Abstract: We detail the magnetic properties of monolayer CrX2 and its Janus counterparts CrXY (X, Y = S, Se, Te, with X not equal Y) using ab initio methods and Landau-Lifshitz-Gilbert magnetization dynamics, and uncover the pronouncedly strong interplay between their structure symmetry and the magnetic order. The relaxation of nonmagnetic chalcogen atoms, that carry large spin-orbit coupling, changes the energetically preferential magnetic order between in-plane antiferromagnetic and tilted ferromagnetic one. The considered Janus monolayers exhibit sizable Dzyaloshinskii-Moriya interaction, in some cases above 20% of the isotropic exchange, and critical temperature of the long-range magnetic order in the vicinity or even significantly above the room temperature.

Abstract: We present a detailed theoretical investigation on the electronic transport properties of n-type monolayer (ML) transition metal dichalcogenides (TMDs) at low temperature in the presence of proximity-induced interactions such as Rashba spin-orbit coupling (RSOC) and the exchange interaction. The electronic band structure is calculated by solving the Schr & ouml;dinger equation with a k <middle dot> p Hamiltonian, and the electric screening induced by electron-electron interaction is evaluated under a standard random phase approximation approach. In particular, the longitudinal and transverse or Hall mobilities are calculated by using a momentum-balance equation derived from a semiclassical Boltzmann equation, where the electron-impurity interaction is considered as the principal scattering center at low temperature. The obtained results show that the RSOC can induce the in-plane spin components for spin-split subbands in different valleys, while the exchange interaction can lift the energy degeneracy for electrons in different valleys. The opposite signs of Berry curvatures in the two valleys would introduce opposite directions of Lorentz force on valley electrons. As a result, the transverse currents from nondegenerate valleys can no longer be canceled out so that the transverse current or Hall mobility can be observed. Interestingly, we find that at a fixed effective Zeeman field, the lowest spin-split conduction subband in ML-TMDs can be tuned from one in the K'-valley to one in the K-valley by varying the Rashba parameter. The occupation of electrons in different valleys also varies with changing carrier density. Therefore, we can change the magnitude and direction of the Hall current by varying the Rashba parameter, effective Zeeman field, and carrier density by, e.g., the presence of a ferromagnetic substrate and/or applying a gate voltage. By taking the ML-MoS2 as an example, these effects are demonstrated and examined. The important and interesting theoretical findings can be beneficial to experimental observation of the valleytronic effect and to gaining an in-depth understanding of the ML-TMD systems in the presence of proximity-induced interactions.

Abstract: We investigated the electron-phonon ( e -ph ) coupling and vibrational thermal conductivity in the representative MAB compounds, namely MoAlB, WAlB, Tc 2 AlB 2 , and Cr 2 AlB 2 . The spectral distribution functions of e -ph interaction, obtained through ab initio linear-response calculations, reveal that the electron-phonon coupling values range from low (0.15) to moderate (0.58). With such e -ph coupling, out of the considered compounds, only Tc 2 AlB 2 exhibits a superconducting transition, at 4 K. We further evaluated the thermal conductivity and associated properties like scattering rates, obtained using ab initio and other methodologies. The latter included the iterative solution of the Peierls-Boltzmann transport equation, using HIPHIVE package for advanced optimization and machine learning techniques, and employing maximum likelihood estimation to approximate scattering rates from a limited set of scattering processes. We found that these methods yield nearly identical predictions for thermal conductivity values, with a significant decrease in the computational cost compared to the first-principles methods. We examined interactions arising from both three-phonon (3 ph ) and four -phonon (4 ph ) scattering processes. The 4 ph interactions demonstrated a smaller yet significant impact on the overall vibrational thermal conductivity, most notably in Tc 2 AlB 2 . Our findings indicate that Cr 2 AlB 2 has the highest thermal conductivity across all considered crystal directions, with the thermal conductivity being spatially anisotropic, most pronouncedly in Tc 2 AlB 2 . Finally, we show that empirical expressions based on Slack models are well suited for screening the thermal conductivity properties of MAB phases, and can be employed to establish upper and lower limits of their thermal conductivity.

Abstract: The tight-binding method is used to investigate the electronic and magnetic properties of borophene nano-ribbons (BNRs) in the presence of a perpendicular magnetic field. Most BNRs exhibit metallic characteristics due to edge bands. Additionally, the appearance of Landau levels (LLs) is strongly influenced by the edge states, contrasting with the sheet platform which produces distinct LLs. We also investigated single atomic vacancy disorders in BNRs and observed localized vacancy states (LVSs) resulting from atomic disorder. Both LVSs and LLs are influenced by the edge states, underscoring that the electronic and magnetic properties of BNRs are strongly edge-dependent. This aspect is crucial for consideration in experimental, theoretical, and computational studies.

Abstract: Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (T-AD) and fieldlike (T-FL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (T-AD) and transverse (T-FL) components of the SOT vector and its efficiency chi(AD) and chi(FL), respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (B-SF) for magnetization reversal by SOT effective fields (B-SOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which chi(AD) and chi(FL) are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find chi(FL) to be (i) twice as large as chi(AD) and (ii) 6 times as large as chi(FL) in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for chi(FL) > chi(AD). Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, T-FL plays a more prominent role in magnetization dynamics than T-AD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of T-FL and chi(FL) amplification merits to be explored.

Abstract: Quantum anomalous Hall, high-Chern number, and axion phases in topological insulators are characterized by its Chern invariant C (respectively, C = 1, integer C > 1, and C = 0 with half-quantized Hall conductance of opposite signs on top and bottom surfaces). They are of recent interest because of novel fundamental physics and prospective applications, but identifying and controlling these phases has been challenging in practice. Here we show that these states can be created and switched between in thin films of Bi2Se3 by Floquet engineering, using irradiation by circularly polarized light. We present the calculated phase diagrams of encountered topological phases in Bi2Se3, as a function of wavelength and amplitude of light, as well as sample thickness, after properly taking into account the penetration depth of light and the variation of the gap in the surface states. These findings open pathways towards energy-efficient optoelectronics, advanced sensing, quantum information processing and metrology.

Abstract: Seminal Goodenough-Kanamori-Anderson (GKA) rules provide an inceptive understanding of the superexchange interaction of two magnetic metal ions bridged with an anion, and suggest fostered ferromagnetic interaction for orthogonal bridging bonds. However, there are no examples of two-dimensional (2D) materials with structure that optimizes the GKA arguments towards enhanced ferromagnetism and its critical temperature. Here we reveal that an ideally planar GKA ferromagnetism is indeed stable in selected tetragonal transition-metal xenes (tTMXs), with Curie temperature above 300 K found in CrC and MnC. We provide the general orbitally-resolved analysis of magnetic interactions that supports the claims and sheds light at the mechanisms dominating the magnetic exchange process in these structures. Furthermore, we propose the set of three GKA-like rules that will guarantee room temperature ferromagetnism. With recent advent of epitaxially-grown tetragonal 2D materials, our findings earmark tTMXs for facilitated spintronic and magnonic applications, or as a desirable magnetic constituent of functional 2D heterostructures.

Abstract: Nitrogen-doped diamond (N-D) is one of the most important carbon-based electronic and optical materials. Here we study the terahertz (THz) magneto-optical (MO) properties of N-D grown by microwave plasma-enhanced chemical vapor deposition. The optical microscope, SEM, XRD, Raman spectrum, FTIR spectroscopy and XPS are used for the characterization of N-D samples. Applying THz time-domain spectroscopy (TDS), in combination with the polarization test and the presence of magnetic field in Faraday geometry, THz MO transmissions through N-D are measured from 0 to 8 T at 80 K. The complex right- and left-handed circular transmission coefficients and MO conductivities for N-D are obtained accordingly. Through fitting the experimental results with theoretical formulas of the dielectric constant and MO conductivities for an electron gas, we are able to determine magneto-optically the key electronic parameters of N-D, such as the static dielectric constant epsilon b, the electron density ne, the electronic relaxation time tau, the electronic localization factor alpha and, particularly, the effective electron mass m* obtained under non-resonant condition. The dependence of these parameters upon magnetic field is examined and analyzed. We find that the MO conductivities of N-D can be described rightly by the MO Drude-Smith formulas developed by us previously. It is shown that N-doping and the presence of the magnetic field can lead towards the larger epsilon b and heavier m* in diamond, while ne/tau/alpha in N-D decreases/increases/decreases with increasing magnetic field. The results obtained from this work are benefit to us in gaining an in-depth understanding of the electronic and optoelectronic properties of N-D.

Abstract: The McMillan-Ginzburg-Landau (MGL) model for charge density waves (CDW) is employed in a systematic phenomenological study of the different phases that have been probed in recent experiments involving transition metal dichalcogenides. We implemented an efficient imaginary time evolution method to solve the MGL equations, which enabled us to investigate the role of different coupling parameters on the CDW patterns and to perform calculations with different energy functionals that lead to several experimentally observed singularities in the CDW phase profiles. In particular, by choosing the appropriate energy functionals, we were able to obtain phases that go beyond the well-known periodic phase slips (discommensurations), exhibiting also topological defects (i.e., vortex-antivortex pairs), domain walls where the CDW order parameter is suppressed, and even CDW with broken rotational symmetry. Finally, we briefly discuss the effect of these different CDW phases on the profile and critical temperature of the competing superconducting state.

Abstract: Symmetry breaking plays a pivotal role in unlocking intriguing properties and functionalities in material systems. For example, the breaking of spatial and temporal symmetries leads to a fascinating phenomenon: the superconducting diode effect. However, generating and precisely controlling the superconducting diode effect pose significant challenges. Here, we take a novel route with the deliberate manipulation of magnetic charge potentials to realize unconventional superconducting flux-quantum diode effects. We achieve this through suitably tailored nanoengineered arrays of nanobar magnets on top of a superconducting thin film. We demonstrate the vital roles of inversion antisymmetry and its breaking in evoking unconventional superconducting effects, namely a magnetically symmetric diode effect and an odd-parity magnetotransport effect. These effects are nonvolatilely controllable through in situ magnetization switching of the nanobar magnets. Our findings promote the use of antisymmetry (breaking) for initiating unconventional superconducting properties, paving the way for exciting prospects and innovative functionalities in superconducting electronics.

Abstract: Transport measurements are readily used to probe different phases in disordered topological insulators (TIs), where determining topological invariants explicitly is challenging. On that note, universal conductance fluctuations (UCF) theory asserts the conductance G for an ensemble has a Gaussian distribution, and that standard deviation 8G depends solely on the symmetries and dimensions of the system. Using a real-space tight -binding Hamiltonian on a system with Anderson disorder, we explore conductance fluctuations in a thin Bi2Se3 film and demonstrate the agreement of their behavior with UCF hypotheses. We further show that magnetic field applied out-of-plane breaks the time -reversal symmetry and transforms the system's Wigner-Dyson class from root symplectic to unitary, increasing 8G by 2. Finally, we reveal that while Bi2Se3 is a strong TI, weak TI and metallic phases can be stabilized in presence of strain and disorder, and detected by monitoring the conductance fluctuations.

Abstract: We provide a comprehensive analysis of the prominent tight-binding (TB) models for transition metal dichalcogenides (TMDs) available in the literature. We inspect the construction of these TB models, discuss their parameterization used and conduct a thorough comparison of their effectiveness in capturing important electronic properties. Based on these insights, we propose a novel TB model for TMDs designed for enhanced computational efficiency. Utilizing MoS2 as a representative case, we explain why specific models offer a more accurate description. Our primary aim is to assist researchers in choosing the most appropriate TB model for their calculations on TMDs.

Abstract: Utilizing magnetic tunnel junctions (MTJs) for write/read and fast spin-orbit-torque (SOT)-driven domain-wall (DW) motion for propagation, enables non-volatile logic and majority operations, representing a breakthrough in the implementation of nanoscale DW logic devices. Recently, current-driven DW logic gates have been demonstrated via magnetic imaging, where the Dzyaloshinskii-Moriya interaction (DMI) induces chiral coupling between perpendicular magnetic anisotropy (PMA) regions via an in-plane (IP) oriented region. However, full electrical operation of nanoscale DW logic requires electrical write/read operations and a method to pattern PMA and IP regions compatible with the fabrication of PMA MTJs. Here, we study the use of a Hybrid Free Layer (HFL) concept to combine an MTJ stack with DW motion materials, and He+ ion irradiation to convert the stack from PMA to IP. First, we investigate the free layer thickness dependence of 100-nm diameter HFL-MTJ devices and find an optimal CoFeB thickness, from 7 to 10 angstrom, providing high tunneling magnetoresistance (TMR) readout and efficient spin-transfer torque (STT) writing. We then show that high DMI materials, like Pt/Co, can be integrated into an MTJ stack via interlayer exchange coupling with the CoFeB free layer. In this design, DMI values suitable for SOT-driven DW motion are measured by asymmetric bubble expansion. Finally, we demonstrate that He+ irradiation reliably converts the coupled free layers from PMA to IP. These findings offer a path toward the integration of fully electrically controlled DW logic circuits.