can be used to determine the open circuit potential $V_{\rm oc}$ of an illuminated junction for a given short-circuit current density $J_{\rm sc}$ . A precise relation between the grain boundary properties and $V_{\rm oc}$ is provided, advancing the understanding of the influence of grain boundaries on the efficiency of thin film polycrystalline photovoltaics like CdTe and $\mathrm{Cu(In,Ga)Se_2}$.

cond-mat.mtrl-sci↗

arXiv2016

Depletion region surface effects in electron beam induced current measurements

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Current models of EBIC assume that excitations in the $p$-$n$ junction depletion region result in perfect charge collection efficiency. However we find that in CdTe and Si samples prepared by focused ion beam (FIB) milling, there is a reduced and nonuniform EBIC lineshape for excitations in the depletion region. Motivated by this, we present a model of the EBIC response for excitations in the depletion region which includes the effects of surface recombination from both charge-neutral and charged surfaces. For neutral surfaces we present a simple analytical formula which describes the numerical data well, while the charged surface response depends qualitatively on the location of the surface Fermi level relative to the bulk Fermi level. We find the experimental data on FIB-prepared Si solar cells is most consistent with a charged surface, and discuss the implications for EBIC experiments on polycrystalline materials.

cond-mat.mes-hall↗

arXiv2016

Circular Photogalvanic Effect in Organometal Halide Perovskite CH$_3$NH$_3$PbI$_3$

We study the circular photogalvanic effect in the organometal halide perovskite solar cell absorber CH$_3$NH$_3$PbI$_3$. For crystal structures which lack inversion symmetry, the calculated photocurrent density is about $10^{-9}$ A/W, comparable to the previously studied quantum well and bulk Rashba systems. Because of the dependence of the circular photogalvanic effect on inversion symmetry breaking, the degree of inversion asymmetry at different depths from the surface can be probed by tuning the photon energy and associated penetration depth. We propose that measurements of this effect may clarify the presence or absence of inversion symmetry, which remains a controversial issue and has been argued to play an important role in the high conversion efficiency of this material.

cond-mat.mtrl-sci↗

arXiv2016

Minority-carrier dynamics in semiconductors probed by two-photon microscopy

Two-photon time-resolved photoluminescence has been recently applied to various semiconductor devices to determine carrier lifetime and surface recombination velocities. So far the theoretical modeling activity has been mainly limited to the commonly used one-photon counterpart of the technique. Here we provide the analytical solution to a 3D diffusion equation that describes two-photon microscopy in the low-injection regime. We focus on a system with a single buried interface with enhanced recombination, and analyze how transport, bulk and surface recombinations influence photoluminescence decays. We find that bulk measurements are dominated by diffusion at short times and by bulk recombination at long times. Surface recombination modifies bulk signals when the optical spot is less than a diffusion length away from the probed interface. In addition, the resolution is increased as the spot size is reduced, which however makes the signal more sensitive to diffusion.

cond-mat.mes-hall↗

arXiv2016

Interfacial magnetic anisotropy from a 3-dimensional Rashba substrate

We study the magnetic anisotropy which arises at the interface between a thin film ferromagnet and a 3-d Rashba material. The 3-d Rashba material is characterized by the spin-orbit strength $α$ and the direction of broken bulk inversion symmetry $\hat n$. We find an in-plane uniaxial anisotropy in the $\hat{z}\times\hat{n}$ direction, where $\hat z$ is the interface normal. For realistic values of $α$, the uniaxial anisotropy is of a similar order of magnitude as the bulk magnetocrystalline anisotropy. Evaluating the uniaxial anisotropy for a simplified model in 1-d shows that for small band filling, the in-plane easy axis anisotropy scales as $α^4$ and results from a twisted exchange interaction between the spins in the 3-d Rashba material and the ferromagnet. For a ferroelectric 3-d Rashba material, $\hat n$ can be controlled with an electric field, and we propose that the interfacial magnetic anisotropy could provide a mechanism for electrical control of the magnetic orientation.

cond-mat.mes-hall↗

arXiv2016

Optical Spintronics in Organic-Inorganic Perovskite Photovoltaics

Organic-inorganic halide CH$_3$NH$_3$PbI$_3$ solar cells have attracted enormous attention in recent years due to their remarkable power conversion efficiency. When inversion symmetry is broken, these materials should exhibit interesting spin-dependent properties as well, owing to their strong spin-orbit coupling. In this work, we consider the spin-dependent optical response of CH$_3$NH$_3$PbI$_3$. We first use density functional theory to compute the ballistic spin current generated by absorption of unpolarized light. We then consider diffusive transport of photogenerated charge and spin for a thin CH$_3$NH$_3$PbI$_3$ layer with a passivated surface and an Ohmic, non-selective contact. The spin density and spin current are evaluated by solving the drift-diffusion equations for a simplified 3-dimensional Rashba model of the electronic structure of the valence and conduction bands. We provide analytic expressions for the photon flux required to induce measurable spin densities, and propose that these spin densities can provide useful information about the role of grain boundaries in the photovoltaic behavior of these materials. We also discuss the prospects for measuring the optically generated spin current with the inverse spin Hall effect.

cond-mat.mes-hall↗

arXiv2016

Antiferromagnetic domain wall motion driven by spin-orbit torques

We theoretically investigate dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet/heavy metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin-waves in the terahertz frequency range. The interplay between spin orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2014

Electron beam induced current in the high injection regime

Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron-hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the {\it total} electron-hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in 1 and 2 dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the 3 dimensional model.

cond-mat.mtrl-sci↗

arXiv2014

Electron beam induced current in photovoltaics with high recombination

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Ideally, an EBIC measurement reflects the spatially resolved quantum efficiency of the device. In this work, a model for EBIC measurements is presented which applies when recombination within the depletion region is substantial. This model is motivated by cross-sectional EBIC experiments on CdS-CdTe photovoltaic cells which show that the maximum efficiency of carrier collection is less than 100 \% and varies throughout the depletion region. The model can reproduce experimental results only if the mobility-lifetime product $μτ$ is spatially varying within the depletion region. The reduced collection efficiency is speculated to be related to high-injection effects, and the resulting increased radiative recombination.

cond-mat.mtrl-sci↗

arXiv2014

Angular dependence of spin-orbit spin transfer torques

In ferromagnet/heavy metal bilayers, an in-plane current gives rise to spin-orbit spin transfer torque which is usually decomposed into field-like and damping-like torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the field-like torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the field-like torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to the exchange interaction. On the other hand, the damping-like torque acquires nontrivial angular dependence when the Rashba spin-orbit coupling is comparable to or stronger than the exchange interaction. It is related to the combined effects of the Fermi surface distortion and the Fermi sea contribution. The angular dependence is consistent with experimental observations and can be important to understand magnetization dynamics induced by spin-orbit spin transfer torques

cond-mat.mtrl-sci↗

arXiv2013

Current induced torques between ferromagnets and compensated antiferromagnets: symmetry and phase coherence effects

It is shown that the current-induced torques between a ferromagnetic layer and an antiferromagnetic layer with a compensated interface vanish when the ferromagnet is aligned with an axis of spin-rotation symmetry of the antiferromagnet. For properly chosen geometries this implies that the current induced torque can stabilize the out-of-plane (or hard axis) orientation of the ferromagnetic layer. This current-induced torque relies on phase coherent transport, and we calculate the robustness of this torque to phase breaking scattering. From this it is shown that the torque is not linearly dependent on applied current, but has an absolute maximum.

cond-mat.mes-hall↗

arXiv2013

Current-induced torques and interfacial spin-orbit coupling

In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here, we calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism using first principles methods. We focus exclusively on the analogue to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the "field-like" torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.

cond-mat.mes-hall↗

arXiv2013

Current induced torques and interfacial spin-orbit coupling: Semiclassical Modeling

In bilayer nanowires consisting of a ferromagnetic layer and a non-magnetic layer with strong spin-orbit coupling, currents create torques on the magnetization beyond those found in simple ferromagnetic nanowires. The resulting magnetic dynamics appear to require torques that can be separated into two terms, damping-like and field-like. The damping-like torque is typically derived from models describing the bulk spin Hall effect and the spin transfer torque, and the field-like torque is typically derived from a Rashba model describing interfacial spin-orbit coupling. We derive a model based on the Boltzmann equation that unifies these approaches. We also consider an approximation to the Boltzmann equation, the drift-diffusion model, that qualitatively reproduces the behavior, but quantitatively fails to reproduce the results. We show that the Boltzmann equation with physically reasonable parameters can match the torques for any particular sample, but in some cases, it fails to describe the experimentally observed thickness dependences.

cond-mat.mtrl-sci↗

arXiv2012

Theory of ZT enhancement in nanocomposite materials

The effect of interface scattering on the performance of disordered, nanocomposite thermoelectric materials is studied theoretically using effective medium theory and direct numerics. The interfacial electronic and phonon scattering properties which lead to an enhancement of the thermoelectric figure of merit $ZT$ are described. Generally, $ZT$ enhancement requires the interfacial electrical conductance to be within a range of values, and the thermal phonon conductance to be below a critical value. For the systems considered, these requirements on interface scattering for $ZT$ enhancement are expressed in terms of the bulk properties of the high-$ZT$ material, and the ratio of the constituent bulk $Z$ values.

cond-mat.mes-hall↗

arXiv2011

Effective medium theory of binary thermoelectrics

The transport coefficients of disordered media are analyzed using direct numerical simulation and effective medium theory. The results indicate a range of materials parameters for which disorder leads to an enhanced power factor. This increase in power factor is generally not accompanied by an increase in the figure of merit $ZT$, however. It is also found that the effective electrical conductivity and electronic contribution to the thermal conductivity are not generally proportional to each other in the presence of disorder.

cond-mat.mtrl-sci↗