arXiv subjects

Vincent Sacksteder IV

Publications and source records attributed to Vincent Sacksteder IV.

7 recordsLinked to original sources

arXiv2019

Spin Diffusion Equations for Magnetized or Orbital Polarized Systems

Charge and spin transport in spintronics devices can be described by a spin diffusion equation suitable for modelling scales much larger than the scattering and atomic scales. This work concerns the coarse graining procedure used to compute the coefficients of the diffusion equation, which are sensitive the details of individual atoms and impurities. We show with two simple examples that in spintronics devices which have both a spin-orbit interaction and magnetization, standard coarse graining can easily obtain diffusion equations which fail to conserve electronic charge. The same failure can occur in systems with both a spin-orbit interaction and orbital polarization. We show that linear response theory, coupled with the self-consistent Born approximation and ladder diagrams, offers an improved way of calculating diffusion equations. We show that the resulting equations satisfy a Ward-Takahashi identity that guarantees charge conservation.

cond-mat.mes-hall↗

arXiv2019

Quantized Repetitions of the Cuprate Pseudogap Line

The cuprate superconductors display several characteristic temperatures which decrease as the material composition is doped, tracing lines across the temperature-doping phase diagram. Foremost among these is the pseudogap transition. At a higher temperature a peak is seen in the magnetic susceptibility, and changes in symmetry and in transport are seen at other characteristic temperatures. We report a meta-analysis of all measurements of characteristic temperatures well above $T_c$ in strontium doped lanthanum cuprate (LSCO) and oxygen doped YBCO. The experimental corpus shows that the pseudogap line is one of a family of four straight lines which stretches across the phase diagram from low to high doping, and from $T_c$ up to $700$ K. These lines all originate from a single point near the overdoped limit of the superconducting phase and increase as doping is reduced. The slope of the pseudogap lines is quantized, with the second, third, and fourth lines having slopes that are respectively $1/2,\;1/3,$ and $1/4$ of the slope of the highest line. This pattern suggests that the cuprates host a single mother phase controlled by a 2-D sheet density which is largest at zero doping and which decreases linearly with hole density, and that the pseudogap lines, charge density wave order, and superconductivity are all subsidiary effects supported by the mother phase.

cond-mat.dis-nn↗

arXiv2019

Bibliography of Literature on GW Ab Initio Calculations which use Imaginary Time

The GW Approximation is an ab initio approach to calculating electronic structure which avoids using the Local Density (LDA) Approximation, the Generalized Gradient (GGA) Approximation, or similar density functionals. It goes beyond the Hartree-Fock approximation by including screening and excited state effects, and shares conceptual similarities with MP2 and RPA calculations. Because GW includes dynamics and time/frequency dependence of the system's screening and excited state behavior, a pivotal issue in any GW calculation is the question of how to numerically represent and manipulate time/frequency dependence. While earlier GW calculations generally used a representation in real time/frequency, many recent calculations have used a representation on the imaginary time axis. Imaginary time is important not only for numerics but also because it can enable additional physics such as systems where self-consistent GW is needed or that are strongly interacting, integration with DMFT, and scaling of GW to very large systems. This current text reviews the research literature on imaginary time GW codes and briefly discusses the possibilities for memory and CPU optimizations.

cond-mat.mes-hall↗

arXiv2018

Multi-valuedness of the Luttinger-Ward functional in the Fermionic and Bosonic System with Replicas

We study the properties of the Luttinger-Ward functional (LWF) in a simplified Hubbard-type model without time or spatial dimensions, but with $N$ identical replicas located on a single site. The simplicity of this $(0+0)d$ model permits an exact solution for all $N$ and for both bosonic and fermionic statistics. We show that fermionic statistics are directly linked to the fact that multiple values of the noninteracting Green function $G_0$ map to the same value of the interacting Green function $G$, i.e. the mapping $G_0 \mapsto G$ is non-injective. This implies that with fermionic statistics the $(0+0)d$ model has a multiply-valued LWF. The number of LWF values in the fermionic model increases proportionally to the number of replicas $N$, while in the bosonic model the LWF has a single value regardless of $N$. We also discuss the formal connection between the $(0+0)d$ model and the $(0+1)d$ model which was used in previous studies of LWF multivaluedness.

cond-mat.str-el↗

arXiv2015

Spin Response to Localized Pumps: Exciton Polaritons Versus Electrons and Holes

Polariton polarization can be described in terms of a pseudospin which can be oriented along the $x,\,y,$ or $z$ axis, similarly to electron and hole spin. Unlike electrons and holes where time-reversal symmetry requires that the spin-orbit interaction be odd in the momentum, the analogue of the spin-orbit interaction for polaritons, the so-called TE-TM splitting, is even in the momentum. We calculate and compare spin transport of polariton, electron, and hole systems, in the diffusive regime of many scatterings. After dimensional rescaling diffusive systems with spatially uniform particle densities have identical dynamics, regardless of the particle type. Differences between the three particles appear in spatially non-uniform systems, with pumps at a specific localized point. We consider both oscillating pumps and transient (delta-function) pumps. In such systems each particle type produces distinctive spin patterns. The particles can be distinguished by their differing spatial multipole character, their response and resonances in a perpendicular magnetic field, and their relative magnitude which is largest for electrons and weakest for holes. These patterns are manifested both in response to unpolarized pumps which produce in-plane and perpendicular spin signals, and to polarized pumps where the spin precesses from in-plane to out-of-plane and vice versa. These results will be useful for designing systems with large spin polarization signals, for identifying the dominant spin-orbit interaction and measuring subdominant terms in experimental devices, and for measuring the scattering time and the spin-orbit coupling's magnitude.

cond-mat.mes-hall↗

arXiv2013

Disorder induced field effect transistor in bilayer and trilayer graphene

We propose use of disorder to produce a field effect transistor (FET) in biased bilayer and trilayer graphene. Modulation of the bias voltage can produce large variations in the conductance when the disorder's effects are confined to only one of the graphene layers. This effect is based on the bias voltage's ability to select which of the graphene layers carries current, and is not tied to the presence of a gap in the density of states. In particular, we demonstrate this effect in models of gapless ABA-stacked trilayer graphene, gapped ABC-stacked trilayer graphene, and gapped bilayer graphene.

cond-mat.mes-hall↗

arXiv2012

Phase Structure of the Topological Anderson Insulator

We study the disordered topological Anderson insulator in a 2-D (square not strip) geometry. We first report the phase diagram of finite systems and then study the evolution of phase boundaries when the system size is increased to a very large $1120 \times 1120$ area. We establish that conductance quantization can occur without a bulk band gap, and that there are two distinct scaling regions with quantized conductance: TAI-I with a bulk band gap, and TAI-II with localized bulk states. We show that there is no intervening insulating phase between the bulk conduction phase and the TAI-I and TAI-II scaling regions, and that there is no metallic phase at the transition between the quantized and insulating phases. Centered near the quantized-insulating transition there are very broad peaks in the eigenstate size and fractal dimension $d_2$; in a large portion of the conductance plateau eigenstates grow when the disorder strength is increased. The fractal dimension at the peak maximum is $d_2 \approx 1.5$. Effective medium theory (CPA, SCBA) predicts well the boundaries and interior of the gapped TAI-I scaling region, but fails to predict all boundaries save one of the ungapped TAI-II scaling region. We report conductance distributions near several phase transitions and compare them with critical conductance distributions for well-known models.

cond-mat.mes-hall↗