By performing density functional theory-based first-principles calculations, the formation of a thin monolayer structure, namely InSeF, via fluorination of monolayer InSe is predicted. It is shown that strong interaction of F and In atoms leads to the detachment of In-Se layers in monolayer InSe and 1T-like monolayer InSeF structure is formed. Monolayer InSeF is found to be dynamically stable in terms of its phonon band dispersions. In addition, its Raman spectrum is shown to exhibit totally distinctive features as compared to monolayer InSe. The electronic band dispersions reveal that monolayer InSeF is a direct gap semiconductor whose valence and conduction band edges reside at the $\mathrm{\Gamma }$ point. Moreover, the orientation-dependent linear elastic properties of monolayer InSeF are investigated in terms of the in-plane stiffness and Poisson ratio. It is found that monolayer InSeF displays strong in-plane anisotropy in elastic constants and it is slightly softer material as compared to monolayer InSe. Overall, it is proposed that a thin, direct gap semiconducting monolayer InSeF can be formed by full fluorination of monolayer InSe as a new member of the two-dimensional family.

Theoretical strength plays a pivotal role in gauging the maximum stress and inferring structural defects in crystalline materials. However, its very existence and the expected prediction of possible defects in amorphous solids remain elusive. Here, by using a finite deformation theory, we obtain the theoretical strengths for several bulk metallic glasses under pure shear loading. The theoretical strengths obtained are only several times larger than the experimental yield stresses; in contrast, they are three or four orders of magnitude higher in crystalline materials. This striking closeness between the theoretical and experimental strengths suggests the absence of any extended structural defects that can substantially reduce the intrinsic strength in the amorphous metals. Instead, the atoms have to sever each individual bond to deform. Further investigation reveals that the deformation occurring at the theoretical stress proceeds with the mechanical instability with a vanishing shear modulus, or a mechanical spinodal. We deduce from these results that, different fundamentally from crystalline solids, there are no extended structural defects in the amorphous solids and the plastic deformation must be local and sensitive to sample conditions.

The ability to tune the iron chalcogenides ${\mathrm{BaFe}}_2{Q}_3$ from Mott insulators to metals and then superconductors with applied pressure has renewed interest in low-dimensional iron chalcogenides and oxychalcogenides. We report here a combined experimental and theoretical study on the iron oxychalcogenides ${\mathrm{BaFe}}_2{Q}_2\mathrm{O}(Q=\mathrm{S},\text{Se})$ and show that their magnetic behavior results from nearest-neighbor magnetic exchange interactions via oxide and selenide anions of similar strength, with properties consistent with more localized electronic structures than those of ${\mathrm{BaFe}}_2{Q}_3$ systems.

A metamaterial approach is capable of drastically increasing the critical temperature, $T_{\mathrm{c}}$, of composite metal-dielectric superconductors as demonstrated by the tripling of $T_c$ that was observed in bulk Al-${\mathrm{Al}}_2{\mathrm{O}}_3$ core-shell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report an observation of this excitation in Al-${\mathrm{Al}}_2{\mathrm{O}}_3$ core-shell metamaterials using inelastic neutron scattering. This result provides support for this mechanism of superconductivity in metamaterials.

We observe an unconventional superconducting minigap induced into a ferromagnet ${\mathrm{SrRuO}}_3$ from a spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ using an $\mathrm{Au}/{\mathrm{SrTiO}}_3/{\mathrm{SrRuO}}_3/{\mathrm{Sr}}_2{\mathrm{RuO}}_4$ tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density of states with an opening of a superconducting minigap in ${\mathrm{SrRuO}}_3$. Observation of a minigap at the surface of a 15-nm-thick ${\mathrm{SrRuO}}_3$ layer confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance suggest that the even-frequency $p$-wave correlations dominate over odd-frequency $s$-wave correlations. Theoretical calculations support this $p$-wave scenario. Our work provides the density-of-states proof for an induced minigap in a ferromagnet, and it significantly advanced our understanding of the spin-triplet proximity effect between unconventional superconductors and ferromagnets.

Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111, and 122 compounds: Energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here, we revisit the electronic structure of the undoped parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (122 family). Our systematic ARPES study, including the detailed temperature and photon energy dependencies, points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity.

The possibility to exploit quantum coherence to strongly enhance the efficiency of charge transport in solid state devices working at ambient conditions would pave the way to disruptive technological applications. In this work, we tackle the problem of the quantum transport of photogenerated electronic excitations subject to dephasing and on-site Coulomb interactions. We show that the transport to a continuum of states representing metallic collectors can be optimized by exploiting the “superradiance” phenomena. We demonstrate that this is a coherent effect which is robust against dephasing and electron-electron interactions in a parameters range that is compatible with actual implementation in few-monolayer transition-metal-oxide (TMO) heterostructures.

The in-plane coefficient of thermal expansion (CTE) for CdSe nanoplatelets with the zinc-blende structure containing from two to five monolayers is calculated from first principles within the quasiharmonic approximation. A comparison of the obtained results with those for bulk CdSe with both the zinc-blende and wurtzite structures finds a significant increase in the magnitude of negative CTE and the temperature range of its observation in nanoplatelets. The main contribution to the negative thermal expansion in CdSe nanoplatelets is given by the out-of-plane flexural ZA mode and in-plane optical $E$ modes that arise from the folding of TA phonon of bulk CdSe.

We obtain analytical solutions for zero-energy Majorana edge modes in the three-dimensional Kitaev model on the hyperhoneycomb lattice. We consider three types of edges (zigzag, bearded, and armchair) and find the regions in parameter space where each type of edge mode exists. In the gapless phase, we obtain edge-state solutions which are the drumhead surface states associated with nodal-line semimetals. We also find a correspondence between noninteracting complex fermions and Majorana fermions on bipartite lattices, which explains the equivalence of energy spectrum and eigenmodes of Kitaev model and a corresponding tight-binding model for complex fermions on the same lattice.

Inorganic clathrates such as ${\mathrm{Ba}}_8{\mathrm{Ga}}_x{\mathrm{Ge}}_{46-x}$ and ${\mathrm{Ba}}_8{\mathrm{Al}}_x{\mathrm{Si}}_{46-x}$ commonly exhibit very low thermal conductivities. A quantitative computational description of this important property has proven difficult, in part due to the large unit cell, the role of disorder, and the fact that both electronic carriers and phonons contribute to transport. Here, we conduct a systematic analysis of the temperature and composition dependence of low-frequency modes associated with guest species in ${\mathrm{Ba}}_8{\mathrm{Ga}}_x{\mathrm{Ge}}_{46-x}$ and ${\mathrm{Ba}}_8{\mathrm{Al}}_x{\mathrm{Si}}_{46-x}$ (“rattler modes”), as well as thermal transport in stoichiometric ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. To this end, we account for phonon-phonon interactions by means of temperature-dependent effective interatomic force constants, which we find to be crucial in order to achieve an accurate description of the lattice part of the thermal conductivity. While the analysis of the thermal conductivity is often largely focused on the rattler modes, here it is shown that at room temperatures modes with $\hslash \omega \gtrsim 10\mathrm{meV}$ account for 50% of lattice heat transport. Finally, the electronic contribution to the thermal conductivity is computed, which shows the Wiedemann-Franz law to be only approximately fulfilled. As a result, it is crucial to employ the correct prefactor when separating electronic and lattice contributions for experimental data.

Excitons in alloyed nanowire quantum dots exhibit unique spectra, as shown here using atomistic calculations. The bright exciton splitting is triggered solely by alloying, despite the cylindrical quantum dot shape reaching over $15\mu \mathrm{eV}$. These results are contrary to previous theoretical predictions; however, they are in line with experimental data. This splitting can nonetheless be tuned by an electric field to go below the $1\mu \mathrm{eV}$ threshold. The dark exciton optical activity is also strongly affected by alloying that reaches a notable $1/3500$ fraction of the bright exciton and has a large out-of-plane polarized component.

We study a three-terminal setup consisting of a single-level quantum dot capacitively coupled to a quantum point contact. The point contact connects to a source and drain reservoir while the quantum dot is coupled to a single base reservoir. This setup has been used to implement a noninvasive, nanoscale thermometer for the bath reservoir by detecting the current in the quantum point contact. Here, we demonstrate that the device can also be operated as a thermal transistor where the average (charge and heat) current through the quantum point contact is controlled via the temperature of the base reservoir. We characterize the performances of this device both as a transistor and a thermometer and derive the operating condition maximizing their respective sensitivities. The present analysis is useful for the control of charge and heat flow and high precision thermometry at the nanoscale.

The Si(553)-Au system resembles a heteroatomic chain ensemble with a delicate spin-charge interplay. The ordering of the $\times 3$ reconstruction vanishes via a phase transition taking place at $T_c=100$ K. Our directional-dependent surface transport measurements showed that this order-disorder phase transition is not driven by the formation of a charge-density wave, as previously suggested. Instead, at 65 K there is a pronounced increase of the surface-state conductivity along the wires. We attribute this to activated charge transfer between the localized Si dangling bond states and the proximate Au bands revealing a $\times 2$ periodicity. Apparently, a quasiorthogonality between the wave functions of the two proximal reconstructions is also responsible for a missing $\times 6$ periodicity along the wires. The electronic charge transfer is in agreement with recent band-structure calculations.

The Lieb lattice possesses three bands and, with intrinsic spin-orbit coupling $\lambda$, supports topologically nontrivial band insulating phases. At half filling the lower band is fully filled, while the upper band is empty. The chemical potential lies in the flat band (FB) located at the middle of the spectrum, thereby stabilizing a flat band insulator. At this filling, we introduce on-site Hubbard interaction $U$ on all sites. Within a slave rotor mean-field theory we show that, in spite of the singular effect of interaction on the FB, the three bands remain stable up to a fairly large critical correlation strength ($U_{\text{crit}}$), creating a correlated flat band insulator. Beyond $U_{\text{crit}}$, there is a sudden transition to a Mott insulating state, where the FB is destroyed due to complete transfer of spectral weight from the FB to the upper and lower bands. We show that all the correlation driven insulating phases host edge modes with linearly dispersing bands along with a FB passing through the Dirac point, exhibiting that the topological nature of the bulk band structure remains intact in the presence of strong correlation. Furthermore, in the limiting case of $U$ introduced only on one sublattice where $\lambda =0$, we show that the Lieb lattice can support mixed edge modes containing contributions from both spinons and electrons, in contrast to purely spinon edge modes arising in the topological Mott insulator.

This paper presents an atomistic understanding of the nanoscale processes that govern crack nucleation and propagation in amorphous silica using a combination of theoretical calculations and molecular dynamics simulations with the ReaxFF potential. We show that both crack nucleation and propagation are governed by chainlike nanoscale virial stress-fibers formed by an intricate mixture of Si and O atoms. The stress-fibers are of a few nanometers in length, aligned parallel to the loading direction, and spatially localized at the evolving crack front at the nucleating site or a propagating crack tip. They form and break continuously during the crack nucleation and propagation process and are responsible for localizing stress at the crack nucleation site or propagating crack front. As soon as the stress-fibers reach a critical density the material starts nucleating cracks or leads to the propagation of initial crack. Additionally, the virial stress fields in the domain is highly heterogeneous and species-dependent—and the O and Si atoms play fundamentally distinct roles throughout the deformation process. With increased loading, heterogeneity in virial stress for the Si atoms goes up, whereas for the O atoms it goes down. Furthermore, from the virial and Hardy estimates of atomic stress, it is found that the stress field emanating from the crack tip decays as $1/r$. Also, presence of holes or pores within the interaction distance of a crack tip intensifies the stress state of the stress-fibers near the crack front leading to an improved effective toughness compared to the situation where the pore is far from the crack tip. Nucleation and propagation of cracks are strictly mediated by a complex admixture of localized bond rupture processes across a set of interacting stress-fibers. The details of the atomistic process regulating the underlying mechanisms are undetectable from the macroscopic stress-strain data.

The magnetocapacitance and magnetoresistance properties near room temperature of partially disordered double perovskite $\mathrm{L}{\mathrm{a}}_2\mathrm{NiMn}{\mathrm{O}}_6$ are related, at least in part, to coupled ferroelastic and magnetic instabilities that are responsible for a ferromagnetic phase transition near 280 K. A systematic analysis of this coupling from the perspectives of strain and elasticity has revealed a system with biquadratic coupling among three order parameters belonging to irreducible representations of $X_3^{+}, {\mathrm{\Gamma }}_{\text{4}}^{\text{+}}$ and $\text{m}{\mathrm{\Gamma }}_{\text{4}}^{\text{+}}$ of the parent space group $Fm\overline{3}m$. Classical octahedral tilting drives the structural transitions at high temperatures and strong acoustic attenuation through the temperature interval ∼300–500 K, observed by resonant ultrasound spectroscopy from a polycrystalline sample, is consistent with pinning of ferroelastic twin walls by point defects. Below room temperature, stiffening of the shear modulus by up to ∼40% can be understood in terms of biquadratic coupling of the ferromagnetic order parameter with strain. Acoustic attenuation with Debye-like patterns of loss in the temperature interval ∼150–280 K yielded activation energies and relaxation times which match up with AC magnetic and dielectric spectroscopy data reported previously in the literature. The dynamic loss mechanism, perhaps related to hopping of electrons between $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$, is potentially multiferroic, therefore. In addition to the possibilities for tailoring the intrinsic properties of $\mathrm{L}{\mathrm{a}}_2\mathrm{NiMn}{\mathrm{O}}_6$ by controlling oxygen content, $B$-site order or by choice of substrate for imposing a strain on thin films, it should be possible also to engineer extrinsic properties which would respond to applied electric, magnetic, and stress fields.

We analyze magnetic phases of monolayers of transition metal dichalcogenides that are two-valley materials with electron-electron interactions. The exchange intervalley scattering makes two-valley systems less stable to the spin fluctuations but more stable to the valley fluctuations. We predict a first-order ferromagnetic phase transition governed by the nonanalytic and negative cubic term in the free energy that results in a large spontaneous spin magnetization. Finite spin-orbit interaction leads to the out-of-plane Ising order of the ferromagnetic phase. Our theoretical prediction is consistent with the recent experiment on the electron-doped monolayers of ${\mathrm{MoS}}_2$ reported by Roch et al. [Nat. Nanotechnol. 14, 432 (2019)]. The proposed first-order phase transition can also be tested by measuring the linear magnetic field dependence of the spin susceptibility in the paramagnetic phase which is a direct consequence of the nonanalyticity of the free energy.

We study the structure of Abrikosov vortices in two-band superconductors for different external magnetic fields and different parameters of the bands, including the interband scattering rate. The vortex-core size determined by the coherence lengths is found to have qualitatively different behavior from that determined by the quasiparticle density of states spatial variation. These different vortex-core length scales coincide near the upper critical field, while the discrepancy between them becomes quite significant at lower fields. Within the diffusive approximation we demonstrate several generic regimes in the field dependence of the vortex-core sizes determined by the disparity of diffusion constants in the two bands, interband pairing, and interband scattering rate.

The forging of strong correlations on decreasing temperature can take place without the arousal of conventional order. If this happens, as in some geometrically frustrated magnets, disorder can be a phenomenon more interesting than order itself. A Coulomb phase, for example, has critical-like pair-spin correlations, leading to neutron scattering pinch points and emergent electromagnetism. Here we present a peculiar instance of disorder in an Ising pyrochlore lattice: the polarized monopole liquid (PML), a dense monopole fluid with pinch points in the magnetic charge-pair correlations. It is a phase of “monopole matter” which, in principle, can be stabilized in real materials using a magnetic field and uniaxial stress along the [100] direction. To explain how the monopole correlations arise, we show that the PML is a Coulomb phase in which spin fluctuations cannot be assigned either to monopoles or to internal magnetic moments, but necessarily comprehend both degrees of freedom. We develop a simple but nontrivial method to Helmholtz decompose the spin field into a divergenceless and a divergenceful part in magnetic charge disordered pyrochlores that shows the appearance of pinch points associated with the divergenceful component in places where Bragg peaks are observed for the “all-in/all-out” antiferromagnet.