The thermal stability of atomic-scale skyrmions is of high relevance for potential spintronics applications and validation of theoretical models. We investigated Pd/Fe nanoislands on an Ir(111) substrate as a function of temperature and magnetic field. Utilizing noncollinear magnetoresistance contrast in scanning tunneling microscopy, the thermomagnetic phase space is explored up to 3 T within a temperature range between 1 K to 100 K. Evidence is found for the spin spiral, field-polarized, and fluctuating disordered magnetic phases. Evidence for the presence of atomic-scale skyrmions at up to approximately 80 K is found, irrespective of considerable magnetization dynamics arising from thermal agitation.

Here we report the results of the high-pressure Hall coefficient ($R_H$) measurements, combined with the high-pressure resistance measurements, at different temperatures on the putative topological superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$. We find the intimate correlation of sign change of $R_H$, a fingerprint to manifest the reconstruction of Fermi surface, with structural phase transition and superconductivity. Below the critical pressure ($P_{\mathrm{c}}$) of 2.7 GPa, our data reveal that the hole-electron carriers are thermally balanced ($R_H=0$) at a critical temperature ($T^{*}$), where $R_H$ changes its sign from positive to negative, and concurrently a tetragonal-orthorhombic phase transition takes place. Within the pressure range from ambient pressure to $P_{\mathrm{c}}$, $T^{*}$ is continuously suppressed by pressure, while $T_c$ increases monotonically. At about $P_{\mathrm{c}}$, $T^{*}$ is undetectable and $T_c$ reaches a maximum value. Moreover, a pressure-induced sign change of $R_H$ is found at $\sim P_{\mathrm{c}}$ where the orthorhombic-monoclinic phase transition occurs. With further compression, $T_c$ decreases and disappears at ∼ 12 GPa. The correlation among the electron-hole balance, crystal structure, and superconductivity found in the pressurized ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ implies that its nontrivial superconductivity is closely associated with its exotic normal state resulting from the interplay between the reconstruction of the Fermi surface and the change of the structural lattice.

The ${\mathrm{BiS}}_2$-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the ${\mathrm{EuFBiS}}_2$ compound, showing a superconducting transition temperature $T_c\sim 0.3$ K at ambient pressure. Upon increasing external pressure, $T_c$ goes through a large amplification, accompanied by a structural phase transition (SPT) from tetragonal to monoclinic symmetry. Here, we use a combination of Eu $L_3$ - edge x-ray absorption spectroscopy and synchrotron x-ray diffraction to unveil the evolution of the Eu valence and lattice symmetry under high pressure. We find that the average Eu valence increases gradually with pressure, exhibiting a pressure plateau near the SPT, at which the $T_c$ increases sharply. Since in ${\mathrm{EuFBiS}}_2$ the charge carriers are introduced via self-doping induced by the mixed valence of the Eu ions, our findings clearly indicate that the role of the charge doping is marginal in the $T_c$ enhancement. On the other hand, the structural distortions, taking place at the SPT, play a central role in enhancing the superconducting properties of the ${\mathrm{EuFBiS}}_2$ system.

Spinel compounds have exhibited rich functionalities but have rarely shown superconductivity. Here, we report the emergence of superconductivity in the spinel ${\mathrm{MgTi}}_2{\mathrm{O}}_4$, known to be an insulator with a complicated order. The superconductivity is achieved by engineering a superlattice of ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ and ${\mathrm{SrTiO}}_3$. The onset transition temperature in the ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ layer can be tuned from 0 to 5 K in such a geometry, concurrently with a stretched out-of-plane lattice (from 8.51 to 8.53 Å) compared to the bulk material. Such a positive correlation suggests ample room for further enhancement. Intriguingly, the superlattice exhibits an isotropic upper critical field $B_{\mathrm{c}2}$ that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ layer is understood in combination with the electron energy loss spectra and first-principles electronic structure calculations, which point to the birth of superconductivity by suppressing orbital ordering. Our discovery not only provides a platform to explore the interplay between superconductivity and other exotic states, but also opens another window to realize superconductivity in spinel compounds as well as other titanium oxides.

The spin-$1/2$ Heisenberg chain with a ferromagnetic first-neighbor exchange coupling $J_1$ and an antiferromagnetic second-neighbor $J_2$ has a Haldane dimer ground state with an extremely small spin gap. Thus the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in $J_1$ on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-$1/2$ chain. It is found that there are two distinct dimerized spin-gap phases, in one of which the spin gap and the magnetic susceptibility are extremely small around the SU(2) symmetric case and in the other they are moderately large far away from the SU(2) symmetric case. A small alternation in the amplitude of $J_1$ rapidly shortens the pitch of spin correlations towards the four-spin periodicity, as in the limit of $J_1/J_2\to 0$. These effects are not sufficient to quantitatively explain overall experimentally observed magnetic properties in the quasi-one-dimensional spin-gapped magnetoelectric cuprate ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ that exhibits ferroelectricity stabilized by a magnetic field. Our results are also relevant to ${\mathrm{Cs}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$, where the ferromagnetic intrachain and antiferromagnetic interchain order has recently been found, in a single chain level. We also reveal the nature of symmetry-protected topological phase transitions in the model by mapping onto effective spin-1 chain models.

The magnetic and superconducting properties of a series of underdoped ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ (BNFA) single crystals with $0.19\le x\le 0.34$ have been investigated with the complementary muon-spin-rotation ($\mu \mathrm{SR}$) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-Q magnetic order. Besides the collinear state with a spatially inhomogeneous spin-charge-density wave (i-SCDW) order at $x=0.24$ and 0.26, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a “hedgehog”-type spin vortex crystal (SVC) structure at $x=0.32$ and 0.34. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10 K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at $x=0.34$ there is even a partial reentrance into a paramagnetic state at $T\ll T_c$.

Within the Landauer transport formalism we demonstrate that conductance zeros are possible in bipartite systems at half-filling when leads are contacted to different sublattice sites. In particular, we investigate the application of this theory to graphene quantum dots with leads in the armchair configuration. The obtained conductance cancellation is robust in the presence of any single-site impurity.

Shift current is the dominant dc-current response in the bulk photovoltaic effect (BPVE), which is the conversion of solar energy into electricity in the materials with broken inversion symmetry. While the guiding principle of BPVE is a lack of inversion symmetry in a material which also results in ferroelectricity, it is therefore, expected that a significantly large shift current is achieved in ferroelectric materials. In this work, we calculate shift current using first principles in two-dimensional $\alpha \text{−}\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ which has both in-plane and out-of-plane polarization at room temperature. To understand the implications of in-plane and out-of-plane polarization on shift current BPVE, mono- and bilayer structures of $3R$ and $2H$ $\alpha \text{−}\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ are considered in our calculations. It suggests that the in-plane polarization doesn't affect the shift current response of this material. In monolayer, a dominant shift current response of magnitude $750\mu \mathrm{A}/{\mathrm{V}}^2$ is obtained along the direction of out-of-plane polarization under uniform illumination of $zz$-polarized light at a photon energy of 4.16 eV. The doping engineering is further implemented to tune the shift current response to visible light. Bismuth (Bi) is used to substitute the indium element at the tetrahedral site thereby introducing more energy levels in the conduction band which importantly are contributed by $p$ orbitals of Bi and take part in the transition process. Consequently, a giant shift current of $1200\mu \mathrm{A}/{\mathrm{V}}^2$ is obtained at a photon energy of 2.98 eV. The present study would be an instrumental in understanding the shift current BPVE and would pave the path for designing efficient photovoltaic devices based on $\alpha \text{−}\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ and similar material systems.

We study the interfaces between lattice Laughlin states at different fillings. We propose a class of model wave functions for such systems constructed using conformal field theory. We find a nontrivial form of charge conservation at the interface, similar to the one encountered in the field theory works from the literature. Using Monte Carlo methods, we evaluate the correlation function and entanglement entropy at the border. Furthermore, we construct the wave function for quasihole excitations and evaluate their mutual statistics with respect to quasiholes originating on the same or the other side of the interface. We show that some of these excitations lose their anyonic statistics when crossing the interface, which can be interpreted as impermeability of the interface to these anyons. Contrary to most of the previous works on interfaces between topological orders, our approach is microscopic, allowing for a direct simulation of, e.g., an anyon crossing the interface. Even though we determine the properties of the wave function numerically, the closed-form expressions allow us to study systems too large to be simulated by exact diagonalization.

Shape memory effect associated with martensitic transformations is a rapidly developing field in nanotechnologies, where industrial use of systems established on that effect provide greater flexibility on the nanodevice fabrications of various kinds. And therefore it addresses questions to the phase transition phenomena at low scale and its limitations and control. In this report we studied the crystal structure of tapered plates of ${\mathrm{Ti}}_2\mathrm{NiCu}$ alloy and the temperature $T_c$ at which the martensitic transition occurs. We demonstrated that $T_c$ has a strongly descending character as a function of the plate thickness $h$. The critical thickness value at which the transition is completely suppressed is 20 nm. Moreover, the obtained results for $T_c\left(h\right)$ curves indicate the hysteretic nature of the transition. These findings open the pathway for size limit indications and regime modulations where the alloy-based nanomechanical devices can be tuned to operate more efficiently.

Collective excitations in liquids are important for understanding liquid dynamical and thermodynamic properties. Gapped momentum states (GMS) are a notable feature of liquid dynamics predicted to operate in the transverse collective excitations. Here, we combine inelastic neutron scattering experiments, theory, and molecular dynamics modeling to study collective excitations and GMS in liquid Ga in a wide range of temperature and $k$ points. We find that all three lines of enquiry agree for the longitudinal liquid collective dynamics. In the transverse collective dynamics, the experiments agree with theory, modeling, as well as earlier x-ray experiments at larger $k$, whereas theory and modeling agree in a wide range of temperature and $k$ points. We observe the emergence and development of the $k$ gap in the transverse dispersion relation which increases with temperature and inverse of relaxation time as predicted theoretically.

Modulation of probe signals in pump-probe measurements of coherent phonons in dielectrics, with and without spectral resolution, are investigated theoretically taking diamond as an example. Analytical investigation as well as first-principles calculations based on time-dependent density-functional theory is utilized to clarify the mechanism of the modulation of the probe signals. Boundary and bulk effects are investigated systematically, putting emphasis on the phase relation between the modulation and the atomic motion of the coherent phonon. They are summarized as follows: Modulation by the boundary effect is in phase with the coherent phonon amplitude, while that by the bulk effect shows $\pi /2$ phase difference. Strong frequency dependence appears in the modulation by the bulk effect, while no frequency dependence by the boundary effect. First-principles calculations support the reliability of the analytical result.

The 134-type $A$-site ordered ${\mathrm{LaMn}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$, that belongs to the family of $A{A}_3^{\prime }{B}_4{\mathrm{O}}_{12}$ perovskites, with cubic structure and magnetically driven improper polarization is puzzling because the conventional mechanisms leading to the creation of polarization in type-II multiferroics do not apply. By symmetry analysis and first-principles simulations, we discover an original and previously overlooked energetic term that naturally explains the origin of improper polarization in ${\mathrm{LaMn}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$. In addition, we demonstrate the existence of three other energetic invariants in $A{A}_3^{\prime }{B}_4{\mathrm{O}}_{12}$ perovskites, for which other physical quantities (such as oxygen tiltings or shear strain) mediate the interaction between $G$-type antiferromagnetic vectors to create and even enhance improper polarization.

Magnetic fields can give rise to a plethora of phenomena in Kitaev spin systems, such as the formation of nontrivial spin liquids in two and three spatial dimensions. For the original honeycomb Kitaev model, it has recently been observed that the sign of the bond-directional exchange is of crucial relevance for the field-induced physics, with antiferromagnetic couplings giving rise to an intermediate spin liquid regime between the low-field gapped Kitaev spin liquid and the high-field polarized state, which is not present in the ferromagnetically coupled model. Here, by employing a Majorana mean-field approach for a magnetic field pointing along the [001] direction, we present a systematic study of field-induced spin liquid phases for a variety of two and three-dimensional lattice geometries. We find that antiferromagnetic couplings generically lead to (i) spin liquid phases that are considerably more stable in field than those for ferromagnetic couplings, and (ii) an intermediate spin liquid phase which arises from a change in the topology of the Majorana band structure. Close inspection of the mean-field parameters reveal that the intermediate phase occurs due to a field-driven sign change in an ‘effective’ $z$-bond energy parameter. Our results clearly demonstrate the richness of the Majorana physics of the antiferromagnetic Kitaev models, in comparison to their ferromagnetic counterparts.

The phase transitions and thermodynamics of the antiferromagnetic system with the presence of the next-to-nearest-neighbor interaction on the body-centered cubic lattice are studied within an exactly solvable model using the recursive lattice approach. It is shown that, besides the Néel and collinear antiferromagnetic phases and the paramagnetic phase discussed in the literature, the model clearly exhibits the existence of an intermediate phase that separates the antiferromagnetic phases. This new phase is observed in a narrow interval of the frustration parameter for nonzero temperatures and is realized in the form of a discrete (single-point-like) ground state in the zero-temperature limit. Transitions between all model phases are studied and it is shown that all transitions between each antiferromagnetic phase of the model and the paramagnetic one as well as between the antiferromagnetic phases and the intermediate phase have the second-order phase transition nature. The magnetization, entropy, and specific heat capacity properties of all phases of the model are studied in detail. Three different ground states are identified and their macroscopic degeneracies are determined. The anomalous Schottky-type behavior of the specific heat capacity in the vicinity of the highly macroscopically degenerated ground state realized for exactly given value of the frustration parameter is observed and discussed. Besides, the existence of the intermediate phase leads to the nontrivial temperature behavior of the specific heat capacity where, in addition to the Schottky-type anomaly related to the frustration, three discontinuous jumps related to the three consecutive second-order phase transitions between corresponding model phases are observed.

Magnetic correlation in ${\mathrm{YBaCo}}_4{\mathrm{O}}_7$ with kagome and triangular lattices was studied through inelastic-neutron-scattering measurements on a single crystal sample. The broad magnetic excitation was observed below 45 meV and showed dispersions both within the $c$ plane and along the $c$ axis. The wave vector with the minimum energy of the magnetic excitation was totally different from the magnetic Bragg point. Our analysis with the first-moment-sum rule suggests that Co ions in ${\mathrm{YBaCo}}_4{\mathrm{O}}_7$ form a unique three-dimensional network with the geometrically frustration.

The double-perovskite $A_{2}B{B}^{\prime }{\mathrm{O}}_6$ with heavy transition metal ions on the ordered $B^{\prime }$ sites is an important family of compounds to study the interplay between electron correlation and spin-orbit coupling. Here we prepared high-quality ${\mathrm{Sr}}_2{\mathrm{MgReO}}_6$ powder and single-crystal samples and performed nonresonant and resonant synchrotron x-ray diffraction experiments to investigate its magnetic ground state. By combining the magnetic susceptibility and heat capacity measurements, we conclude that ${\mathrm{Sr}}_2{\mathrm{MgReO}}_6$ exhibits a layered antiferromagnetic order at temperatures below $\sim 55$ K with a propagation vector $\mathit{q}=\left(001\right)$, which contrasts the previously suspected spin-glass state. Our works clarify the magnetic order in ${\mathrm{Sr}}_2{\mathrm{MgReO}}_6$ and demonstrate it as a candidate system to look for magnetic octupolar orders and exotic spin dynamics.

We propose a data-driven technique to estimate the spin Hamiltonian, including uncertainty, from multiple physical quantities. Using our technique, an effective model of ${\mathrm{KCu}}_4{\mathrm{P}}_3{\mathrm{O}}_{12}$ is determined from the experimentally observed magnetic susceptibility and magnetization curves with various temperatures under high magnetic fields. An effective model, which is the quantum Heisenberg model on a zigzag chain with eight spins having $J_1=-8.54\pm 0.51\mathrm{meV}$, $J_2=-2.67\pm 1.13\mathrm{meV}$, $J_3=-3.90\pm 0.15\mathrm{meV}$, and $J_4=6.24\pm 0.95\mathrm{meV}$, describes these measured results well. These uncertainties are successfully determined by the noise estimation. The relations among the estimated magnetic interactions or physical quantities are also discussed. The obtained effective model is useful to predict hard-to-measure properties such as spin gap, spin configuration at the ground state, magnetic specific heat, and magnetic entropy.