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Roger K. Lake

Publications and source records attributed to Roger K. Lake.

At least 37 records · Page 2Linked to original sources
arXiv2022

One-Dimensional van der Waals Quantum Materials -- State of the Art and Perspectives

The advent of graphene and other two-dimensional van der Waals materials, with their unique electrical, optical, and thermal properties, has resulted in tremendous progress for fundamental science. Recent developments suggest that taking one more step down in dimensionality - from monolayer, atomic sheets to individual atomic chains - can bring exciting prospects as the ultimate limit in material downscaling is reached while establishing an entirely new field of one-dimensional quantum materials. Here we review this emerging area of one-dimensional van der Waals quantum materials and anticipate its future directions. We focus on quantum effects associated with the charge-density-wave condensate, strongly-correlated phenomena, topological phases, and other unique physical characteristics, which are attainable specifically in van der Waals materials of lower dimensionality. Possibilities for engineering the properties of quasi-one-dimensional materials via compositional changes, vacancies, and defects, as well as the prospects of their applications in composites are also discussed.

cond-mat.mes-hall↗
arXiv2021

Discrete Quantum Geometry and Intrinsic Spin Hall Effect

We show that the quantum geometry of the Fermi surface can be numerically described by a 3-dimensional discrete quantum manifold. This approach not only avoids singularities in the Fermi sea, but it also enables the precise computation of the intrinsic Hall conductivity resolved in spin, as well as any other local properties of the Fermi surface. The method assures numerical accuracy when the Fermi level is arbitrarily close to singularities, and it remains robust when Kramers degeneracy is protected by symmetry. The approach is demonstrated by calculating the anomalous Hall and spin Hall conductivities of a 2-band lattice model of a Weyl semimetal and a full-band ab-initio model of zincblende GaAs.

cond-mat.mtrl-sci↗
arXiv2021

Thermal conductivity of the quasi-1D materials TaSe3 and ZrTe3

The high breakdown current densities and resilience to scaling of the metallic transition metal trichalcogenides TaSe3 and ZrTe3 make them of interest for possible interconnect applications, and it motivates this study of their thermal conductivities and phonon properties. These crystals consist of planes of strongly bonded one-dimensional chains more weakly bonded to neighboring chains. Phonon dispersions and the thermal conductivity tensors are calculated using density functional theory combined with an iterative solution of the phonon Boltzmann transport equation. The phonon velocities and the thermal conductivities of TaSe3 are considerably more anisotropic than those of ZrTe3. The maximum LA velocity in ZrTe3 occurs in the cross-chain direction, and this is consistent with the strong cross-chain bonding that gives rise to large Fermi velocities in that direction. The thermal conductivities are similar to those of other metallic two-dimensional transition metal dichalcogenides. At room temperature, a significant portion of the heat is carried by the optical modes. In the low frequency range, the phonon lifetimes and mean free paths in TaSe3 are considerably shorter than those in ZrTe3. The shorter lifetimes in TaSe3 are consistent with the presence of lower frequency optical branches and zone-folding features in the acoustic branches that arise due to the doubling of the TaSe3 unit cell within the plane.

cond-mat.mtrl-sci↗
arXiv2020

Stacking enabled strong coupling of atomic motion to interlayer excitons in van der Waals heterojunction photodiodes

We reveal stacking-induced strong coupling between atomic motion and interlayer excitons through photocurrent measurements of WSe$_2$/MoSe$_2$ heterojunction photodiodes. Strong coupling manifests as pronounced periodic sidebands in the photocurrent spectrum in frequency windows close to the interlayer exciton resonances. The sidebands, which repeat over large swathes of the interlayer exciton photocurrent spectrum, occur in energy increments corresponding directly to a prominent vibrational mode of the heterojunction. Such periodic patterns, together with interlayer photoconductance oscillations, vividly demonstrate the emergence of extraordinarily strong exciton-phonon coupling - and its impact on interlayer excitations - in stack-engineered van der Waals heterostructure devices. Our results establish photocurrent spectroscopy as a powerful tool for interrogating vibrational coupling to interlayer excitons and suggest an emerging strategy to control vibronic physics in the solid-state.

cond-mat.mes-hall↗
arXiv2020

Effects of filling, strain, and electric field on the Néel vector in antiferromagnetic CrSb

CrSb is a layered antiferromagnet (AFM) with perpendicular magnetic anisotropy, a high Néel temperature, and large spin-orbit coupling (SOC), which makes it interesting for AFM spintronic applications. To elucidate the various mechanisms of Néel vector control, the effects of strain, band filling, and electric field on the magnetic anisotropy energy (MAE) of bulk and thin-film CrSb are determined and analysed using density functional theory. The MAE of the bulk crystal is large (1.2 meV per unit cell). Due to the significant ionic nature of the Cr-Sb bond, finite slabs are strongly affected by end termination. Truncation of the bulk crystal to a thin film with one surface terminated with Cr and the other surface terminated with Sb breaks inversion symmetry, creates a large charge dipole and average electric field across the film, and breaks spin degeneracy, such that the thin film becomes a ferrimagnet. The MAE is reduced such that its sign can be switched with realistic strain, and the large SOC gives rise to an intrinsic voltage controlled magnetic anisotropy (VCMA). A slab terminated on both faces with Cr remains a compensated AFM, but with the compensation occurring nonlocally between mirror symmetric Cr pairs. In-plane alignment of the moments is preferred, the magnitude of the MAE remains large, similar to that of the bulk, and it is relatively insensitive to filling.

cond-mat.mtrl-sci↗
arXiv2020

Strain controlled superconductivity in few-layer NbSe2

The controlled tunability of superconductivity in low-dimensional materials may enable new quantum devices. Particularly in triplet or topological superconductors, tunneling devices such as Josephson junctions etc. can demonstrate exotic functionalities. The tunnel barrier, an insulating or normal material layer separating two superconductors, is a key component for the junctions. Thin layers of NbSe2 have been shown as a superconductor with strong spin orbit coupling, which can give rise to topological superconductivity if driven by a large magnetic exchange field. Here we demonstrate the superconductor-insulator transitions in epitaxially grown few-layer NbSe2 with wafer-scale uniformity on insulating substrates. We provide the electrical transport, Raman spectroscopy, cross-sectional transmission electron microscopy, and X-ray diffraction characterizations of the insulating phase. We show that the superconductor-insulator transition is driven by strain, which also causes characteristic energy shifts of the Raman modes. Our observation paves the way for high quality hetero-junction tunnel barriers to be seamlessly built into epitaxial NbSe2 itself, thereby enabling highly scalable tunneling devices for superconductor-based quantum electronics.

cond-mat.supr-con↗
arXiv2020

Interfacial Dzyaloshinskii-Moriya Interaction of Antiferromagnetic Materials

The interface between a ferromagnet (FM) or antiferromagnet (AFM) and a heavy metal (HM) results in an antisymmetric exchange interaction known as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) which favors non-collinear spin configurations. The iDMI is responsible for stabilizing noncollinear spin textures such as skyrmions in materials with bulk inversion symmetry. Interfacial DMI values have been previously determined theoretically and experimentally for FM/HM interfaces, and, in this work, values are calculated for the metallic AFM MnPt and the insulating AFM NiO. The heavy metals considered are W, Re, and Au. The effects of the AFM and HM thicknesses are determined. The iDMI values of the MnPt heterolayers are comparable to those of the common FM materials, and those of NiO are lower.

cond-mat.mtrl-sci↗
arXiv2020

Electron transport through antiferromagnetic spin textures and skyrmions in a magnetic tunnel junction

An ideal layered $\hat{x}$-polarized antiferromagnet (AFM) between two antialigned $\pm \hat{z}$ polarized ferromagnetic (FM) contacts transmits no current due to a $π$ phase difference of the matrix elements coupling the spin degenerate states to the two FM contacts. The ratio of the two matrix elements coupling the two spin degenerate $\hat{x}$ AFM states to a $\hat{z}$ FM contact is determined by the exchange energy and the eigenstate energy. Inserting a normal metal layer or tunnel barrier layer between one FM contact and the AFM alters this phase difference, and, due to the unequal weighting of the two spins at the interface, it also breaks the spin degeneracy of the two AFM states. The broken symmetry of the matrix elements combined with the broken degeneracy of the AFM states, result in a Fano resonance in the transmission and a turn-on of the $T_{\uparrow,\downarrow}$ transmission channel. Such a magnetic tunnel junction geometry with two antialigned $\pm \hat{z}$ FM contacts can electrically detect an AFM skyrmion. The AFM skyrmion serves as an analogue of the oblique polarizer in the triple polarizer experiment. Resistances and resistance ratios are calculated and compared for FM and AFM skyrmions in a magnetic tunnel junction.

cond-mat.mes-hall↗
arXiv2020

Large spin-Hall effect in Si at room temperature

Silicon's weak intrinsic spin-orbit coupling and centrosymmetric crystal structure are a critical bottleneck to the development of Si spintronics, because they lead to an insignificant spin-Hall effect (spin current generation) and inverse spin-Hall effect (spin current detection). Here, we undertake current, magnetic field, crystallography dependent magnetoresistance and magneto thermal transport measurements to study the spin transport behavior in freestanding Si thin films. We observe a large spin-Hall magnetoresistance in both p-Si and n-Si at room temperature and it is an order of magnitude larger than that of Pt. One explanation of the unexpectedly large and efficient spin-Hall effect is spin-phonon coupling instead of spin-orbit coupling. The macroscopic origin of the spin-phonon coupling can be large strain gradients that can exist in the freestanding Si films. This discovery in a light, earth abundant and centrosymmetric material opens a new path of strain engineering to achieve spin dependent properties in technologically highly-developed materials.

cond-mat.mes-hall↗
arXiv2020

Properties of Shape-Engineered Phoxonic Crystals: Brillouin-Mandelstam Spectroscopy and Ellipsometry Study

We report the results of Brillouin-Mandelstam spectroscopy and Mueller matrix spectroscopic ellipsometry of the nanoscale "pillar with the hat" periodic silicon structures, revealing intriguing phononic and photonic properties. It has been theoretically shown that periodic structures with properly tuned dimensions can act simultaneously as phononic and photonic - phoxonic - crystals, strongly affecting the light-matter interactions. Acoustic phonon states can be tuned by external boundaries, either as a result of phonon confinement effects in individual nanostructures, or as a result of artificially induced external periodicity, as in the phononic crystals. The shape of the nanoscale pillar array was engineered to ensure the interplay of both effects. The Brillouin-Mandelstam spectroscopy data indicated strong flattening of the acoustic phonon dispersion in the frequency range from 2 GHz to 20 GHz and the phonon wave vector extending to the higher-order Brillouin zones. The specifics of the phonon dispersion dependence on the pillar arrays orientation suggest the presence of both periodic modulation and spatial localization effects for the acoustic phonons. The ellipsometry data reveal a distinct scatter pattern of four-fold symmetry due to nanoscale periodicity of the pillar arrays. Our results confirm the dual functionality of the nanostructured shape-engineered structure and indicate a possible new direction for fine-tuning the light-matter interaction in the next generation of photonic, optoelectronic, and phononic devices.

physics.app-ph↗
arXiv2019

Small Angle and Non-Monotonic Behavior of the Thermal Conductivity in Twisted Bilayer Graphene

Nothing is known about the thermal conductivity in twisted bilayer graphene (TBG) at small twist angles, and how it approaches its aligned value as the twist angle approaches 0 degrees. To provide insight into these questions, we perform large scale non-equilibrium molecular dynamics calculations on commensurate TBG structures with angles down to 1.87 degrees. The results show a smooth, non-monotonic behavior of the thermal conductivity with respect to the commensurate lattice constant. As the commensurate lattice constant increases, the thermal conductivity initially decreases by 50%, and then it returns to 90% of its aligned value as the angle is reduced to 1.89 degrees. These same qualitative trends are followed by the trends in the shear elastic constant, the wrinkling intensity, and the out-of-plane ZA2 phonon frequency. The picture that emerges of the physical mechanism governing the thermal conductivity is that misorientation reduces the shear elastic constant; the reduced shear elastic constant enables greater wrinkling; and the greater wrinkling reduces the thermal conductivity. The small-angle behavior of the thermal conductivity raises the question of how do response functions approach their aligned values as the twist angle approaches 0 degrees. Is the approach gradual, discontinuous, or a combination of the two?

physics.comp-ph↗
arXiv2019

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductor Materials

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS3 and MnPS3 semiconductors. The Raman spectroscopy was conducted using three different excitation lasers with the wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). The resonant UV-Raman spectroscopy reveals new spectral features, which are not detectable via visible Raman light scattering. The thermal conductivity of FePS3 and MnPS3 thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS3 to be 1.35 W/mK at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS3 is 0.85 W/mK and 2.7 W/mK, respectively. The films of MnPS3 have higher thermal conductivity of 1.1 W/mK through-plane and 6.3 W/mK in-plane. The data obtained by both techniques reveal strong thermal anisotropy of the films and the dominant contribution of phonons to heat conduction. Our results are important for the proposed applications of the antiferromagnetic semiconductor thin films in spintronic devices.

cond-mat.mtrl-sci↗
arXiv2019

Topological Spin Hall Effect due to Magnetic Skyrmions

The intrinsic spin Hall effect (SHE) originates from the topology of the Bloch bands in momentum space. The duality between real space and momentum space calls for a spin Hall effect induced from a real space topology in analogy to the topological Hall effect (THE) of skyrmions. We theoretically demonstrate the topological spin Hall effect (TSHE) in which a pure transverse spin current is generated from a skyrmion spin texture.

cond-mat.mes-hall↗
arXiv2019

Planar Hall effect in antiferromagnetic MnTe thin films

We show that the spin-orbit coupling (SOC) in alpha-MnTe impacts the transport behavior by generating an anisotropic valence-band splitting, resulting in four spin-polarized pockets near Gamma. A minimal k-dot-p model is constructed to capture this splitting by group theory analysis, a tight-binding model and ab initio calculations. The model is shown to describe the rotation symmetry of the zero-field planer Hall effect (PHE). The upper limit of the PHE percentage is shown to be fundamentally determined by the band shape, and is quantitatively estimated to be roughly 31% by first principles.

cond-mat.mtrl-sci↗
arXiv2019

Strain control of the Neel vector in Mn-based antiferromagnets

Control of the Neel vector in antiferromagnetic materials is one of the challenges preventing their use as active device components. Several methods have been investigated such as exchange bias, electric current, and spin injection, but little is known about strain-mediated anisotropy. This study of the antiferromagnetic L10-type MnX alloys MnIr, MnRh, MnNi, MnPd, and MnPt shows that a small amount of strain effectively rotates the direction of the Neel vector by 90 degrees for all of the materials. For MnIr, MnRh, MnNi, and MnPd, the Neel vector rotates within the basal plane. For MnPt, the Neel vector rotates from out-of-plane to in-plane under tensile strain. The effectiveness of strain control is quantified by a metric of efficiency and by direct calculation of the magnetostriction coefficients. The values of the magnetostriction coefficients are comparable with those from ferromagnetic materials. These results indicate that strain is a mechanism that can be exploited for control of the Neel vectors in this family of antiferromagnets.

physics.app-ph↗
arXiv2018

Charged impurity scattering in two-dimensional materials with ring-shaped valence bands: GaS, GaSe, InS, and InSe

The singular density of states and the two Fermi wavevectors resulting from a ring-shaped or "Mexican hat" valence band give rise to unique trends in the charged impurity scattering rates and charged impurity limited mobilities. Ring shaped valence bands are common features of many monolayer and few-layer two-dimensional materials including the III-VI materials GaS, GaSe, InS, and InSe. The wavevector dependence of the screening, calculated within the random phase approximation, is so strong that it is the dominant factor determining the overall trends of the scattering rates and mobilities with respect to temperature and hole density. Charged impurities placed on the substrate and in the 2D channel are considered. The different wavevector dependencies of the bare Coulomb potentials alter the temperature dependence of the mobilities. Moving the charged impurities 5 $Å$ from the center of the channel to the substrate increases the mobility by an order of magnitude.

cond-mat.mes-hall↗
arXiv2018

Spin superfluid Josephson oscillator

The magnetic analogue of the Josephson effect can be exploited to develop a new class of nano-spin oscillators that we denote as spin superfluid Josephson oscillators. Such a device, consisting of two exchange coupled easy-plane metallic ferromagnets separated by a thin normal metal spacer, is proposed and analyzed. A spin chemical potential difference drives a $2π$ precession of the in-plane magnetization of each ferromagnet. The $2 π$ precession angle gives maximum values of the giant magnetoresistance, resulting in large output power compared to conventional spin Hall oscillators. An applied ac current results in a time-averaged magnetoresistance with Shapiro-like steps. The multistate mode-locking behavior exhibited by the spin Shapiro steps may be explored for applications in neuromorphic computing. As an experimental characterization method, electrical measurements of spin superfluid Josephson junctions can provide additional signatures of spin superfluidity.

cond-mat.mes-hall↗