Journal of Physics: Condensed Matter

The study of heat-to-work conversion has garnered significant attention in recent years, underscoring the potential of nanoscale systems to achieve energy conversion in steady-state devices without the involvement of macroscopic moving parts. The operation of these devices relies on the steady-state flows of quantum particles, including electrons, photons, and phonons. This review explores the theoretical frameworks that govern these steady-state flows within various mesoscopic or nanoscale devices, such as thermoelectric heat engines, with a particular focus on quantum dot (QD) Aharonov–Bohm (AB) interferometric configurations. Quantum interference effects, in particular, show great promise for enhancing the thermoelectric transport properties of these quantum devices. By enabling precise control over energy levels and transport pathways, such effects can significantly improve heat-to-work conversion efficiency. Driven QD AB networks provide an ideal platform for studying these engines due to their ability to maintain quantum coherence and offer precise experimental control. Unlike bulk systems, nanoscale systems such as QDs exhibit unique quantum interference phenomena, including sharp features in transmission spectra and Fano resonances. This review highlights the distinction between optimization methods that produce boxcar functions and coherent control methods that yield complex interference patterns. It demonstrates that the effective design of thermoelectric heat engines requires the careful tailoring of quantum interference and magnetic field-induced effects to enhance performance. Additionally, it addresses fundamental questions regarding the bounds of these thermoelectric machines, with particular emphasis on how magnetic fields can alter the limits of power or efficiency and the interplay between quantum transport theories and the laws of thermodynamics. Thermoelectric devices with broken time-reversal symmetry provide valuable insights into directional dependencies and asymmetries in quantum transport. This review offers a comprehensive overview of past and present research on quantum thermoelectric heat engines utilizing the AB effect. Special attention is given to three-terminal AB heat engines, where broken time-reversal symmetry can induce a coherent diode effect. Furthermore, the review examines bounds on power and efficiency in systems with broken time-reversal symmetry. We conclude by presenting open questions, summarizing key findings, and offering insights into future directions in the field of quantum thermoelectric heat engines.

We report the mechanisms of atomic ordering in Fe–Pt bimetallic alloys using density functional theory (DFT) and machine-learning interatomic potential Monte Carlo (MLIP-MC) simulations. We clarified that the formation enthalpy of the ordered phase was significantly enhanced by spin polarization compared to that of the disordered phase. Analysis of the density of states indicated that coherence in local potentials in the ordered phase brings energy gain over the disordered phase, when spin is considered. MLIP-MC simulations were performed to investigate the phase transition of atomic ordering at finite temperatures. The model trained using the DFT dataset with spin polarization exhibited quantitatively good agreement with previous experiments and thermodynamic calculations across a wide range of Pt compositions. In contrast, the model without spin significantly underestimated the transition temperature. Through this study, we clarified that spin polarization is essential for accurately accounting for the ordered phase in Fe–Pt bimetallic alloys, even above the Curie temperature, possibly because of the remaining short-range spin order.

We compare the driven dynamics of skyrmions, antiskyrmions, and skyrmionium interacting with random disorder, circular defects, and asymmetric potentials. When interacting with a line defect at a constant drive, skyrmions and antiskyrmions show an acceleration effect for motion along the wall and a drop in velocity when they can cross the barrier. In contrast, skyrmionium travels at a reduced velocity when moving along a wall, and exhibits an increase in velocity once it can cross the barrier. For point defects, skyrmionium can be pinned for a finite fixed period of time, while for skyrmions and antiskyrmions, the Magnus force creates a deflection from the defect and an acceleration effect. For a given drive, skyrmionium moves twice as fast as skyrmions; however, skyrmionium is more susceptible to pinning effects than skyrmions and antiskyrmions. Additionally, there is a critical threshold where the skyrmionium transforms to a skyrmion that is associated with a drop in the velocity of the texture. We show that all three textures exhibit diode and ratchet effects when interacting with an asymmetric substrate, but skyrmions and antiskyrmions show a stronger ratcheting effect than skyrmionium due to the Magnus force.

In the framework of density functional theory, we present a methodology that is as ab initio as possible for calculating the elastic constants in pressure and temperature. In this context, elastic constants are derived via the strain-fluctuation formalism involving Born, kinetic and stress fluctuation terms. ab initio molecular dynamic trajectories in the isokinetic (NVT) ensemble are performed using the Abinit software to evaluate each term. Stress fluctuations are obtained directly from the trajectories. The Born term, on the other hand, is obtained by extracting several uncorrelated configurations from the trajectories and applying the energy–strain method. Bayesian inference is used to quantify the uncertainties associated with this procedure. As a result, the methodology enables elastic constants and their uncertainties to be evaluated for a wide range of materials. Admittedly, the whole approach has a high computational cost. In this paper, the method is then applied to solid lead in the fcc and hcp phases at various pressures and temperatures. The elastic constants obtained are linear as a function of temperature and pressure, and are qualitatively consistent with the experimental results available for the fcc phase. The major computational effort involved in obtaining a numerical ab initio reference database for lead can be used to test the accuracy of other approaches using surrogate models.

Bulk photovoltaic effect is a non-linear response in noncentrosymmetric materials that converts light into DC current. In this work, we investigate the optical linear and non-linear responses in a chalcopyrite semiconductor ZnGeP₂. The reference point for chemical potential (E_f) appears at the valence band maximum of high symmetry Γ point in Brillouin zone for ZnGeP₂. We report large bulk photovoltaic namely shift and circular photogalvanic current conductivities which are 4.46 µA V⁻² and −5.49 µA V⁻² respectively with the incident photo energy around ∼5 eV at the chemical potential of E_f = 0 eV which increase about 38% and 81% respectively at a chemical potential of E_f = 1.52 eV. The systematic evolution of the bulk Fermi surface along with the high symmetry points in three dimensional Brillouin zone reveals the enhancement of bulk photovoltaic with the chemical potential in ZnGeP₂. We further explore the distribution of bulk projected band and surface Fermi surface distribution in the energy landscape using tight binding Hamiltonian within semi infinite slab geometry. The augmentation of bulk photovoltaic with the chemical potential is due to the projected bulk bands along the high symmetry $\Gamma-Z$ direction in Brillouin zone. Our thorough and detailed study not only provide a deeper understanding about the role of Fermi surface contribution to the bulk photovoltaic responses with chemical potential, but also suggest ZnGeP₂ as an ideal candidate for optoelectronics and bulk photovoltaic.

The study of heat-to-work conversion has garnered significant attention in recent years, underscoring the potential of nanoscale systems to achieve energy conversion in steady-state devices without the involvement of macroscopic moving parts. The operation of these devices relies on the steady-state flows of quantum particles, including electrons, photons, and phonons. This review explores the theoretical frameworks that govern these steady-state flows within various mesoscopic or nanoscale devices, such as thermoelectric heat engines, with a particular focus on quantum dot (QD) Aharonov–Bohm (AB) interferometric configurations. Quantum interference effects, in particular, show great promise for enhancing the thermoelectric transport properties of these quantum devices. By enabling precise control over energy levels and transport pathways, such effects can significantly improve heat-to-work conversion efficiency. Driven QD AB networks provide an ideal platform for studying these engines due to their ability to maintain quantum coherence and offer precise experimental control. Unlike bulk systems, nanoscale systems such as QDs exhibit unique quantum interference phenomena, including sharp features in transmission spectra and Fano resonances. This review highlights the distinction between optimization methods that produce boxcar functions and coherent control methods that yield complex interference patterns. It demonstrates that the effective design of thermoelectric heat engines requires the careful tailoring of quantum interference and magnetic field-induced effects to enhance performance. Additionally, it addresses fundamental questions regarding the bounds of these thermoelectric machines, with particular emphasis on how magnetic fields can alter the limits of power or efficiency and the interplay between quantum transport theories and the laws of thermodynamics. Thermoelectric devices with broken time-reversal symmetry provide valuable insights into directional dependencies and asymmetries in quantum transport. This review offers a comprehensive overview of past and present research on quantum thermoelectric heat engines utilizing the AB effect. Special attention is given to three-terminal AB heat engines, where broken time-reversal symmetry can induce a coherent diode effect. Furthermore, the review examines bounds on power and efficiency in systems with broken time-reversal symmetry. We conclude by presenting open questions, summarizing key findings, and offering insights into future directions in the field of quantum thermoelectric heat engines.

The seminal Haldane model brings up a paradigm beyond the quantum Hall effect to look for a plethora of topological phases in the honeycomb and other lattices. Here we dwell into this model considering a full parameter space in the presence of spin–orbit interaction as well as Zeeman field such that the flavour of Kane-Mele model is invoked. Adopting this extended Haldane model as an example, we elucidate, in a transparent manner, a number of topological features in a pedagogical manner. First, we describe various first order topological insulator phases and their characterizations while explaining various anomalous quantum Hall effects and quantum spin Hall effects in the extended Haldane model. Second, we demonstrate the concepts of higher order topological insulator phases along with the topological invariants in the anisotropic limit of the extended Haldane model. At the end, we discuss various open issues involving emergent or extended symmetries that might lead to a broader understanding of various topological phases and the associated criteria behind their emergence.

Since the discovery of high-temperature superconductivity in cuprates, understanding the unconventional pairing mechanism has remained one of the most significant challenges. The upper critical field ($H_{\mathrm{c2}}$) is an essential parameter for obtaining information on the pair-breaking mechanism, coherence length ξ, and pairing symmetry, all of which are crucial for understanding unconventional superconducting mechanisms. Here, we provide a brief review of studies on $H_{\mathrm{c2}}$ in several representative series of cuprate, iron-based, and nickelate superconductors. By comparing the behavior of $H_{\mathrm{c2}}$ as a function of temperature, doping concentration, and anisotropy across these three major classes of superconductors, we hope to contribute to a better understanding of the complex pairing interactions in high-temperature superconductors.

Active agents, which convert energy into directed motion, are inherently non-equilibrium systems. Inspired by living organisms and polymer physics, connected active agents with various topologies have recently garnered significant attention. These agents have positional degrees of freedom with well-defined topologies, while activity introduces extra degrees of freedom. The intricate interplay of activity, elasticity, noise, and conformational degrees of freedom gives rise to novel non-equilibrium behaviors in chain-like structures. This review categorizes active agents into three types based on their alignment mechanisms: Active Brownian agents, Vicsek-type agents, and self-aligning agents. It further provides the results when these agents are connected through different topological structures in two-dimensional spaces, at interfaces, in three-dimensional environments, and under confinement. The goal is to shed light on the fundamental physics that govern their non-equilibrium behavior at the level of individual chains and to highlight potential research directions. These findings hold significant potential for advancing the design of metamaterials and swarm robotics.

The transition from planar to three-dimensional (3D) magnetic nanostructures represents a significant advancement in both fundamental research and practical applications, offering vast potential for next-generation technologies like ultrahigh-density storage, memory, logic, and neuromorphic computing. Despite being a relatively new field, the emergence of 3D nanomagnetism presents numerous opportunities for innovation, prompting the creation of a comprehensive roadmap by leading international researchers. This roadmap aims to facilitate collaboration and interdisciplinary dialogue to address challenges in materials science, physics, engineering, and computing. The roadmap comprises eighteen sections, roughly divided into three blocks. The first block explores the fundamentals of 3D nanomagnetism, focusing on recent trends in fabrication techniques and imaging methods crucial for understanding complex spin textures, curved surfaces, and small-scale interactions. Techniques such as two-photon lithography and focused electron beam-induced deposition enable the creation of intricate 3D architectures, while advanced imaging methods like electron holography and synchrotron x-ray tomography provide nanoscale spatial resolution for studying magnetization dynamics in three dimensions. Various 3D magnetic systems, including coupled multilayer systems, artificial spin-ice, magneto-plasmonic systems, topological spin textures, and molecular magnets are discussed. The second block introduces analytical and numerical methods for investigating 3D nanomagnetic structures and curvilinear systems, highlighting geometrically curved architectures, interconnected nanowire systems, and other complex geometries. Finite element methods are emphasized for capturing complex geometries, along with direct frequency domain solutions for addressing magnonic problems. The final block focuses on 3D magnonic crystals and networks, exploring their fundamental properties and potential applications in magnonic circuits, memory, and spintronics. Computational approaches using 3D nanomagnetic systems and complex topological textures in 3D spintronics are highlighted for their potential to enable faster and more energy-efficient computing.

Accurately modelling nonlinear optical experiments such as second-harmonic scattering and hyper-Raman scattering requires the hyperpolarizability $\boldsymbol{\beta}$, a nonlinear dielectric response to an applied electric field. The hyperpolarizability tensor is a computationally expensive quantity to calculate, making it a natural target for machine-learning methods. We test a family of recently developed models for the hyperpolarizability of water, trained on small clusters containing up to 8 water molecules. These models are able to predict $\boldsymbol{\beta}$ for larger clusters, with more complex structures than those observed in the training set. For configurations of bulk water, the agreement is not so straightforward: while the total hyperpolarizability is quite well described, the predicted \textit{molecular} $\boldsymbol{\beta}$ tensors vary wildly between models. This means that while experiments whose outputs depend on total hyperpolarizability can be accurately modelled, those that require molecular quantities will require improved models.

The depletion of fossil fuels and the environmental impact of chemical batteries, coupled with the rapid proliferation of portable electronic devices and the Internet of Things (IoT), have created an urgent demand for high-performance, lightweight, and sustainable energy systems. Flexible triboelectric nanogenerators (TENGs) have emerged as a promising technology for powering self-sufficient devices, offering advantages such as simple structure, flexibility, low cost, and environmental adaptability. In particular, electrospun nanofiber-based TENGs stand out due to their enhanced surface area, superior charge collection capabilities, and improved mechanical durability. This review presents a comprehensive overview of recent advancements in electrospun nanofiber-based TENGs, focusing on material selection, structural design, fabrication techniques, and their integration into applications ranging from self-powered sensors to wearable electronics. Furthermore, the review discusses the challenges and future directions in optimizing the performance and scalability of TENGs to meet the growing demands of next-generation, energy-efficient technologies. It is hoped that this review will help researchers to gain a deeper understanding of this field and promote its development to a new stage.

Recent theoretical and experimental studies on the frustration-induced skyrmion crystal (SkX) in centrosymmetric magnets are reviewed, with some emphasis on their symmetry and topological aspects. Special importance of frustration and chirality is highlighted. Theories cover the studies based on both the spin models and the electronic models. In the former, the frustrated Heisenberg models on the triangular or the square lattices interacting either via the long-range RKKY interaction or via the competing short-range exchange interactions are treated, where frustration is borne by the oscillating nature of the long-range RKKY interaction or by the competition between the shorter-range exchange interactions. Special attention is paid to the role played by the magnetic anisotropy including the dipolar interaction. The electronic models discussed are mainly the Kondo lattice model on the triangular lattice, which reduces to the RKKY Heisenberg in the weak-coupling limit. Experiments on centrosymmetric SkX-hosting magnets cover the hexagonal magnets Gd$_2$PdSi$_3$ (triangular) and Gd$_3$Ru$_4$Al$_{12}$ (breathing kagome), and the tetragonal magnets GdRu$_2$Si$_2$ and EuAl$_4$. Various experimental data, including magnetization or susceptibility, specific heat, Hall resistivity, resonant magnetic $x$-ray scattering, neutron scattering, Lorentz transmission electron miscroscopy, etc are reviewed and discussed in conjunction with the theoretical results. The nature of a variety of phases surrounding the SkX phase in the phase diagram, many of which are of multiple-$q$ character, is also examined. Finally, some discussion is given about the physical origin of the centrosymmetric SkX formation, its unique features in comparison with the non-centrosymmetric SkX induced by the antisymmetric Dzaloshinskii-Moriya interaction, together with some open and challenging problems for the future.

Lead-free perovskite halide CsSnI_{3} has emerged as a promising material for optoelectronic applications due to its direct bandgap (1.3–1.4 eV), high charge carrier mobility, and strong visible-spectrum absorption. Among its polymorphs, the green phase, with a favorable bandgap of \sim1.24 eV, demonstrates enhanced structural stability and resistance to phase degradation under ambient conditions. In this study, we investigate the green polymorph of CsSnI_{3} and observe pyroelectric behavior, indicative of ferroelectric-like properties despite its globally centrosymmetric (Pa\overline{3}) cubic structure. Utilizing Piezo-force microscopy, dielectric measurements, impedance spectroscopy, and Raman spectroscopy, we identified local non-centrosymmetry influencing hysteresis and conduction properties. Impedance spectroscopy further reveals the interaction of grains and grain boundaries under a low AC electric field, both before and after light exposure and poling. A reduction in relaxation time with increasing temperature in poled samples is observed, while the combined effects of light exposure and poling result in an increased relaxation time. Our results indicate that local non-centrosymmetry plays a critical role in influencing hysteresis and conduction behavior. These findings highlight the importance of phase transitions and vibrational mode dynamics in optimizing the performance of CsSnI_{3}-based devices, paving the way for their broader application in advanced optoelectronic technologies. mechanism and I-V hysteresis. This study highlights that local alterations in vibration modes significantly impact the current-voltage hysteresis and conduction behavior of perovskite halides, suggesting the presence of local non-centrosymmetry within the globally centrosymmetric CsSnI_{3}.

Lipidic mesophases (LMPs) are lyotropic liquid crystals formed by the self-assembly of lipid in water, offering diverse phase symmetries with unique physicochemical properties. However, a fundamental understanding of how the dynamics relate to the composition and structure remains limited. In this study, we substitute water with glycerol, which closely resembles the headgroup structure of phytantriol, as the solvent to explore phytantriol-based LMPs in a pure glycerol environment. The non-crystallizing nature of both phytantriol and glycerol enables phase studies at sub-zero temperatures. Combined small-angle x-ray scattering and differential scanning calorimetry analyses confirm the formation of reverse micelles (L2), which undergo a phase transition to lamellar phase (Lα) upon cooling. Broadband dielectric spectroscopy (BDS) reveals how the dynamics of phytantriol are governed by the composition and symmetry of the LMP: Increased glycerol content decreases the relaxation time of the Debye- and α‍-‍relaxation, therefore exerting a plasticizing effect. The change in long-range order of phytantriol during the L2 – Lα phase transition reveals a decrease of the conductivity relaxation time. The introduction of a net orientation of phytantriol further reveals a new relaxation process—the dipole-matrix interaction—exclusive to the Lα phase. Our results highlight the value of combining BDS with structural and thermal analyses for a deeper understanding of the dynamics in soft matter systems.

Accurately modelling nonlinear optical experiments such as second-harmonic scattering and hyper-Raman scattering requires the hyperpolarizability $\boldsymbol{\beta}$, a nonlinear dielectric response to an applied electric field. The hyperpolarizability tensor is a computationally expensive quantity to calculate, making it a natural target for machine-learning methods. We test a family of recently developed models for the hyperpolarizability of water, trained on small clusters containing up to 8 water molecules. These models are able to predict $\boldsymbol{\beta}$ for larger clusters, with more complex structures than those observed in the training set. For configurations of bulk water, the agreement is not so straightforward: while the total hyperpolarizability is quite well described, the predicted \textit{molecular} $\boldsymbol{\beta}$ tensors vary wildly between models. This means that while experiments whose outputs depend on total hyperpolarizability can be accurately modelled, those that require molecular quantities will require improved models.

Lipidic mesophases (LMPs) are lyotropic liquid crystals formed by the self-assembly of lipid in water, offering diverse phase symmetries with unique physicochemical properties. However, a fundamental understanding of how the dynamics relate to the composition and structure remains limited. In this study, we substitute water with glycerol, which closely resembles the headgroup structure of phytantriol, as the solvent to explore phytantriol-based LMPs in a pure glycerol environment. The non-crystallizing nature of both phytantriol and glycerol enables phase studies at sub-zero temperatures. Combined small-angle x-ray scattering and differential scanning calorimetry analyses confirm the formation of reverse micelles (L2), which undergo a phase transition to lamellar phase (Lα) upon cooling. Broadband dielectric spectroscopy (BDS) reveals how the dynamics of phytantriol are governed by the composition and symmetry of the LMP: Increased glycerol content decreases the relaxation time of the Debye- and α‍-‍relaxation, therefore exerting a plasticizing effect. The change in long-range order of phytantriol during the L2 – Lα phase transition reveals a decrease of the conductivity relaxation time. The introduction of a net orientation of phytantriol further reveals a new relaxation process—the dipole-matrix interaction—exclusive to the Lα phase. Our results highlight the value of combining BDS with structural and thermal analyses for a deeper understanding of the dynamics in soft matter systems.

MAX phases are a family of atomically laminated materials with various potential applications. Mn2GaC is a prototype magnetic MAX phase, where complex magnetic behaviour arises due to competing interactions. We have resolved the room temperature magnetic structure of Mn2GaC by neutron diffraction from single-crystal thin films and we propose a magnetic model for the low temperature phase. It orders in a helical structure, with a rotation angle that changes gradually between 120° and 90° depending on temperature.

We report evidence of a finite density of states at the Fermi level at the surface of epitaxial thin films of the narrow bandgap Mott insulator Sr₃Ir₂O₇(001). The Brillouin zone critical points for Sr₃Ir₂O₇(001) thin films have been determined by a comparison of the band mapping from angle-resolved photoemission spectroscopy and low energy electron diffraction. Angle-resolved X-ray photoemission studies reveal the surface termination of Sr₃Ir₂O₇(001) is Sr-O. The absence of dispersion with photon energy, or changing wave vector along the surface normal, indicates the two-dimensional character of the bands contributing to the density of states close to the Fermi level for Sr₃Ir₂O₇(001) thin films, thus is attributed to surface states or surface resonances. This appearance of a finite density of states at the Fermi level is consistent with the increased conductivity with decreasing film thickness for ultrathin Sr₃Ir₂O₇(001) films.

We report on the observation of a flat band situated at the Fermi level $E_{\mathrm{F}}$ along with the structural, electrical transport, and magnetic properties of BaCo₂P₂ that crystallizes in the ThCr₂Si₂-type body-center tetragonal structure. This compound has the largest inter-layer pnictide (Pn) distance $d_{\mathrm{Pn-Pn}}$ as well as the largest $c/a$ ratio among all the known ACo$_2 Pn_2$ (A = alkaline earth metal) compounds, where a and c are the tetragonal lattice parameters. Hence, the magnetic and electronic properties of this compound are expected to have a quasi-two-dimensional character. Despite the evidence of the presence of sizable magnetic interactions, magnetic susceptibility $\chi(T)$ of BaCo₂P₂ does not show magnetic ordering down to 1.8 K. The material shows good metallic conduction with a large residual resistivity ratio $\rho_{\mathrm{300\,K}}/\rho_{\mathrm{1.8\,K}} \approx 70$ and a Fermi liquid behavior at low temperature. Kadowaki–Woods ratio $R_{\mathrm{KW}}$ of BaCo₂P₂ suggests the presence of sizable electronic correlations within this system. Additionally, a large many-body enhancement of 2.3 of the experimental density of states $D^{\gamma}(E_{\mathrm{F}})$ over the band-structure $D^{\mathrm{band}}(E_{\mathrm{F}})$ is inferred to arise from sizable electron-electron and/or electron-phonon interactions leading to a substantial deviation from the free-electron behavior.

The paper theoretically studies thin ferromagnetic films obtained by successive deposition of layers of easy-axis and easy-plane materials. It is shown that in films of this type, under certain conditions, effective anisotropy can arise, leading to the appearance of new directions of easy magnetization, set of which is divided into two independent orbits. The structure of domain walls arising as a result of the transition of the magnetization vector from one orbit to another was studied. It has been proven that in the presence of perforations in the films under consideration, topologically protected inhomogeneities can arise, which are localized in the vicinity of two closely spaced holes and can be in one of six nonequivalent states, as a result of which paired perforations can be considered as memory cells for recording data in a base 6 number system. A numerical experiment has demonstrated that two adjacent cells can actually independently encode two digits in the specified number system due to the possibility of forming thin domain walls between the cells.

We present a comprehensive photoemission study of two Vanadium-based quaternary Heusler alloys, CrFeVGa and CoFeVSb, which are highly promising candidates for spintronics and topological quantum applications. CrFeVGa exhibits large anomalous Hall conductivity due to the large Berry curvature originating from its non-trivial topological bands. In contrast, CoFeVSb displays a spin-valve-like behavior alongside excellent thermoelectric properties, such as ultra-low thermal conductivity and high power factor at room temperature. By utilizing synchrotron x-ray photoemission spectroscopy and resonant photoemission spectroscopy, we have investigated the core levels and valence band of both the alloys. Our analysis shows that the V 3d states are primarily responsible for the electronic states at the Fermi level which result in the high spin polarization, consistent with our theoretical predictions. The presence of the Fermi edge in the valence band spectra in both the systems confirms the predicted metallic or half/semi-metallic features. The observed spectra match qualitatively with our simulated partial density of states. A close inspection of the temperature dependent valence band spectra indicates that some of the intriguing bulk properties reported earlier on these two systems are intimately connected with their unique band structure topology. This in turn facilitate a deeper insight into the origin of such interesting properties of these alloys. Such direct measurements of electronic structure provide a guiding platform towards a better understanding of the anomalous properties of any material in general.

The squeezing of a Ge planar quantum dot enhances the Rabi frequency of electric dipole spin resonance by several orders of magnitude due to a strong Direct Rashba spin–orbit interaction in such geometries (Bosco et al 2021 Phys. Rev. B 104 115425). We investigate the geometric effect of an elliptical (squeezed) confinement and its interplay with the polarization of driving field in determining the Rabi frequency of a heavy-hole qubit in a planar Ge quantum dot. To calculate the Rabi frequency, we consider only the p-linear SOIs viz. electron-like Rashba, hole-like Rashba and hole-like Dresselhaus which are claimed to be the dominant ones by recent studies on planar Ge heterostructures. We derive approximate analytical expressions of the Rabi frequency using a Schrieffer–Wolff transformation for small SOI and driving strengths. Firstly, for an out-of-plane magnetic field with magnitude B, we get an operating region with respect to B, squeezing and polarization parameters where the qubit can be operated to obtain 'clean' Rabi flips. On and close to the boundaries of the region, the higher orbital levels strongly interfere with the two-level qubit subspace and destroy the Rabi oscillations, thereby putting a limitation on squeezing of the confinement. The Rabi frequency shows different behaviour for electron-like and hole-like Rashba SOIs. It vanishes for right (left) circular polarization in presence of purely electron-like (hole-like) Rashba SOI in a circular confinement. For both in- and out-of-plane magnetic fields, higher Rabi frequencies are achieved for squeezed configurations when the ellipses of polarization and the confinement equipotential have their major axes aligned but with different eccentricities. We also deduce a simple formula to calculate the effective heavy hole mass by measuring the Rabi frequencies using this setup.

Approaching the problem of understanding fundamental physical constants (FPCs) started with discussing the role these constants play in high-energy nuclear physics and astrophysics. Condensed matter physics was relatively unexplored in this regard. More recently, it was realised that FPCs set lower or upper bounds on key condensed matter properties. Here, we discuss a much wider role played by FPCs in condensed matter physics: at given environmental conditions, FPCs set the observability and operation of entire physical effects and phenomena. We discuss structural and superconducting phase transitions and transitions between different states of matter, with implications for life processes. We also discuss metastable states, transitions between them, chemical reactions and their products. A byproduct of this discussion is that the order of magnitude of the transition temperature can be calculated from FPCs only. We show that the new states emerging as a result of various transitions increase the phase space and entropy. Were FPCs to take different values, these transitions would become inoperative at our environmental conditions and the new states due to these transitions would not emerge. This suggests that the current values of FPCs, by enabling various transitions and reactions which give rise to new states, promote entropy increase. Based on this entropy increase and the associated increase of statistical probability, we conjecture that entropy increase is a selection principle for FPCs considered to be variable in earlier discussions.

Andreev reflection in DNA molecules terminated by a d-wave superconductor is investigated for demonstrating advantages in using DNA as the probe for the spectroscopy of the superconductor. DNA molecules are incorporated in the simulations using a two-leg ladder model with a simplification as constructed by homopolymers. The increase of the Andreev reflection probability at zero bias originating from the midgap surface states of d-wave superconductors appears even when the DNA molecule is coupled strongly. The zero-bias peak is enhanced by orders of magnitude when the coupling is weakened. The one-dimensional transport in DNA strands gives rise to the remarkable sensitivity in the spectroscopy, where the changes of the reflection probability caused by the midgap states are also in orders of magnitude when the voltage bias is varied and when the orientation of the d-wave symmetry is inclined with respect to the superconductor surface. The quantum interference of the transport between the two strands in DNA modifies the zero-bias increase. A narrow dip occurs at zero bias with a plateau sandwiched by two peaks in the immediate vicinity of the zero bias. The characteristics of these transmission resonances are dependent on the parameters describing the model molecules, and so the width of the resonance peaks, for instance, enables us to evaluate the strength of the inter-strand coupling.