arXiv subjects

Anderson Janotti

Publications and source records attributed to Anderson Janotti.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous magneto-optical response at $\mathrm{RuO_2 / WSe_2}$ van der Waals interface

Ruthenium dioxide ($\mathrm{RuO_2}$) has been proposed as an altermagnetic candidate, although its magnetic ground state remains controversial. Here, we probe weak interfacial magnetic states at the surface of (001)-oriented $\mathrm{RuO_2}$ films using the magnetic proximity effect (MPE) in a van der Waals heterostructure consisting of monolayer tungsten diselenide ($\mathrm{WSe_2}$) atop $\mathrm{RuO_2}$. Temperature-dependent magneto-optical spectroscopy reveals an anomalous excitonic energy shift and a deviation from conventional Varshni behavior below 55 K that are absent in an encapsulated $\mathrm{WSe_2}$ control sample. The anomalous shift reverses sign upon field cooling with opposite magnetic field polarity, indicating a magnetic origin. Polarization-resolved measurements further show a nearly field-independent and fluctuating valley splitting in $\mathrm{WSe_2 / RuO_2}$ in strong contrast to the conventional linear Zeeman splitting observed in the control bare $\mathrm{WSe_2}$ sample. These results suggest that the valley states are governed predominantly by interfacial exchange fields associated with weak surface magnetic states in $\mathrm{RuO_2}$, which do not produce a conventional linear Zeeman response within the applied magnetic field range. Importantly, this approach enables direct optical probing of emergent surface magnetism without introducing an additional ferromagnetic layer, positioning MPE-based optical probing as a tool for investigating weak surface magnetism and offering new possibilities for studying magnetic materials with controversial magnetic states.

cond-mat.mtrl-sci↗

arXiv2026

Strong electron correlations and ligand hybridization for altermagnetism

Spin-band splitting is a hallmark of altermagnetism, intrinsically linked to magnetic ordering driven by electron correlations. However, recent inconsistencies in the detection of altermagnetism in strongly correlated altermagnet candidates have cast doubt on the robustness of this phenomenon and its dependence on many-body effects. Here, density functional theory combined with dynamical mean-field theory (DFT+DMFT), we dissect the electronic origins of altermagnetism in three prototypical candidates: MnF$_2$, MnTe, and RuO$_2$. In MnF$_2$, we identify pronounced local electron correlations within Mn-3$d$ states and uncover a distinct Mott gap in the visible range. The strong correlations markedly localize the Mn-3$d$ electrons, leading to a narrowing of the spin-resolved bandwidth and, consequently, a suppression of spin-band splitting. By contrast, MnTe provides an ideal platform for altermagnetism, exhibiting substantial local Mn-3$d$ magnetic moments due to the strong correlations and pronounced spin-band splitting, enabled by robust Mn-3$d$--Te-5$p$ orbital hybridization. RuO$_2$ manifests as a Pauli paramagnet with vanishing local moments, even in its antiferromagnetic phase. Nonetheless, it exhibits significant spin-band splitting, indicative of itinerant altermagnetic behavior. Our results reveal that both strong local electron correlations and judicious ligand selection to promote orbital hybridization are key prerequisites to realizing altermagnetism in strongly correlated systems. These insights pave the way for the rational design and discovery of novel altermagnetic materials.

cond-mat.str-el↗

arXiv2026

Deterministic Realization of Complex Local Strain Fields and Bandgap Profiles in Two-Dimensional Materials

Emerging classical and quantum device concepts demand precise spatial control over the optoelectronic properties of two-dimensional (2D) materials, but deterministic engineering via local multiaxial strain distributions remains challenging. Using Ga$_2$Se$_2$, we demonstrate a material-agnostic platform in which nanostructure geometry deterministically prescribes in-plane strain profiles in suspended van der Waals membranes. We first use hyperspectral photoluminescence mapping and experimentally-constrained finite element analysis to quantify the experimental biaxial and uniaxial strain gauge factors that relate strain to the change in bandgap. We next show that a two-component analytical model can predict, with less than 12% error, spatially-resolved bandgap shifts arising from multiaxial strain distributions in complex geometries, including the interactions between adjacent nanostructures. Finally, we demonstrate that this approach can be extended to other materials. The results demonstrate that nanostructure design provides a quantitative, deterministic framework for the realization of designed strain and bandgap distributions in 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

Resonance-enhanced super-superexchange yields giant chiral magnon splitting in rutile altermagnets

Altermagnets host momentum-selective spin splitting and chiral-split magnonic excitations despite vanishing net magnetization, enabling spin transport without ferromagnetism. In rutile structures, establishing altermagnetism spectroscopically has been challenging, motivating the search for a rutile platform with a resolvable exchange-driven chiral magnon splitting. Here we combine hybrid-functional first-principles calculations with linear spin-wave theory to show that rutile CuF$_2$ exhibits a meV-scale splitting between magnon modes of opposite chirality along momentum directions dictated by its $d$-wave altermagnetic symmetry. The splitting originates from an anomalously strong long-range super-superexchange channel Cu--F$\cdots$F--Cu, which enhances the symmetry-allowed difference between seventh-neighbour exchanges, $J_{7b} - J_{7a}$, controlling the chiral-mode splitting. We identify an orbital-resonance mechanism: energetic alignment between Cu $3d_{z^2}$ and F $2p_z$ states strengthens virtual hopping along the Cu--F$\cdots$F--Cu path and amplifies the anisotropic long-range exchange. Rutile CuF$_2$ therefore provides an ideal platform to validate rutile altermagnetism and suggests an orbital-energy description for engineering large chiral magnon splittings in insulating altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

Sequential Quenching to Predict Semiconductor Defect Concentrations from Formation & Migration Energies: The Case of CdTe:As Doping

Defect concentrations in semiconductors are strongly influenced by thermal history during growth and cooldown, yet most defect calculations assume either instantaneous quenching from high temperature or that full-equilibrium is maintained - two limiting cases rarely approached in reality. Here, we introduce sequential quenching (SQ) as a 3rd type of defect calculation utilizing defect formation and migration energies to model defect concentrations subject to diffusion-limited kinetics in samples cooled at finite rates. In SQ, the concentration of each defect is frozen at a characteristic temperature determined by its diffusion rate, distance to sources/sinks, and cooling rate. Because different charge-states interact through charge neutrality but freeze at different temperatures, the sequence of freeze-in events is non-commuting. Critically, not all room-temperature SQ solutions can be predicted from full equilibrium (EQ) or full-quenching (FQ) calculations - erroneous predictions are likely without SQ. We illustrate SQ using the example of As-doped CdTe, for which experimental data show differences in doping with cooling rate and between polycrystalline thin-films for photovoltaics and bulk crystals. SQ calculations reveal that fast-diffusing defects such as Cd-interstitials remain mobile to lower temperatures and freeze-in at larger characteristic distances, leading to strong compensation and n-type behavior in rapidly cooled or bulk samples. Slower cooling and reduced characteristic distances suppress donor freeze-in and enhance p-type activation. These results establish SQ as a physically transparent and computationally efficient framework for connecting cooling conditions, sample geometry, and defect kinetics to dopant activation in CdTe and related materials.

cond-mat.mtrl-sci↗

arXiv2026

Effects of uniaxial strain on monolayer transition-metal dichalcogenides revisited

Using hybrid density functional calculations including spin-orbit coupling, we compute the strain evolution of the band structure of monolayer 1H-phase transition-metal dichalcogenides, MX$_2$ (M= Mo, W; X= S, Se, Te), emphasizing an accurate reproduction of the quasiparticle band gap (as opposed to the excitonic optical gap). We show that tensile uniaxial strain applied along either the armchair or zigzag directions leads to a pronounced reduction of the fundamental gap, with the conduction-band edge generally exhibiting the stronger strain response. Both the conduction-band electron valleys (CBM) and the valence-band hole valleys (VBM) remain degenerate under uniaxial strain, while simultaneously drifting away from the high-symmetry $K$ point under strain ("valley drift"), such that the band extrema occur at nearby off-symmetry wave vectors. A minimal tight-binding model rationalizes the valley drift and the unequal electron- and hole-valley drift rates in the presence of strain, leading to indirect band gaps. In particular, for MoS$_2$ the indirectness increases with tensile strain, providing a natural explanation for the experimentally observed decrease in photoluminescence intensity under uniaxial deformation. These results provide quantitative guidance for tailoring band structures for optoelectronic and quantum-defect applications.

cond-mat.mtrl-sci↗

arXiv2026

Quantifying the Relationship Between Strain and Bandgap in Thin Ga$_2$Se$_2$

We present a rigorous analysis that combines theory, simulation, and experimental measurements to quantify the relationship between strain and bandgap in two dimensional gallium selenide (Ga$_2$Se$_2$). Experimentally, we transfer thin Ga$_2$Se$_2$ flakes onto patterned substrates to deterministically induce multiaxial localized strain. We quantify the local strain using a combination of atomic force microscopy (AFM) measurements and COMSOL Multiphysics simulation. We then experimentally map the strain-induced bandgap shifts using high-resolution hyperspectral PL imaging to generate a robust and statistically significant dataset. We systematically fit this data to extract gauge factors that relate the bandgap shift to the local uniaxial and biaxial strain. We then compute the uniaxial and biaxial strain gauge factors via density functional theory (DFT) and find excellent agreement with the experimentally-determined values. Finally, we show that a simple model that computes bandgap shifts from the local uniaxial and biaxial strain predicts the observed multiaxial bandgap shift with less than 10\% error. The combined results provide a framework for deterministic realization of tailored bandgap profiles induced by controlled strain applied to Ga$_2$Se$_2$, with implications for the future realization of localized quantum emitters for quantum photonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Quantum geometric origins of the orbital degrees of freedom of hybrid bosonic quasiparticles in magnetic systems

The orbital degree of freedom has recently attracted significant attention due to the novel phenomena it enables in condensed matter systems. However, the interpretation of the orbital degree of freedom in bosonic quasiparticles remains conceptually ambiguous and the mechanisms governing the transfer of orbital angular moment (OAM) between distinct quasiparticles, such as magnons and phonons, are not yet fully understood. We investigate orbital dynamics in bosonic systems and identify two origins of OAM: (i) global rotational motion of the system, and (ii) the quantum geometry of wavefunctions. Focusing on the latter, we study strongly coupled magnon-phonon systems in two-dimensional antiferromagnets as a test case. We uncover finite OAM arising from quantum geometric effects via two mechanisms: (a) time-parity symmetry breaking, yielding intra band OAM, and (b) interband coupling, generating interband OAM. We propose that an electrical detection scheme based on the transverse voltage generated by hybrid magnon phonon modes can be used to experimentally probe the bosonic orbital degree of freedom. Our results establish a foundation for the emerging field of phonon orbitronics, providing both a conceptual bridge between phonon and magnon orbitronics and a tool for better understanding magnon-polarons. They also advance a unified framework for harnessing orbital degrees of freedom in bosonic systems and pave the way toward electrical control of magnetization and phononic transport.

cond-mat.mes-hall↗

arXiv2025

Probing Magnetic Properties of RuO$_{2}$ Heterostructures Through the Ferromagnetic Layer

RuO$_{2}$ has been proposed as the prototypical altermagnetic material. However, several reports have recently questioned its intrinsic magnetic ordering, leading to conflicting findings, especially in thin film heterostructures pointing to possible interface effects being convoluted with supposed antiferromagnetic/altermagnetic signatures. Here, extensive magnetometry measurements were performed on two independently grown thin film heterostructures of RuO$_{2}$ interfaced with either NiFe or Fe acting as the ferromagnetic layer. Below about 15 K, both samples exhibit exchange bias fields when cooled to approximately 2 K in a $+$1 T field, and a spin transitional feature is observed around 31 K. Magneto-Raman measurements on RuO$_{2}$ thin films only reveal a magnon mode when there is a NiFe layer, suggesting that RuO$_{2}$ does not intrinsically possess long range magnetic ordering.. When in contact with a ferromagnet, RuO$_2$ displays effects that could be ascribed to antiferromagnetism. However, the lack of intrinsic magnon modes points toward possible diffusion between the layers or spin disorder at the interface as seen by density functional theory (DFT) calculations.

cond-mat.mtrl-sci↗

arXiv2025

Lifshitz transition in correlated topological semimetals

Topological quasiparticles, arising when the chemical potential is near the band crossing, are pivotal for the development of next-generation quantum devices. They are expected to exist in half-Heusler correlated topological semimetals. However, the emergence of hole carriers, which alter the chemical potential away from the quadratic-band-touching points is not yet understood. Here, we investigated the electronic structure of YPtBi and GdPtBi through ab initio many-body perturbation GW theory combined with dynamical mean-field theory and revealed that the correlation effects of 4$d$ or 4$f$ electrons can lead to the formation of hole carriers. In YPtBi, the weakly correlated Y-4$d$ electrons constitute the topological bands, and the quadratic-band-touching point is at the Fermi level at high temperatures. At low temperatures, enhanced correlations of Y-4$d$ renormalize the topological bands, leading to the formation of hole pocket. In GdPtBi, the strongly correlated Gd-4$f$ electrons form the Hubbard-like bands originate from self-energy effects associated with a topological singularity. These local bands encompass itinerant 4$f$ bands, which hybridize with topological bands to induce pronounced hole bands. This concerted effect reduces the hole doping, bringing the chemical potential closer to the quadratic-band-touching points as the temperature is lowered. The temperature-induced Lifshitz transition should be responsible for the large hole bands observed in both topological semimetals in angle-resolved photoemission spectroscopy measurements at low temperatures. Our findings indicate that the integration of correlated fermions within a topological framework can modulate the energy landscape of topological bands.

cond-mat.str-el↗

arXiv2025

Tunable magnon band topology and magnon orbital Nernst effect in noncollinear antiferromagnets

We theoretically investigate the intrinsic magnon orbital Nernst effect (ONE) in noncollinear antiferromagnets with Kagom\'e spin systems. Our analysis reveals that an externally applied magnetic field induces topological phase transitions in the magnonic system, characterized by the closing and reopening of the band gap between distinct magnon bands. These transitions enable tunable control of the magnon orbital Nernst effect with applied magnetic field, with a pronounced enhancement in magnon orbital Nernst conductivity near the phase transition points. This tunability presents a promising direction for experimental detection of the magnon ONE.

cond-mat.mtrl-sci↗

arXiv2025

Structural phase transitions between layered Indium Selenide for inte-grated photonic memory

The primary mechanism of optical memristive devices relies on the phase transitions between amorphous-crystalline states. The slow or energy hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the devices scalability and performance. Leveraging the integrated photonic platform, we demonstrate a single nanosecond pulse triggered nonvolatile and reversible switching between two layered structures of indium selenide (In2Se3). High resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With inter-layer shear glide and isosymmetric phase transition, the switching between alpha and beta structural states contain low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular beam epitaxy-grown thin films and compared to ab initials calculations. The nonlinear resonator transmission spectra measure an incremental linear loss rate of 3.3 GHz introduced by 1.5 micrometer long In2Se3 covered lay-er, resulting from the combinations of material absorption and scattering.

physics.optics↗

arXiv2025

Weyl semimetal phases and intrinsic spin-Hall conductivity in SbAs ordered alloys

Using density functional theory calculations we investigated possible Weyl semimetal (WSM) phases in antimony arsenide ordered alloys Sb1-xAsx (x=0, 1/6, 1/3, 1/2, 2/3, 5/6, 1). We find WSM phases for all As compositions of Sb1-xAsx with broken inversion symmetry, in contrast to Bi1-xSbx where only compositions x=1/2 and 5/6 were predicted to exhibit WSM phases. The WSM phases in Sb1-xAsx are characterized by the presence of 12 Weyl points, located within 55 meV from the Fermi level in the case of x=1/2. The robust spin-orbit coupling strength and Berry curvature in these alloys produce large spin-Hall conductivity in the range of 176-602 (hbar/e)(S/cm), comparable to that in the BiSb alloys. Finally, Sb0.5As0.5 is predicted to be almost lattice-matched to GaAs(111), with the Fermi level within the gap of the semiconductor, facilitating growth and characterization, and thus, offering promising integration with conventional semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry-breaking induced surface magnetization in non-magnetic RuO$_2$

Altermagnetism is a newly identified phase of magnetism distinct from ferromagnetism and antiferromagnetism. RuO$_2$ has been considered a prototypical metallic altermagnet with a critical temperature higher than room temperature. Previous interpretations of the unusual magnetic properties of RuO$_2$ relied on the theoretical prediction that local moments on two Ru sublattices, which are connected by four-fold rotational symmetry, are quite significant (approximately 1 $\mu_B$), leading to long-range antiferromagnetic order. However, accumulated experimental data suggest that local moments on Ru in RuO$_2$ are vanishingly small, indicating that the bulk material is likely non-magnetic. This observation is consistent with the delocalized nature of the 4$d$ electrons of Ru and the strong screening effect in the metallic state. In this work, we show that despite the non-magnetic bulk, the RuO$_2$(110) surface exhibits spontaneous magnetization. We attribute this effect to the breaking of local symmetry, which results in electronic redistribution and magnetic moment enhancement. The emergence of surface magnetism gives rise to interesting spectroscopic phenomena, including spin-polarized surface states, spin-polarized scanning probe microscopy images, and potentially spin-dependent transport effects. These findings highlight the important role of surface magnetic structures in the otherwise non-magnetic bulk RuO$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Band-gap reduction and band alignments of dilute bismide III--V alloys

Adding a few atomic percent of Bi to III--V semiconductors leads to significant changes in their electronic structure and optical properties. Bismuth substitution on the pnictogen site leads to a large increase in spin-orbit splitting $\Delta_{\rm SO}$ at the top of the valence band ($\Gamma_{8v}-\Gamma_{7v}$) and a large reduction in the band gap, creating unique opportunities in semiconductor device applications. Quantifying these changes is key to the design and simulation of electronic and optoelectronic devices. Using hybrid functional calculations, we predict the band gap of III--Vs (III=Al, Ga, In and V=As, Sb) with low concentrations of Bi (3.125\% and 6.25\%), the effects of adding Bi on the valence- and conduction-band edges, and the band offset between these dilute alloys and their III--V parent compounds. As expected, adding Bi raises the valence-band maximum (VBM). However, contrary to previous assumptions, the conduction-band minimum (CBM) is also significantly lowered, and both effects contribute to the sizable band-gap reduction. Changes in band gap and $\Delta_{\rm SO}$ are notably larger in the arsenides than in the antimonides. We also predict cases of band-gap inversion ($\Gamma_{6c}$ below $\Gamma_{8v}$) and $\Delta_{\rm SO}$ larger than the band gap, which are key parameters for designing topological materials and for minimizing losses due to Auger recombination in infrared lasers.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic proximity coupling to defects in a two-dimensional semiconductor

The ultrathin structure and efficient spin dynamics of two-dimensional (2D) antiferromagnetic (AFM) materials hold unprecedented opportunities for ultrafast memory devices, artificial intelligence circuits, and novel computing technology. For example, chromium thiophosphate (CrPS4) is one of the most promising 2D A-type AFM materials due to its robust stability in diverse environmental conditions and net out-of-plane magnetic moment in each layer, attributed to anisotropy in crystal axes (a and b). However, their net zero magnetic moment poses a challenge for detecting the Neel state that is used to encode information. In this study, we demonstrate the detection of the Neel vector by detecting the magnetic order of the surface layer by employing defects in tungsten diselenide (WSe2). These defects are ideal candidates for optically active transducers to probe the magnetic order due to their narrow linewidth and high susceptibility to magnetic fields. We observed spin-polarized charge transfer in the heterostructure of bulk CrPS4 and single-layer WSe2 indicating type-II band alignment as supported by density functional theory (DFT) calculations. In the A-type AFM regime, the intensity of both right-handed and left-handed circularly polarized light emanating from the sample remains constant as a function of the applied magnetic field, indicating a constant polarized transition behavior. Our results showcase a new approach to optically characterizing the magnetic states of 2D bulk AFM material, highlighting avenues for future research and technological applications.

cond-mat.mtrl-sci↗

arXiv2024

Role of chalcogen vacancies and hydrogen in the optical and electrical properties of bulk transition-metal dichalcogenides

Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS$_2$ flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.

cond-mat.mtrl-sci↗

arXiv2023

Fermi-level pinning in ErAs nanoparticles embedded in III-V semiconductors

Embedding rare-earth pnictide (RE-V) nanoparticles into III-V semiconductors enables unique optical, electrical, and thermal properties, with applications in THz photoconductive switches, tunnel junctions, and thermoelectric devices. Despite the high structural quality and control over growth, particle size, and density, the underlying electronic structure of these nanocomposite materials has only been hypothesized. Basic questions about the metallic or semiconducting nature of the nanoparticles (that are typically < 3 nm in diameter) have remained unanswered. Using first-principles calculations, we investigated the structural and electronic properties of ErAs nanoparticles in AlAs, GaAs, InAs, and their alloys. Formation energies of the ErAs nanoparticles with different shapes and sizes (i.e., from cubic to spherical, with 1.14 nm, 1.71 nm, and 2.28 nm diameters) show that spherical nanoparticles are the most energetically favorable. As the diameter increases, the Fermi level is lowered from near the conduction band to the middle of the gap. For the lowest energy nanoparticles, the Fermi level is pinned near the mid-gap, at about 0.8 eV above the valence band in GaAs and about 1.2 eV in AlAs, and it is resonant in the conduction band in InAs. Our results show that the Fermi level is pinned on an absolute energy scale once the band alignment at AlAs/GaAs/InAs interfaces is considered, offering insights into the rational design of these nanocomposite materials.

cond-mat.mtrl-sci↗