Search Results (46)

Inelastic electron tunneling spectroscopy (IETS) has emerged as a powerful vibrational spectroscopy technique for molecular electronic junctions, providing unique insights into molecular vibrations and electron–phonon coupling at the nanoscale. In this review, we present a comprehensive overview of IETS in molecular junctions, tracing its development from foundational principles to the latest advances. We begin with the theoretical background, detailing the mechanisms by which inelastic tunneling processes generate vibrational fingerprints of molecules, and highlighting how IETS complements optical spectroscopies by accessing electrically driven vibrational excitations. We then discuss recent progress in experimental techniques and device architectures that have broadened the applicability of IETS. Central focus is given to emerging applications of IETS over the last decade: molecular sensing (identification of chemical bonds and conformational changes in junctions), thermoelectric energy conversion (probing vibrational contributions to molecular thermopower), molecular switches and functional devices (monitoring bias-driven molecular state changes via vibrational signatures), spintronic molecular junctions (detecting spin excitations and spin–vibration interplay), and advanced data analysis approaches such as machine learning for interpreting complex tunneling spectra. Finally, we discuss current challenges, including sensitivity at room temperature, spectral interpretation, and integration into practical devices. This review aims to serve as a thorough reference for researchers in physics, chemistry, and materials science, consolidating state-of-the-art understanding of IETS in molecular junctions and its growing role in molecular-scale device characterization. Full article

We present a brief overview of the field of charge density waves (CDW) in condensed systems with focus set to the underlying mechanisms behind the CDW ground state. Our intention in this short review is not to count all related facts from the vast volume of literature about this decades-old and still developing field, but rather to pinpoint the most important, mostly theoretical ones, presenting the development of the field. Starting from the “early days”, mainly based on weakly coupled, chain-like quasi-1D systems and Peierls instability, in which the Fermi surface nesting has been the predominant and practically paradigmatic mechanism of the CDW ground state stabilisation, we track the change in paradigms while entering the field of layered quasi-2D systems, with Fermi surface far away from the nesting regime, in which rather strong, essentially momentum-dependent interactions and particular reconstructions of the Fermi surface become essential. Examples of real quasi-1D materials, such as organic and inorganic conductors like Bechgaard salts or transition metal trichalcogenides and bronzes, in which experiment and theory have been extremely successful in providing detailed understanding, are contrasted to layered quasi-2D materials, such as ${\mathrm{high-T}}_c$ superconducting cuprates, intercalated graphite compounds or transition metal dichalcogenides, for which the theory explaining an onset of the CDWs constitutes a frontier of this fast-evolving field, strongly boosted by development of modern ab initio calculation methods. Full article

Research on two dimensional (2D) antiferromagnetic materials and heterobilayers is gaining prominence in spintronics. This study focuses on MPS₃ monolayers and their van der Waals heterobilayers with GaN monolayers. We systematically investigated the structural stability, electronic properties, and magnetic characteristics of MPS₃ (M = Mn, Fe, and Ni) monolayers via first-principles calculations, and explored their potential applications in optoelectronics and spintronics. Through phonon spectrum analysis, the dynamic stability of MPS₃ monolayers was confirmed, and their bond lengths, charge distributions, and wide-bandgap semiconductor properties were analyzed in detail. In addition, the potential applications of MPS₃ monolayers in UV detection were explored. Upon constructing the MPS₃/GaN heterobilayer structure, a significant reduction in the bandgap was observed, thereby expanding its potential applications in the visible light spectrum. The intrinsic antiferromagnetic nature of MPS₃ monolayers was confirmed through calculations, with the magnetic moments of the magnetic atoms M being 4.560, 3.672, and 1.517, respectively. Moreover, the heterobilayer structures further enhanced the magnetic moments of these elements. The magnetic properties of MPS₃ monolayers were further analyzed using spin-orbit coupling (SOC), confirming their magnetic anisotropy. These results provide a theoretical basis for the design of novel two-dimensional spintronic and optoelectronic devices based on MPS₃. Full article

On the basis of a microscopic model and employing Green’s function technique, the effects of temperature, size, and ion doping on the magnetization and phonon energy of the A_1g mode in double perovskites Ba₂FeReO₆ and Sr₂CrReO₆—both in bulk and nanoscale samples—are investigated for the first time. The Curie temperature T_C and magnetization M decrease as nanoparticle size is reduced. Doping with rare-earth ions such as Sm, Nd, or La at the Ba or Sr sites further reduces M. This behavior originates from the compressive strain induced by the smaller ionic radii of the dopant ions compared to the host ions. As a result, the antiferromagnetic superexchange interaction between Fe or Cr and Re ions is enhanced, along with an increase in the magnetic moment of the Re ion. The dependence of the band gap energy of Sr₂CrReO₆ on temperature, size, and doping is also studied. Near the magnetic-phase-transition temperature T_C, anomalies in phonon energy and damping indicate strong spin–phonon coupling. The theoretical calculations show good qualitative agreement with experimental data. Full article

A new compound [Y₂(sq)₃(H₂O)₄] (Y-sq; sq = squarate (C₄O₄^2–)) was prepared and structurally characterized. Since the RE-sq family (RE = Y, Dy, Yb, Lu) gave isostructural crystals, the objective of this study is to explore the effects of diamagnetic dilution on the SIM behavior through systematic investigation and comparison of diamagnetically diluted and undiluted forms. The 1%-Diluted Dy compounds, Dy@Y-sq and Dy@Lu-sq, showed AC magnetic susceptibility peaks without any DC bias field (H_DC), whereas undiluted Dy-sq showed no AC out-of-phase response under the same conditions. The Orbach and Raman mechanisms are assumed in the Arrhenius plots, giving U_eff/k_B = 139(5) and 135(8) K for Dy@Y-sq and Dy@Lu-sq, respectively, at H_DC = 0 Oe. In contrast, Yb@Y-sq and Yb@Lu-sq behaved different; Yb@Y-sq can be regarded as a field-induced SIM because AC out-of-phase response was recorded only when H_DC was present. On the other hand, Yb@Lu-sq showed a relaxation independent from temperature around 2 K at H_DC = 0 Oe, possibly ascribed to a quantum-tunneling-magnetization mechanism. Applying H_DC = 400 Oe afforded U_eff = 61.2(14) and 62.5(16) K for Yb@Y-sq and Yb@Lu-sq, respectively. The Y/Lu matrix dependence may be related to spin–phonon coupling. The dilution technique is a facile and versatile tool for modification of SIM characteristics. Full article

For more than a decade, the unusual distribution of electrons observed in ARPES (angle-resolved photoemission spectroscopy) data within the energy range of ~30 meV to ~300 meV below the Fermi level, known as the ARPES energy range, has remained a puzzle in the field of iron-based superconductivity. As the electron–phonon coupling of FeSe/SrTiO₃ is very strong, our investigation is centered on exploring the synergistic interplay between spin-density waves (SDW) and charge-density waves (CDW) with differential phonons at the interface between antiferromagnetic maxima and minima under wave interference. Our analysis reveals that the synergistic energy is proportional to the ARPES energy range, as seen in the comparison between FeSe and FeSe/SrTiO₃. This finding may suggest that the instantaneous interplay between these intricate phenomena may play a role in triggering the observed energy range in ARPES. Full article

With the help of a microscopic model and Green’s function technique, we studied the multiferroic and phonon properties of the recently reported new multiferroic Pr₂FeAlO₆ (PFAO) compound, which belongs to the double perovskite A₂BB’O₆ family. The magnetization decreases with the increase in temperature and disappears at the ferromagnetic Curie temperature $T_C^{FM}$. The polarization increases with the application of an external magnetic field, indicating strong magnetoelectric coupling and confirming the multiferroic behavior of PFAO. In the curves of dependence of the phonon energy and their damping with respect to temperature, a kink is observed at $T_C^{FM}$. This is due to the strong anharmonic spin–phonon interactions, which play a crucial role below $T_C^{FM}$ and are frequently observed in other double perovskite compounds. Above $T_C^{FM}$, only anharmonic phonon–phonon coupling remains. The phonon mode is controlled by an external magnetic field. Full article

Phonon blockade is an important quantum effect for revealing the quantum behaviors of mechanical systems. For a nitrogen-vacancy center spin strongly coupled to a mechanical resonator via the second-order magnetic gradient, we show that the qubit driving can lead to the implementation of the two-phonon blockade, while the usual mechanical driving only allows for the appearance of a single-phonon blockade. As a signature, we investigate three-phonon antibunching with a simultaneous two-phonon bunching process by numerically calculating the second-order and third-order correlation functions. We also analyze in detail the influence of the system parameters (including the qubit driving strength, the dephasing rate of the qubit, as well as the thermal phonon number) on the quality of the two-phonon blockade effect. Our work provides an alternative method for extending the concept of a phonon blockade from a single phonon to multiphonon. It is of direct relevance for the engineering of multiphonon quantum coherent devices and thus has potential applications in quantum information processing. Full article

The present study focuses on the spin-dependent vibrational properties of HKUST-1, a metal–organic framework with potential applications in gas storage and separation. Employing density functional theory (DFT), we explore the consequences of spin couplings in the copper paddle wheels (as the secondary building units of HKUST-1) on the material’s vibrational properties. By systematically screening the impact of the spin state on the phonon bands and densities of states in the various frequency regions, we identify asymmetric -COO- stretching vibrations as being most affected by different types of magnetic couplings. Notably, we also show that the DFT-derived insights can be quantitatively reproduced employing suitably parametrized, state-of-the-art machine-learned classical potentials with root-mean-square deviations from the DFT results between 3 cm⁻¹ and 7 cm⁻¹. This demonstrates the potential of machine-learned classical force fields for predicting the spin-dependent properties of complex materials, even when explicitly considering spins only for the generation of the reference data used in the force-field parametrization process. Full article

Transition metal dichalcogenides (TMDs) have unique absorption and emission properties that stem from their large excitonic binding energies, reduced-dielectric screening, and strong spin–orbit coupling. However, the role of substrates, phonons, and material defects in the excitonic scattering processes remains elusive. In tungsten-based TMDs, it is known that the excitons formed from electrons in the lower-energy conduction bands are dark in nature, whereas low-energy emissions in the photoluminescence spectrum have been linked to the brightening of these transitions, either via defect scattering or via phonon scattering with first-order phonon replicas. Through temperature and incident-power-dependent studies of WS₂ grown by CVD or exfoliated from high-purity bulk crystal on different substrates, we demonstrate that the strong exciton–phonon coupling yields brightening of dark transitions up to sixth-order phonon replicas. We discuss the critical role of defects in the brightening pathways of dark excitons and their phonon replicas, and we elucidate that these emissions are intrinsic to the material and independent of substrate, encapsulation, growth method, and transfer approach. Full article

A review is provided here about the thermal effects on optical chirality. To this goal, chiral objects dispersed in an embedding fluid are examined for their magnetoelectric coupling. Thermal effects on several chiral meta-atoms and their ensembles are examined. To this goal, DNA-like helical structures are examined in detail. The mechanical aspect of thermo-elasticity is reviewed along with transverse deformations while drawing analogies from condensed-matter physics. In this respect, the chirality-induced spin selection is reviewed along with the temperature-mediated electron–phonon interactions. A wide range of materials, such as polymers and biological cells, are also examined for temperature effects. A transition temperature delineating a sign flip in the chirality parameter is identified as well. Chirality-associated functionalities such as ratchet motions, switching, and modulations are investigated for their respective thermal effects. Issues of fabricating chiral meta-atoms are also discussed. Full article

We report on the far-infrared, temperature-dependent optical properties of a CrI${}_3$ transition metal halide single crystal, a van der Waals ferromagnet (FM) with a Curie temperature of 61 K. In addition to the expected phonon modes determined by the crystalline symmetry, the optical reflectance and transmittance spectra of CrI${}_3$ single crystals show many other excitations as a function of temperature as a consequence of the combination of a strong lattice anharmonicity and spin–phonon coupling. This complex vibrational spectrum highlights the presence of entangled interactions among the different degrees of freedom in CrI${}_3$. Full article

The study of the human psyche has elucidated a bipartite structure of logic reflecting the quantum–classical nature of the world. Accordingly, we posited an approach toward studying the brain by means of the quantum–classical dynamics of a mixed Weyl symbol. The mixed Weyl symbol can be used to describe brain processes at the microscopic level and, when averaged over an appropriate ensemble, can provide a link to the results of measurements made at the meso and macro scale. Within this approach, quantum variables (such as, for example, nuclear and electron spins, dipole momenta of particles or molecules, tunneling degrees of freedom, and so on) can be represented by spinors, whereas the electromagnetic fields and phonon modes can be treated either classically or semi-classically in phase space by also considering quantum zero-point fluctuations. Quantum zero-point effects can be incorporated into numerical simulations by controlling the temperature of each field mode via coupling to a dedicated Nosé–Hoover chain thermostat. The temperature of each thermostat was chosen in order to reproduce quantum statistics in the canonical ensemble. In this first paper, we introduce a general quantum–classical Hamiltonian model that can be tailored to study physical processes at the interface between the quantum and the classical world in the brain. While the approach is discussed in detail, numerical calculations are not reported in the present paper, but they are planned for future work. Our theory of brain dynamics subsumes some compatible aspects of three well-known quantum approaches to brain dynamics, namely the electromagnetic field theory approach, the orchestrated objective reduction theory, and the dissipative quantum model of the brain. All three models are reviewed. Full article

Multilayered van der Waals heterostructures based on transition metal dichalcogenides are suitable platforms on which to study interlayer (dipolar) excitons, in which electrons and holes are localized in different layers. Interestingly, these excitonic complexes exhibit pronounced valley Zeeman signatures, but how their spin-valley physics can be further altered due to external parameters—such as electric field and interlayer separation—remains largely unexplored. Here, we perform a systematic analysis of the spin-valley physics in MoSe${}_2$/WSe${}_2$ heterobilayers under the influence of an external electric field and changes of the interlayer separation. In particular, we analyze the spin ($S_z$) and orbital ($L_z$) degrees of freedom, and the symmetry properties of the relevant band edges (at K, Q, and $\mathsf{\Gamma }$ points) of high-symmetry stackings at 0° (R-type) and 60° (H-type) angles—the important building blocks present in moiré or atomically reconstructed structures. We reveal distinct hybridization signatures on the spin and the orbital degrees of freedom of low-energy bands, due to the wave function mixing between the layers, which are stacking-dependent, and can be further modified by electric field and interlayer distance variation. We find that H-type stackings favor large changes in the g-factors as a function of the electric field, e.g., from $-5$ to 3 in the valence bands of the H${}_{\mathrm{h}}^{\mathrm{h}}$ stacking, because of the opposite orientation of $S_z$ and $L_z$ of the individual monolayers. For the low-energy dipolar excitons (direct and indirect in k-space), we quantify the electric dipole moments and polarizabilities, reflecting the layer delocalization of the constituent bands. Furthermore, our results show that direct dipolar excitons carry a robust valley Zeeman effect nearly independent of the electric field, but tunable by the interlayer distance, which can be rendered experimentally accessible via applied external pressure. For the momentum-indirect dipolar excitons, our symmetry analysis indicates that phonon-mediated optical processes can easily take place. In particular, for the indirect excitons with conduction bands at the Q point for H-type stackings, we find marked variations of the valley Zeeman (∼4) as a function of the electric field, which notably stands out from the other dipolar exciton species. Our analysis suggests that stronger signatures of the coupled spin-valley physics are favored in H-type stackings, which can be experimentally investigated in samples with twist angle close to 60°. In summary, our study provides fundamental microscopic insights into the spin-valley physics of van der Waals heterostructures, which are relevant to understanding the valley Zeeman splitting of dipolar excitonic complexes, and also intralayer excitons. Full article

Layered magnetic metal-organic frameworks (MOFs) are an emerging class of materials that can combine the advantages of both MOFs and 2D magnetic crystals. The recent discovery of large coercivity and long-range magnetic ordering up to 515 K in a layered MOF of general formula MCl₂(pyz)₂ (M = transition metal, pyz = pyrazine) offers an exciting versatile platform to achieve high-T_C magnetism at the 2D limit. In this work, we investigate the exfoliation feasibility down to the monolayer of VCl₂(pyz)₂ and CrCl₂(pyz)₂ by means of first-principles calculations. We explore their structural, electronic, magnetic and vibrational properties, as well as the effect of halide substitution. Then, we provide a full analysis of the spin-phonon coupling (SPC) in both 2D derivatives. Our calculations reveal a low SPC and thermal evolution of the magnetic exchange interactions and single-ion anisotropy mainly governed by low-frequency phonon modes. Finally, we provide chemical insights to improve the performance of these magnetic 2D MOFs based on the effective manipulation of the phonon modes that can present a major impact on their magnetic properties. Full article