arXiv subjects

Sinisa Coh

Publications and source records attributed to Sinisa Coh.

At least 19 recordsLinked to original sources

arXiv2024

Electron-phonon coupling in ferromagnetic Fe-Co alloys from first principles

The measured magnetization dynamics of ferromagnetic iron--cobalt Fe$_{1-x}$Co$_x$ alloys show a strong dependence on the alloy composition, especially near $x=0.25$. Here, we calculate from first principles the electron-phonon coupling strength in Fe$_{1-x}$Co$_x$ alloys for compositions ranging from $x=0$ to $x=0.75$. We find a strong, spin-dependent variation of the electron-phonon coupling strength with alloy composition, with a minimum near $x=0.25$. We analyze the variation of the electron-phonon interaction with composition, as a function of electron spin, density of states, electron-phonon matrix elements, character of electron wavefunction at the Fermi level, orbital-resolved strength of the phonon perturbing potential, and phonon frequencies. We calculate the electron-phonon energy transfer coefficients, and find that they are in qualitative agreement with the phenomenological electron-phonon energy transfer coefficient deduced from magnetization dynamics experiments. Our findings show that variations in the composition of ferromagnetic alloys can significantly alter the magnetization dynamics and transport properties.

cond-mat.mtrl-sci↗

arXiv2023

Orbital magnetization of a metal is not a bulk property in the mesoscopic regime

We find that, in the mesoscopic regime, modification of the material's surface can induce an extensive change of the material's magnetic moment. In other words, perturbation of order $N^2$ atoms on the surface of a 3-dimensional solid can change the magnetic moment proportionally to $N^3$. When the solid's surface is perturbed, it triggers two changes in the magnetization. One arises from variations of the electron wavefunction and energy, while the other arises from a modification in the kinetic angular momentum operator. In the macroscopic regime of our model, these two bulk effects cancel each other, resulting in no impact of the surface perturbation on the magnetization - consistent with prior work. In the mesoscopic regime, we find a departure from this behavior, as the cancelation of two terms is not complete.

cond-mat.mes-hall↗

arXiv2023

Specular Inverse Faraday Effect in Transition Metals

The inverse Faraday effect is an opto-magnetic phenomenon that describes the ability of circularly polarized light to induce magnetism in solids. The capability of light to control magnetic order in solid state materials and devices is of interest for a variety of applications, such as magnetic recording, quantum computation and spintronic technologies. However, significant gaps in understanding about the effect persist, such as what material properties govern the magnitude of the effect in metals. In this work, we report time-resolved measurements of the specular inverse Faraday effect in non-magnetic metals, i.e., the magneto-optic Kerr effect induced by circularly polarized light. We measure this specular inverse Faraday effect in Cu, Pd, Pt, W, Ta, and Au at a laser wavelength of 783 nm. For Ta and W, we investigate both {\alpha} and \{beta} phases. We observe that excitation of these metals with circularly polarized light induces significant circular dichroism. This nonlinear magneto-optical response to circularly polarized light is an order of magnitude larger in {\alpha}-W than other metals, e.g., Pt, Au, and is greater than nearly all other reported values for IFE in other materials. Our results benchmark the range of IFE that can be observed in non-magnetic metals and provide insight into what material properties govern the inverse Faraday effect in metals.

cond-mat.mtrl-sci↗

arXiv2022

Spin contribution to the inverse Faraday effect of non-magnetic metals

We formulate the spin contribution to the inverse Faraday effect of non-magnetic metals. We deal with the role of the inversion symmetry, which forces all electronic bands to be at least twice degenerate at every point in the Brillouin zone. We show both analytically and numerically that our formulation of the inverse Faraday effect is invariant under unitary rotation within the doubly degenerate set of bands. In addition, we show the importance of resonance-like features in the band structure for the inverse Faraday effect. Our first-principles computed spin component of the inverse Faraday effect in a simple metal such as Au is reminiscent of its optical absorption, with a characteristic d-s resonance in the optical spectrum.

cond-mat.mtrl-sci↗

arXiv2022

Frequency splitting of chiral phonons from broken time reversal symmetry in CrI$_3$

Conventional approaches for lattice dynamics based on static interatomic forces do not fully account for the effects of time-reversal-symmetry breaking in magnetic systems. Recent approaches to rectify this involve incorporating the first-order change in forces with atomic velocities under the assumption of adiabatic separation of electronic and nuclear degrees of freedom. In this work, we develop a first-principles method to calculate this velocity-force coupling in extended solids, and show via the example of ferromagnetic CrI$_3$ that, due to the slow dynamics of the spins in the system, the assumption of adiabatic separation can result in large errors for splittings of zone-center chiral modes. We demonstrate that an accurate description of the lattice dynamics requires treating magnons and phonons on the same footing.

cond-mat.mtrl-sci↗

arXiv2022

Electric field control of phonon angular momentum in perovskite BaTiO$_3$

We find that in BaTiO$_3$ the phonon angular momentum is dominantly pointing in directions perpendicular to the electrical polarization. Therefore, external electric field in ferroelectric BaTiO$_3$ does not control only the direction of electrical polarization, but also the direction of phonon angular momentum. This finding opens up the possibility for electric-field control of physical phenomena that rely on phonon angular momentum. We construct an intuitive model, based on our first-principles calculations, that captures the origin of the relationship between phonon angular momentum and electric polarization.

cond-mat.mtrl-sci↗

arXiv2022

Nonadiabatic Born effective charges in metals and the Drude weight

In insulators, Born effective charges describe the electrical polarization induced by the displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where the macroscopic polarization is ill-defined. Here we show that, in clean conductors, going beyond the adiabatic approximation results in nonadiabatic Born effective charges that are well defined in the low-frequency limit. In addition, we find that the sublattice sum of the nonadiabatic Born effective charges does not vanish as it does in the insulating case, but instead is proportional to the Drude weight. We demonstrate these formal results with density functional perturbation theory calculations of Al, and electron-doped SnS$_2$ and SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Terahertz magneto-optical response in ferromagnetic Fe-Co-Al alloys

We study the magneto-optical properties of Fe-Co-Al ordered alloys in the terahertz range of frequencies. Using the standard Kubo-based approach to compute intrinsic part of the $\sigma_{xy}(\omega)$ we find a strong dependence of $\sigma_{xy}$ on $\omega$ in the terahertz range. For example, we find that below 10~THz Co$_3$Al has nearly constant $\sigma_{xy}$ and that above 10~THz it is reduced by about 50 times. Furthermore, we find a strong dependence of $\sigma_{xy}$ on the chemical composition. For example, we find that the addition of Al to Fe changes the sign of $\sigma_{xy}$, while the addition of Co to Fe leads to a nonmonotonic dependence of $\sigma_{xy}$ on Co concentration.

cond-mat.mtrl-sci↗

arXiv2022

Magneto-optical Kerr spectra of gold induced by spin accumulation

We report the magneto optic Kerr effect (MOKE) angle of Au magnetically excited by spin accumulation. We perform time resolved polar MOKE measurements on Au/Co heterostructures. In our experiment, the ultrafast optical excitation of the Co drives spin accumulation into an adjacent Au layer. The spin accumulation, together with spin-orbit coupling, leads to non-zero terms in the off-diagonal conductivity tensor of Au, which we measure by recording the polarization and ellipticity of light reflected from the Au surface for photon energies between 1.3 and 3.1 eV. At energies near the interband transition threshold of Au, the Kerr rotation per A/m exceeds 1 urad. Typical values for Kerr rotation per moment in transition ferromagnetic metals like Ni are < 10 nrad per A/m, while predicted values for heavy metals like Pt or W are < 13 nrad per A/m. The exceptional sensitivity of the optical properties of Au to spin magnetic moments make Au to be an exceptionally sensitive optical magnetometer, with potential applications in the development of optospintronic technologies.

cond-mat.mtrl-sci↗

arXiv2021

Differentiating Contributions of Electrons and Phonons to the Thermoreflectance Spectra of Gold

To better understand the many effects of temperature on the optical properties of metals, we experimentally and theoretically quantify the electron vs. phonon contributions to the thermoreflectance spectra of gold. We perform a series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths between 400 and 1000 nm. At all wavelengths, we find that changes in phonon temperature, not electron temperature, are the primary contributor to the thermoreflectance of Au. The thermoreflectance is most sensitive to the electron temperature at wavelength of ~480 nm due to interband transitions between d-states and the Fermi-level. In the near infrared, the electron temperature is responsible for only ~2% of the total thermoreflectance. We also compute the thermoreflectance spectra of Au from first principles. Our calculations further confirm that phonon temperature dominates thermoreflectance of Au. Most of Au's thermoreflectance is due to the effect of the phonon population on electron lifetime.

cond-mat.mtrl-sci↗

arXiv2021

Electron-Phonon Scattering governs both Ultrafast and Precessional Magnetization Dynamics in Co-Fe Alloys

Recent investigations have advanced the understanding of how structure-property relationships in ferromagnetic metal alloys affect the magnetization dynamics on nanosecond time-scales. A similar understanding for magnetization dynamics on femto- to pico-second time-scales does not yet exist. To address this, we perform time-resolved magneto optic Kerr effect (TRMOKE) measurements of magnetization dynamics in Co-Fe alloys on femto- to nano-second regimes. We show that Co-Fe compositions that exhibit low Gilbert damping parameters also feature prolonged ultrafast demagnetization upon photoexcitation. We analyze our experimental TR-MOKE data with the three-temperature-model (3TM) and the Landau-Lifshitz-Gilbert equation. These analyses reveal a strong compositional dependence of the dynamics across all time-scales on the strength of electron-phonon interactions. Our findings are beneficial to the spintronics and magnonics community, and will aid in the quest for energy-efficient magnetic storage applications.

cond-mat.mtrl-sci↗

arXiv2021

Large cross-polarized Raman signal in CrI$_3$: A first-principles study

We find unusually large cross-polarized (and anti-symmetric) Raman signature of A$_{\rm g}$ phonon mode in CrI$_3$, in agreement with experiments. The signal is present only when the following three effects are considered in concert: ferromagnetism on Cr atoms, spin-orbit interaction, and resonant effects. Somewhat surprisingly, we find that the relevant spin-orbit interaction potential originates from iodine atoms, despite magnetism being mostly on chromium atoms. We analyze the Raman signature as a function of magnetic order, the direction of the magnetic moment, energy and polarization of light used for Raman scattering, as well as carrier lifetime. In addition to a strong cross-polarized Raman signal, we also find unusually strong phonon modulated magneto-optical Kerr effect (MOKE) in CrI$_3$.

cond-mat.mtrl-sci↗

arXiv2020

Signature of multilayer graphene strain-controlled domain walls in quantum Hall effect

Domain walls, topological defects that define the frontier between regions of different stacking in multilayer graphene, have proved to host exciting physics. The ability of tuning these topological defects in-situ in an electronic transport experiment brings a wealth of possibilities in terms of fundamental understanding of domain walls as well as for electronic applications. Here, we demonstrate through a MEMS (micro-electromechanical system) actuator and magnetoresistance measurements the effect of domain walls in multilayer graphene quantum Hall effect. Reversible and controlled uniaxial strain triggers these topological defects, manifested as new quantum Hall effect plateaus as well as a discrete and reversible modulation of the current across the device. Our findings are supported by theoretical calculations and constitute the first indication of the in-situ tuning of topological defects in multilayer graphene probed through electronic transport, opening the way to the use of reversible topological defects in electronic applications.

cond-mat.mes-hall↗

arXiv2020

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, and others. Here, we report simple expressions that capture the interplay of e-e and e-p interactions on electron distribution relaxation times. We observe a dependence of the dynamics on e-e and e-p interaction strengths that is universal to most metals and is also counterintuitive. While only e-p interactions reduce the total energy stored by excited electrons, the time for energy to leave the electronic subsystem also depends on e-e interaction strengths because e-e interactions increase the number of electrons emitting phonons. The effect of e-e interactions on energy-relaxation is largest in metals with strong e-p interactions. Finally, the time high energy electron states remain occupied depends only on the strength of e-e interactions, even if e-p scattering rates are much greater than e-e scattering rates.

cond-mat.mtrl-sci↗

arXiv2020

Hydrogen plasma favored modification of anatase TiO$_2$ (001) surface with desirable water splitting performance

We show that when TiO$_2$ anatase (001) is exposed to hydrogen plasma that the pristine surface termination becomes unfavorable to another, slightly modified, surface. On this modified surface the topmost TiO$_2$ layer is intact but out of registry with the bottom layers. Nevertheless, the modified surface has significantly improved ability to split water under exposure to sunlight. We show by explicit calculation of the water splitting reaction that the energy barrier that exists on a pristine surface is not present on the modified surface. The valence band maximum of the surface is raised relative to the pristine surface, which is a favorable way of adjusting the band gap in TiO$_2$ to the solar spectrum.

physics.chem-ph↗

arXiv2020

Comparison of GW band structure to semi-empirical approach for an FeSe monolayer

We present the G$_0$W$_0$ band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn Sham density functional theory (DFT) with the PBE functional. We find the GW correction increases the bandwidth of the states forming the $M$ pocket near the Fermi energy, while leaving the $Γ$ pocket roughly unchanged. We then compare the G$_0$W$_0$ quasiparticle band energies with the band structure from a simple empirical +A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory (DFPT). We show that this empirical correction succeeds in approximating the GW non-local and dynamical self energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

cond-mat.mtrl-sci↗

arXiv2019

Classification of materials with phonon angular momentum and microscopic origin of angular momentum

We group materials into five symmetry classes and determine in which of these classes phonons carry angular momentum in the Brillouin zone, away from a high-symmetry point, line, or plane. In some materials phonons acquire angular momentum via the forces induced by relative displacements of atoms out of their equilibrium positions. However, for other materials, such as ferromagnetic iron, phonon angular momentum arises from the forces induced by relative velocities of atoms. These effects are driven by the spin-orbit interaction.

cond-mat.mtrl-sci↗

arXiv2018

Pressure-induced spin reorientation transition in layered ferromagnetic insulator Cr2Ge2Te6

Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of the applied pressure. At zero applied pressure, the easy axis is along the c-direction or perpendicular to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the c-axis to the ab-plane at zero magnetic field, which amounts to a giant magnetic anisotropy energy change (>100%). As the temperature is increased across the Curie temperature, the characteristic AMR effect gradually decreases and disappears. Our first-principles calculations confirm the giant magnetic anisotropy energy change with moderate pressure and assign its origin to the increased off-site spin-orbit interaction of Te atoms due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation transition is very rare in three-dimensional ferromagnets, but it may be common to other layered ferromagnets with similar crystal structures to CGT, and therefore offers a unique way to control magnetic anisotropy.

cond-mat.mtrl-sci↗