Intrinsic In-plane Research Articles

Symmetry-based requirement for the measurement of electrical and thermal Hall conductivity under an in-plane magnetic field

The in-plane (thermal) Hall effect is an unconventional transverse response when the applied magnetic field is in the (heat) current plane. In contrast to the normal Hall effect, the in-plane Hall effect requires the absence of certain crystal symmetries, and possibly manifests a non-trivial topology of quantum materials. An accurate estimation of the intrinsic in-plane (thermal) Hall conductivity is crucial to identify the underlying mechanisms as in the case of the Kitaev spin-liquid candidate alpha-RuCl3. Here, we give the symmetry conditions for the in-plane Hall effect and discuss the implications that may impede the experimental evaluation of the in-plane (thermal) Hall conductivity within the single-device measurement. First, the lack of symmetry in crystals can create merohedral twin domains that cancel the total Hall signal. Second, even in a twin-free crystal, the intrinsic response is potentially contaminated by the out-of-plane conduction in three-dimensional systems, which is systematically unavoidable in the in-plane Hall systems. Third, even in a quasi-two-dimensional system, the conversion of (thermal) resistivity, rho (lambda), to (thermal) conductivity, sigma (kappa) requires protocols beyond the widely-used simplified formula due to the lack of in-plane-rotational symmetry. In principle, two independent sample devices are necessary to accurately estimate the s_xy (k_xy). As a case study, we discuss the half-integer quantization of the in-plane thermal Hall effect in the spin-disordered state of alpha-RuCl3. For an accurate measurement of the thermal Hall effect, it is necessary to avoid crystals with the merohedral twins contributing oppositely to k_xy, while the out-of-plane transport may have a negligible effect. To deal with the field-induced rotational-symmetry breaking, we propose two symmetry-based protocols, improved single-device and two-device methods.

Effect of fabric structure on in-plane and through-plane hydraulic properties of nonwoven geotextiles

The intrinsic in-plane (IP) permeability (K∥) and through-plane (TP) permeability (K⊥) of nonwoven geotextiles were measured experimentally. Also, the realistic 3D images of the samples were acquired by the nondestructive μCT technique and their microstructural characteristics were measured. This information was used to simulate virtual fibrous structures resembling the microstructure of geotextiles. The flow of fluid was then simulated through realistic and virtual structures and K∥ and K⊥ were calculated. The experimental and predicted results were compared with each other and those of some empirical, analytical and numerical studies. A non-linear relation between both the K∥ and K⊥ and geotextile porosity was observed. It was found that for the same porosity, coarser fibers provide higher K∥ and K⊥ and this is more pronounced at higher porosity values. It was observed that by increasing the IP fiber orientation in the flow direction, K∥ increases exponentially, while K⊥ is independent of IP fiber orientation. The anisotropic permeability ratios λ=K∥/K⊥ of samples were calculated and it was established that nonwoven geotextiles show highly anisotropic behavior with λ tending to unity with the increase of geotextile porosity. It was shown that K⊥ increases with the increase of fiber orientation through the thickness direction.

Tunable band gap of diamond twin boundaries by strain engineering

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.

Molding Broadband Dispersion in Twisted Trilayer Hyperbolic Polaritonic Surfaces.

Recent advancement in twisted layered metasurfaces can be employed to control the nanoscale flow of light, including the exotic hyperbolic-to-elliptic topological transitions in twisted bilayers (tBL). Such topological transitions can only occur to limited frequency ranges, restricted by the intrinsic in-plane dispersion of individual hyperbolic surfaces. Here, we report that, by controlling interlayer evanescent coupling in twisted polaritonic trilayers, moldable topological transitions of light can be achieved in broadband. We reveal that the required minimum open angle of the individual hyperbolic polaritonic surface for open-to-close topological transitions can be significantly lowered compared to that of the twisted bilayer counterpart. This increases the degree of freedom to enhance and control near-field light-matter interactions and energy management. As an example, we demonstrate a knob to manipulate near-field radiative heat transfer (NFRHT). By rotating the relative angles of trilayers, exotic and tunable thermal conductance can be achieved. Our findings enrich the controllability of light at the nanoscale in broadband, bringing twisted optical materials one step closer to practical applications.

Vacancy-engineered half-metallicity and magnetic anisotropy in CrSI semiconductor monolayer

By virtue of their complete spin polarization at the Fermi level, two-dimensional (2D) magnetic half-metallic materials are emerging as one of the latest wonder building blocks for spintronic applications. Using first-principles calculations, we explored how vacancy defects affect the electronic structure and magnetic properties of CrSI semiconductor monolayer. Our results indicate that the magnetic semiconductor monolayer becomes metallic with the presence of single Cr vacancies, VCr, or paired vacancies made up of Cr and nearby S atoms, VCr-S, while by introducing VS, VI, VCr-I and VS-I vacancies, the monolayer becomes a half-metallic ferromagnet with Curie temperature TC above room temperature. Compared with the pristine case that exhibits an intrinsic in-plane magnetic anisotropy with an easy axis along the [100] direction, the six types of vacancies considered in this study either enhance the in-plane anisotropy or switch it to out-of-plane. Our work implies that vacancy engineering could be a viable approach for achieving both half-metallicity and perpendicular magnetic anisotropy in structurally similar magnetic semiconductor monolayers as well as in relevant designer van der Waals heterostructures for 2D spintronic applications.

Newly discovered dimensional effects of electrodes on liquid crystal THz phase shifters enable novel switching between in-plane and out-of-plane

To unveil a novel switching mechanism in liquid crystal (LC)-based phase shifters for the THz range, we analyse how the dimensions of the electrode structures enable a new type of switching, namely, THz in-plane and THz out-of-plane (TIP–TOP) switching. Specifically, we determine how varying these electrode dimensions influences the LC in-plane states with the corresponding phase shifts by calculating these effects in virtual devices. Interestingly, we found that significant dimensional effects of the in-plane electrode structures statically and dynamically influence the phase shift and response time of LC switching. Analysing the electromagnetic fields in the TIP–TOP cell clearly reveals that these dimensional effects are due to changes in the electric field strengths caused by lateral bus-line electrodes that were originally assumed not to contribute to the switching. Further, we discover that the ultimate dimensional effect produces a novel type of LC switching, which results in hexadirectional switching between the initial, intrinsic in-plane, and out-of-plane reorientations of the LCs, suggesting a broader range of phase shifts while maintaining a rapid response.

The effect of shape and size in the stability of triangular Janus MoSSe quantum dots

Asymmetric Janus transition metal dichalcogenide MoSSe is a promising catalytic material due to the intrinsic in-plane dipole of its opposite faces. The atomic description of the structures observed by experimental techniques is relevant to tuning and optimizing its surface reaction processes. Furthermore, the experimentally observed triangular morphologies in MoSSe suggest that an analysis of the chemical environment of its edges is vital to understand its reactivity. Here we analyze the size-shape stability among different triangular structures-quantum- dots proposed from the ideal S(-1010) and Mo(10-10) terminations. Our stability analysis evidenced that the S–Se termination is more stable than Mo; moreover, as the size of the quantum dot increases, its stability increases as well. Besides, a trend is observed, with the appearance of elongated Mo-S/Se bonds at symmetric positions of the edges. Tersoff–Hamann scanning tunneling microscopy images for both faces of the stablest models are presented. Electrostatic potential isosurfaces denote that the basal plane on the S face of both configurations remains the region with more electron density concentration. These results point toward the differentiated activity over both faces. Finally, our study denotes the exact atomic arrangement on the edges of MoSSe quantum dots corresponding with the formation of S/Se dimers who decorates the edges and their role along with the faces as catalytic sites.

Characterization of room-temperature in-plane magnetization in thin flakes of CrTe2 with a single-spin magnetometer

We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the $\mathrm{Cr}{\mathrm{Te}}_{2}$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\ensuremath{\sim}27$ kA/m for flakes with thicknesses down to 20 nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized $\mathrm{Cr}{\mathrm{Te}}_{2}$ flakes, which suggests the existence of a non-negligible magnetocrystalline anisotropy. These results make $\mathrm{Cr}{\mathrm{Te}}_{2}$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date that offers an intrinsic in-plane magnetization and a Curie temperature above 300 K in thin flakes.

Strongly enhanced electromechanical coupling in atomically thin transition metal dichalcogenides

Flexoelectricity in thin films has emerged as an effective electromechanical response owing to appealing scaling law and universal existence. However, current studies show limited out-of-plane converse flexoelectric effect (CFE) of ultra-thin transition metal dichalcogenides (TMDs) when compared to their conventional in-plane piezoresponse. Here, we report converse flexoresponse of atomically thin TMDs such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) which exceeds their intrinsic in-plane piezoresponses. Our piezoresponse force microscopy (PFM) measurements revealed strongly enhanced CFE of the atomically thin MoS2 and WSe2 than their bulk counterpart (∼700% enhancement in MoS2, ∼400% enhancement in WSe2). We observed an anomalous reduction in converse flexoresponse in the monolayer structure attributed to a puckering deformation. By inducing a built-in in-plane tension to reduce puckering, we estimated the CFE of monolayer WSe2 to be 8.14 pm/V, the highest among the atomically thin TMDs.

Thickness-Dependent In-Plane Polarization and Structural Phase Transition in van der Waals Ferroelectric CuInP2 S6.

van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-density storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in-plane polarization in vdW ferroelectrics CuInP2 S6 single crystals is reported, whereas below a critical thickness between 90 and 100 nm, the in-plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.

The equivalent Young's modulus prediction for vacancy defected graphene under shear stress

The uncertain and unavoidable vacancy defects in graphene have the inevitable influence in the extraordinary intrinsic in-plane strength. In this paper, the equivalent Young's modulus is derived from the strain energy as an important factor to evaluate the stiffness of the entire graphene based on the mechanical molecular theory. The location of vacancy defects in graphene is discussed in the regular deterministic and uncertain patterns. In terms of the boundary condition, shear stress is loaded in armchair and zigzag edges, respectively. The results show that the center concentrated vacancy defects evidently deteriorate the elastic stiffness under shear stress. The influences of periodic and regular vacancy defects are sensitive to the boundary condition. By the Monte Carlo based finite element method, vacancy defects are dispersed randomly and propagated. The results of the equivalent Young's modulus are compared with the original values in pristine graphene. The interval and mean values of Young's modulus, total strain and energy density are also provided and discussed. Compared with the results of graphene with vacancy defects under uniaxial tension, the enhancement effects of vacancy defects are less evident in the graphene under shear stress.

Polar phonons in polycrystalline SrTiO3 thin films: The role of the substrate

Recently we have demonstrated that out-of-plane dielectric properties of polycrystalline SrTiO3 (ST) films depend on the strain/stress induced by the substrates due to thermal expansion misfit. Here, we present a complementary infra-red (IR) spectroscopy investigation of the role of Al2O3 and MgO substrates on the lattice dynamics and in-plane dielectric properties of ST thin films also prepared by sol-gel. We verify that ST films deposited on Al2O3 substrates, being under a tensile stress, present the soft mode behaviour similar to that of ST single crystals, deviating just below 150 K. In contrast, ST films deposited on MgO substrates, being under compressive stress, have higher soft mode frequency from 10 to 300 K. It is thus proved that for polycrystalline ST films the intrinsic in-plane relative permittivity may be tuned through an induced stress state.

Tensile mechanical characteristics and deformation mechanism of metal-graphene nanolayered composites

Metal-graphene nanolayered composites (MGNLCs), composed of alternating layers of metal (copper here) and graphene layers, are new emerging engineering materials with outstandingly enhanced mechanical properties thanks to the high intrinsic in-plane strength and modulus of one-atom-thick layers of graphene. In this paper, mechanical behavior and elastic-plastic deformation mechanisms of MGNLCs subjected to uniaxial tensile loading were investigated based on molecular dynamics (MD) simulations. Stress-strain curves of the composite samples and their elastic properties were obtained and compared with their pure metal counterparts. In addition, influence of thickness of the metal layers on the composite performance was determined which seems tough or even impossible to be dealt with by the experimental techniques. Graphene inclusion outstandingly increased strength and failure strain of the composites and remarkably improved their stiffness and toughness. Graphene layers could provide effective barriers against shearing flows and dislocation propagation of the metal layers and made the applied strain energy spread out more evenly inside the material.

Emerging two-dimensional ferromagnetism in silicene materials

The appeal of ultra-compact spintronics drives intense research on magnetism in low-dimensional materials. Recent years have witnessed remarkable progress in engineering two-dimensional (2D) magnetism via defects, edges, adatoms, and magnetic proximity. However, intrinsic 2D ferromagnetism remained elusive until recent discovery of out-of-plane magneto-optical response in Cr-based layers, stimulating the search for 2D magnets with tunable and diverse properties. Here we employ a bottom-up approach to produce layered structures of silicene (a Si counterpart of graphene) functionalized by rare-earth atoms, ranging from the bulk down to one monolayer. We track the evolution from the antiferromagnetism of the bulk to intrinsic 2D in-plane ferromagnetism of ultrathin layers, with its characteristic dependence of the transition temperature on low magnetic fields. The emerging ferromagnetism manifests itself in the electron transport. The discovery of a class of robust 2D magnets, compatible with the mature Si technology, is instrumental for engineering new devices and understanding spin phenomena.

Magnetic order in laser-irradiated kagome antiferromagnets

Dispersionless ‘zero energy mode’ is one of the hallmarks of frustrated kagomé antiferromagnets (KAFMs). It points to extensive classically degenerate ground-states. The ‘zero energy mode’ can be observed experimentally when lifted to a flat mode at finite energy by a strong intrinsic magnetic anisotropy. In this letter, we study the effects of irradiation of laser light on the KAFMs. We adopt the magnon picture without loss of generality. It is shown that circularly or linearly polarized light lifts the ‘zero energy mode’, stabilizes magnetic order, and induces energy gaps in the KAFMs. We find that the circularly polarized light-induced anisotropies have similar features as the intrinsic in-plane and out-of-plane Dzyaloshinskii–Moriya interaction in KAFMs. The former stabilizes long-range magnetic order and the latter induces spin canting out-of-plane with nonzero scalar spin chirality. The Floquet thermal Hall effect shows that the synthetic magnetic excitation modes in the case of circularly polarized light are topological, whereas those of linearly polarized light are not.

Electronic thermal conduction in suspended graphene

The electronic thermal conductivity (ETC), κe, of suspended graphene (SG) is studied for 15<T<400K, following the Boltzmann transport formalism. The electrons are considered to be scattered from defects along with the intrinsic in-plane acoustic phonons, out-of-plane flexural phonons (FPs) and optical phonons. The ETC is evaluated by computing the first-order perturbation distribution function by directly solving the linearized Boltzmann equation by an iterative method. Numerical calculations of the temperature and concentration dependences of κe show the dominance of charged impurity scattering at lower temperatures (T<75K) and of FPs at higher temperatures. The results are compared with the commonly used low-temperature and high-energy relaxation time approximations. Our calculations are in good agreement with recent κe data extracted for high-mobility SG samples. The validity of Wiedemann–Franz law is also discussed.

Thermal property of bent graphene nanorribons

Comparing the thermal conductance features and mechanism between carbon nanotubes and graphene nanoribbons reveal the key role of intrinsic in-plane and out-of-plane phonon modes coupling in effectively influencing the materials’ thermal conductivity. The finding allows us to successfully propose an analytic intrinsic phonon mode-coupling-tuning model to control thermal properties of bent graphene nanorribons (BGNRs) with bending angle varying. The derived thermal conductivity using this method is larger in zigzag-edged and smaller in armchair-edged BGNRs than that of the corresponding single wall carbon nanotubes, respectively. The decrease of both thermal conductivity and specific heat as bending angle of BGNRs increasing due to the growing coupling between the intrinsic in-plane and out-of-plane modes present enormous potential of the BGNRs in acting as excellent components in important thermal devices.

Inducing topological superconductivity at the LaAlO3/SrTiO3 interface

The discovery of superconductivity in the two-dimensional electron gas at the interface between two band insulators SrTiO3 and LaAlO3 has opened up possibilities for novel physics. We show that due to the presence of Rashba spin-orbit interaction at the interface, a topological superconductivity can be induced by applying a magnetic field perpendicular to the interface plane even in the presence of the intrinsic in-plane magnetization. The inplane magnetization favours a finite momentum pairing of electrons and therefore weakens the topological superconducting phase. We show that the tunable Rashba spin-orbit interaction can help in stabilizing the topological superconducting state against the deleterious in-plane magnetization.

Spin Polarization of Gapped Dirac Surface States Near the Topological Phase Transition inTlBi(S1−xSex)2

We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S(1-x)Se(x))(2) which undergoes a topological phase transition at x ~ 0.5. In TlBiSe(2) (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of ~0.8. The spin polarization still survives in the gapped surface states at x > 0.5, although it gradually weakens upon approaching x = 0.5 and vanishes in the nontopological phase. No evidence for the out-of-plane spin polarization was found, irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions.

Diffusion thermopower in suspended graphene: Effect of strain

Diffusion thermopower, Sd, is studied in pure suspended graphene (SG) monolayers in equipartition regime (for 100 K &amp;lt; T &amp;lt; 300 K). Electrons are considered to be scattered by the intrinsic in-plane and out-of-plane flexural acoustic phonons. Our numerical calculations of the temperature, T and carrier concentration, ns, dependences of electron mobility and Sd bring out the relative importance of the two scattering mechanisms in SG in the presence and absence of strain. For free-standing SG, Sd is found to vary almost linearly with temperature with dominant contribution from flexural phonons. This contribution is found to be suppressed in strained SG resulting in enhancement in mobility and reduction in Sd. Dependences of mobility and Sd on T and ns are modified. The relevance of Nordheim-Gorter rule in SG is also studied.