arXiv subjects

Benjamin M. Fregoso

Publications and source records attributed to Benjamin M. Fregoso.

At least 19 recordsLinked to original sources

arXiv2026

Average density of Bloch electrons in a homogeneous magnetic field: A second-order response

We compute the average density of a three-dimensional multiband crystal of arbitrary symmetry, metal or insulator, to first and second order in a weak homogeneous magnetic field. To linear order and for insulators, the density follows the well-known Streda formula, but for metals there is an extra contribution from the orbital magnetic moments at the Fermi surface. To second order the average density depends on several microscopic processes. Among these, the quantum metric tensor plays an important role by generating a pseudo-magnetic moment resulting from the rotation of the Bloch wave functions in the complex projective plane. We also discuss the implications of our results for the volume and pressure. The method we develop is explicitly gauge invariant, considers intraband and interband processes on equal footing, accommodates relaxation processes, and can be readily extended to other observables.

cond-mat.mtrl-sci↗

arXiv2024

Nonlinear photomagnetization in insulators

Nonlinear photomagnetization is a process by which an oscillating electric field induces a static magnetization. We show that all 32 crystallographic point groups admit such spin polarization using circularly polarized electric fields to second order (as in the usual spin orientation or inverse Faraday effect) but only 29 points groups admit spin polarization using linearly polarized electric fields to second order. The excluded point groups are the highly symmetric $m$-3$m$, -$43m$ and 432. Using density functional theory we compute the spectrum of the second-order electric spin susceptibility of prototypical semiconductors Te, Se, SnS$_2$, GaAs, InSb and Si which corresponds to nonmagnetic materials with and without inversion symmetry. We show that nonlinear photomagnetization can be comparable to those of naturally occurring ferromagnets.

cond-mat.mtrl-sci↗

arXiv2023

Topological Mixed Valence Model in Magic-Angle Twisted Bilayer Graphene

We develop a model to describe the mixed valence regime in magic-angle twisted bilayer graphene (MATBG) using the recently developed heavy-fermion framework. By employing the large-$N$ slave-boson approach, we derive the self-consistent mean field equations and solve them numerically. We find that the SU(8) symmetry constraint moiré system exhibits novel mixed-valence properties which are different from conventional heavy-fermions systems. We find the solutions describing the physics at the filling near the Mott insulator regime in the limit of strong Coulomb interactions between the flat-band fermions. Our model can provide additional insight into the possible microscopic origin of unconventional superconductivity in MATBG.

cond-mat.supr-con↗

arXiv2022

Bulk photospin effect: Calculation of electric spin susceptibility to second order in an electric field

We compute the electric spin susceptibility of Bloch electrons with spin-orbit coupling to second order. We find that it is possible to generate a nonequilibrium spin polarization in the bulk of non-magnetic inversion-symmetric materials using linearly polarized electric fields, but the process depends on interband coherence and produces heating. It may be possible to avoid heating with circular polarization in certain scenarios. The standard Edelstein effect and spin orientation effects are recovered in appropriate limits within the formalism. Finally, the electric spin susceptibility of metals has contributions proportional to spin multipole moments of the Fermi sea that dominate the low frequency spin response.

cond-mat.mes-hall↗

arXiv2022

Energy relaxation dynamics in a nodal-line semimetal

We study the temperature relaxation dynamics of nodal-line semimetals after a sudden excitation in the presence of acoustic and optical phonon modes. We find that the nodal line constrains the electron momenta in scattering processes and, as a result, the temperature relaxation due to acoustic phonons is exponential as a function of time. However, depending on initial conditions, other functional forms are possible. In typical pump-probe experiments, the temperature relaxation is linear due to acoustic phonons with rates that vary as $\sim n^{1/2}$ with density. The temperature relaxation due to optical phonons is also linear with rates $\sim n^{-1/2}$ or $\sim n$.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast relaxation of acoustic and optical phonons in a topological nodal-line semimetal ZrSiS

Despite being the most studied nodal line semimetal, a clear understanding of the transient state relaxation dynamics and the underlying mechanism in ZrSiS is lacking. Using time and angle resolved photoemission spectroscopy, we study the ultrafast relaxation dynamics in ZrSiS and reveal a unique relaxation in the bulk nodal-line state which is well captured by a simple model based on optical and acoustic phonon cooling. We find linear decay processes for both optical and acoustic phonon relaxations with acoustic cooling suppressed at high temperatures. Our results reveal different decay mechanisms for the bulk and surface states and pave a way to understand the mechanism of conduction in this material.

cond-mat.mes-hall↗

arXiv2020

Pure spin current injection of single-layer monochalcogenides

We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin orbit coupling. The velocity of spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 250, 210, 180 and 154 Km/s for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., CdSe and GaAs. Interestingly, the spin velocity is independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-velocity injection for spintronics applications.

cond-mat.mes-hall↗

arXiv2020

Terahertz radiation of jerk photocurrent

We compute the jerk current tensor of GaAs, Si, and ferroelectric single-layer GeS, GeSe, SnS, and SnSe. We find peak values of the order of $10^{14}$ mA/V$^3$s$^2$ in GaAs and Si within the visible energy spectrum and an order of magnitude larger in single-layer GeS, GeSe, SnSe and SnS. We show that the detailed knowledge of this tensor and its large value in single-layer GeS, GeSe, SnSe and SnS make it possible to predict the magnitude and angle of rotation of polarization of intense terahertz pulses generated in photoconductive switches and point to alternative functionalities of these devices.

cond-mat.mtrl-sci↗

arXiv2020

Colloquium: Physical properties of group-IV monochalcogenide monolayers

We survey the state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally-driven two-dimensional structural transformations. Additionally, these 2D materials also display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry---such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies---is provided as well. The physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in non-volatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D materials toolbox.

cond-mat.mes-hall↗

arXiv2019

Injection current in ferroelectric group-IV monochalcogenide monolayers

We study the injection current response tensor (also known as circular photogalvanic effect or ballistic current) in ferrolectric monolayer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization and could reach peak (bulk) values of the order of $10^{10}$A/V$^{2}$s in the visible spectrum. The magnitude of the injection current is the largest reported in the literature to date for a two dimensional material. To rationalize the large injection current, we correlate the injection current spectrum with the joint density of states, electric polarization, strain, etc. We find that various factors such as anisotropy, in-plane polarization and wave function delocalization are important in determining the injection current tensor in these materials. We also find that compression along the polar axis can increase the injection current (or change its sign), and hence strain can be an effective control knob for their nonlinear optical response. Conversely, the injection current can be a sensitive probe of the crystal structure.

cond-mat.mes-hall↗

arXiv2019

Bulk photovoltaic effects in the presence of a static electric field

This paper presents a study of dc photocurrents in biased insulators to the third order in the electric field. We find three photocurrents which are characterized by physical divergences of the third-order free-electron polarization susceptibility. In the absence of momentum relaxation and saturation effects, these dc photocurrents grow as $t^n$ $(n=2,1,0)$ with illumination time. The photocurrents are dubbed \textit{jerk}, third-order injection, and third-order shift current, respectively, and are generalizations of the second-order injection and shift currents of the bulk photovoltaic effect. We also revisit the theory of the bulk photovoltaic effect and include Fermi surface contributions which are important in metals. Finally, we show that injection, shift, and jerk currents admit simple physical interpretations in terms of semiclassical wave packet dynamics in electric fields. Experimental signatures and extensions to higher-order susceptibilities are also discussed.

cond-mat.mes-hall↗

arXiv2018

Jerk current: A novel bulk photovoltaic effect

We investigate a physical divergence of the third order polarization susceptibility representing a photoinduced current in biased crystalline insulators. This current grows quadratically with illumination time in the absence of momentum relaxation and saturation; we refer to it as the \textit{jerk current}. Two contributions to the current are identified. The first is a hydrodynamic acceleration of optically injected carriers by the static electric field, and the second is the change in the carrier injection rate in the presence of the static electric field. The jerk current can have a component perpendicular to the static field, a feature not captured by standard hydrodynamic descriptions of carriers in electric fields. We suggest an experiment to detect the jerk current and some of its interesting features.

cond-mat.mes-hall↗

arXiv2017

Strong second harmonic generation in two-dimensional ferroelectric IV-monochalcogenides

The two-dimensional ferroelectrics GeS, GeSe, SnS and SnSe are expected to have large spontaneous in-plane electric polarization and enhanced shift-current response. Using density functional methods, we show that these materials also exhibit the largest effective second harmonic generation reported so far. It can reach magnitudes up to $10$ nm/V which is about an order of magnitude larger than that of prototypical GaAs. To rationalize this result we model the optical response with a simple one-dimensional two-band model along the spontaneous polarization direction. Within this model the second-harmonic generation tensor is proportional to the shift-current response tensor. The large shift current and second harmonic responses of GeS, GeSe, SnS and SnSe make them promising non-linear materials for optoelectronic applications.

cond-mat.mes-hall↗

arXiv2017

Large bulk photovoltaic effect and spontaneous polarization of single-layer monochalcogenides

We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current ($\sim$100 $μ$A/V$^2$), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective three-dimensional electric polarization ($\sim$1.9 C/m$^2$). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials

cond-mat.mes-hall↗

arXiv2017

Quantitative relationship between polarization differences and the zone-averaged shift photocurrent

A relationship is derived between differences in electric polarization between bands and the "shift vector" that controls part of a material's bulk photocurrent, then demonstrated in several models. Electric polarization has a quantized gauge ambiguity and is normally observed at surfaces via the surface charge density, while shift current is a bulk property and is described by shift vector gauge invariant at each point in momentum space. They are connected because the same optical transitions that are described in shift currents pick out a relative gauge between valence and conduction bands. We also discuss subtleties arising when there are points at the Brillouin zone where optical transitions are absent. We conclude that two-dimensional materials with significant interband polarization differences should have high bulk photocurrent, meaning that the modern theory of polarization can be used as a straightforward way to search for bulk photovoltaic material candidates.

cond-mat.mes-hall↗

arXiv2017

Design principles for shift current photovoltaics

While the basic principles and limitations of conventional solar cells are well understood, relatively little attention has gone toward maximizing the potential efficiency of photovoltaic devices based on shift currents. In this work, we outline simple design principles for the optimization of shift currents for frequencies near the band gap, derived from the analysis of a general effective model. The use of a novel sum rule allows us to express the band edge shift current in terms of a few model parameters and to show it depends explicitly on wavefunctions via Berry connections in addition to standard band structure. We use our approach to identify two new classes of shift current photovoltaics, ferroelectric polymer films and single-layer orthorhombic monochalcogenides such as GeS. We introduce tight-binding models for these systems, and show that they exhibit the largest shift current responsivities at the band edge reported so far. Moreover, exploring the parameter space of these models we find photoresponsivities that can exceed $100$ mA/W. Our results show how the study of the shift current via effective models allows one to improve the possible efficiency of devices based on this mechanism and better grasp their potential to compete with conventional solar cells.

cond-mat.mes-hall↗

arXiv2016

Structural phase transition and material properties of few-layer monochalcogenides

GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle onto a square at $T_c$, the electronic, spin, optical, and piezo-electric properties dramatically depart from earlier predictions. Indeed, the $Y-$ and $X-$points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement has been drawn by theoretical predictions of giant piezo-electricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3\times 10^{-12}$ $C/K m$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides

cond-mat.mtrl-sci↗

arXiv2016

Non-adiabatic bulk-surface oscillations in driven topological insulators

Recent theoretical and experimental work has suggested the tantalizing possibility of opening a topological gap upon driving the surface states of a three-dimensional strong topological insulator (TI) with circularly polarized light. With this motivation, we study the response of TIs to a driving field that couples to states near the surface. We unexpectedly find coherent oscillations between the surface and the bulk and trace their appearance to unavoidable resonances caused by photon absorption from the drive. We show how these resonant oscillations may be captured by the Demkov- Osherov model of multi-level Landau-Zener physics, leading to non-trivial consequences such as the loss of adiabaticity upon slow ramping of the amplitude. We numerically demonstrate that these oscillations are observable in the time-dependent Wigner distribution, which is directly measurable in time-resolved ARPES experiments. Our results apply generically to any system with surface states in the presence of a gapped bulk, and thus suggest experimental signatures of a novel surface-bulk coupling mechanism that is fundamental for proposals to engineer non-trivial states by periodic driving.

cond-mat.mes-hall↗