Under pressure, a quasi-two-dimensional electron gas can collapse toward the true two-dimensional (2D) limit. In this limit, the exact exchange-correlation energy per electron has a known finite limit, but general-purpose semilocal approximate density functionals, such as the local density approximation (LDA) and the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE GGA), are known to diverge to minus infinity. Here we consider a model density for a noninteracting electron gas confined to a thickness $L$ by infinite-barrier walls, with a fixed 2D density $1/\left[\pi {\left(r_s^{\text{2D}}\right)}^2\right]$ and $r_s^{\text{2D}}=4$ Bohr. We estimate that LDA, PBE, and the strongly constrained and appropriately normed (SCAN) meta-GGA are accurate for the exchange-correlation energy over a wide quasi-2D range, $1.5<L/r_s^{\text{2D}}<3.85$, but not for smaller $L$. Of these functionals, only SCAN tends to a finite limit when $L$ tends to 0. Since the noninteracting kinetic energy, treated exactly in Kohn-Sham theory, dominates in this limit within a deformable jellium model, all of the general-purpose functionals can estimate the pressure required to achieve any thickness (with SCAN and LDA better than PBE). This pressure vanishes around $L/r_s^{\text{2D}}=3.85$, where the 3D electron density is roughly that of the valence electrons in metallic potassium, and it reaches about 20 GPa at $L/r_s^{\text{2D}}=1.5$ and 400 GPa at $L/r_s^{\text{2D}}=0.6$.

We study quantum phase transitions associated with splitting nodal Fermi points, motivated by topological phase transitions between Dirac and Weyl semimetals. A Dirac point in Dirac semimetals may be split into two Weyl points by breaking a lattice symmetry or time-reversal symmetry, and the Lifshitz transition is commonly used to describe the phase transitions. Here, we show that the Lifshitz description is fundamentally incorrect in quantum phase transitions with splitting nodal Fermi points. We argue that correlations between fermions, order parameter, and the long-range Coulomb interaction must be incorporated from the beginning. One of the most striking correlation effects we find is infinite anisotropy of physical quantities, which cannot appear in a Lifshitz transition. By using the standard renormalization group method, two types of infinitely anisotropic quantum criticalities are found in three spatial dimensions, varying with the number of the Dirac points $\left(N_f\right)$. For $N_f=1$, the ratio of the fermion velocity to the velocity of order-parameter excitations becomes universal $\left(1+\sqrt{2}\right)$ along the Dirac point splitting direction. For $N_f>1$, we find that fermions are parametrically faster than order-parameter excitations in all directions. Our renormalization group analysis is fully controlled by the fact that order parameter and fermion fluctuations are at the upper critical dimension, and thus our stable fixed points demonstrate the presence of weakly coupled quantum criticalities with infinite anisotropy.

Our ability to understand and tailor metal-organic interfaces is mandatory to functionalize organic complexes for next generation electronic and spintronic devices. For magnetic data storage applications, metal-carrying organic molecules, the so-called single molecular magnets (SMM) are of particular interest as they yield the possibility to store information on the molecular scale. In this work, we focus on the adsorption properties of the prototypical SMM ${\mathrm{Sc}}_3\mathrm{N}@{\mathrm{C}}_{80}$ grown in a monolayer film on the Ag(111) substrate. We provide clear evidence of a pyramidal distortion of the otherwise planar ${\mathrm{Sc}}_3\mathrm{N}$ core inside the carbon cage upon the adsorption on the Ag(111) surface. This adsorption-induced structural change of the ${\mathrm{Sc}}_3\mathrm{N}@{\mathrm{C}}_{80}$ molecule can be correlated to a charge transfer from the substrate into the lowest unoccupied molecular orbital of ${\mathrm{Sc}}_3\mathrm{N}@{\mathrm{C}}_{80}$, which significantly alters the charge density of the fullerene core. Our comprehensive characterization of the ${\mathrm{Sc}}_3\mathrm{N}@{\mathrm{C}}_{80}$-Ag(111) interface hence reveals an indirect coupling mechanism between the ${\mathrm{Sc}}_3\mathrm{N}$ core of the fullerene molecule and the noble metal surface mediated via an interfacial charge transfer. Our work shows that such an indirect coupling between the encapsulated metal centers of SMM and metal surfaces can strongly affect the geometric structure of the metallic centers and thereby potentially also alters the magnetic properties of SMMs on surfaces.

We consider the superconducting and Mott-insulating states for twisted bilayer graphene, modeled as a two-narrow-band system of electrons with appreciable intra-atomic Coulomb interactions. The interaction induces kinetic exchange which leads to real space, either triplet- or singlet-spin pairing, in direct analogy to heavy fermions and high-temperature superconductors. By employing the statistically consistent Gutzwiller method, we construct explicitly the phase diagram as a function of electron concentration for the spin-triplet $d_{x^2-y^2}+i{d}_{xy}$ paired case, as well as determine the topological edge states. The model reproduces principal features observed experimentally in a semiquantitative manner. The essential role of electronic correlations in driving both the Mott-insulating and superconducting transitions is emphasized. The transformation of the spin-triplet state into its spin-singlet analog is also analyzed, as well as the appearance of the phase-separated superconducting+Mott-insulating state close to the half-filling.

One-dimensional metal-halide compounds provide model systems to investigate the manner in which coupling between elementary degrees of freedom—here, electronic and vibrational—result in instabilities that give rise to both chemical and structural rearrangements. Here, we employ “single-shot” pump-probe spectroscopy to examine a one-dimensional platinum iodide compound (PtI(en)) under far-from-equilibrium conditions where repeated photoexcitation results in sample damage. It presents evidence for a distinct collective excited state lasting more than 100 ps upon self-trapped exciton generation at high densities, as measured by electronic signal amplitudes and phonon properties.

Analytical solution has been found for the second-order effective anisotropy of magnetic nanoparticles of a cubic shape due to the surface anisotropy (SA) of the Néel type. Similarly to the spherical particles, for the simple cubic lattice the grand-diagonal directions $\left(\pm 1,\pm 1,\pm 1\right)$ are favored by the effective cubic anisotropy but the effect is twice as strong. Uniaxial core anisotropy and applied magnetic field cause screening of perturbations from the surface at the distance of the domain-wall width and reduce the effect of SA near the energy minima. However, screening disappears near the uniaxial energy barrier, and the uniform barrier state of larger particles may become unstable. For these effects the analytical solution is obtained as well, and the limits of the additive formula with the uniaxial and effective cubic anisotropies for the particle are established. Thermally-activated magnetization-switching rates have been computed by the pulse-noise technique for the stochastic Landau-Lifshitz equation for a system of spins.

We present the results of a muon-spin spectroscopy investigation of ${\mathrm{GaV}}_4{\mathrm{S}}_{8-y}{\mathrm{Se}}_y$ with $y=0,2,4$, and 8. Zero-field measurements suggest that ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ and ${\mathrm{GaV}}_4{\mathrm{S}}_8$ have distinct magnetic ground states, with the latter material showing an anomalous temperature dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and 4 materials showing glassy magnetic behavior at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of ${\mathrm{GaV}}_4{\mathrm{Se}}_8$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

In this paper, we discuss a micromagnetic modeling approach to describe the ultrafast spin-transfer torque excitation of coherent and incoherent magnons on the nanoscale. Implementing the action of a femtosecond spin current pulse entering an orthogonally magnetized thin ferromagnetic film, we reproduce recent experimental results and reveal the factors responsible for the unequal excitation efficiency of various spin waves. Our findings are in an excellent agreement with the results of an analytical description of spin-wave excitation based on classical kinetic equations. Furthermore, we suggest an experimental design allowing for the excitation of laterally propagating spin waves beyond the optical diffraction limit. Our findings demonstrate that the classical micromagnetic picture retains its predictive and interpretative power for modeling the reaction of the magnetization on dynamical stimuli that reside on femtosecond temporal and nanometer spatial scales.

The search for exotic quantum spin liquid states in simple yet realistic spin models remains a central challenge in the field of frustrated quantum magnetism. Here we consider the canonical nearest-neighbor kagome Heisenberg antiferromagnet restricted to a quasi-one-dimensional strip consisting entirely of corner-sharing triangles. Using large-scale density matrix renormalization group calculations, we identify in this model an extended gapless quantum phase characterized by central charge $c=2$ and power-law decaying spin and bond-energy correlations which oscillate at tunably incommensurate wave vectors. We argue that this intriguing spin liquid phase can be understood as a marginal instability of a two-band spinon Fermi surface coupled to an emergent U(1) gauge field, an interpretation which we substantiate via bosonization analysis and Monte Carlo calculations on model Gutzwiller variational wave functions. Our results represent one of the first numerical demonstrations of emergent fermionic spinons in a simple SU(2) invariant nearest-neighbor Heisenberg model beyond the strictly one-dimensional (Bethe chain) limit.

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth (${\lambda }_{\mathrm{ab}}$) in underdoped, optimally doped, and overdoped samples. The results suggest an abrupt increase of ${\lambda }_{\mathrm{ab}}$ as doping is increased from $x_{\mathrm{opt}}$, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for $x=0.19$ and 0.58 is compatible with previously observed structural symmetries at those doping levels.

Proposed effective Hamiltonians from the literature for the material $\alpha -\mathrm{RuCl}{}_3$ are used to compute the magnon thermal Hall conductivity ${\kappa }_{xy}$ using linear spin-wave theory for the magnetically ordered state. No model previously proposed that was tested explains the published experimental data. Models with Kitaev interaction $K>0$ are seen to predict ${\kappa }_{xy}\gtrsim 0$, which is inconsistent with the data. Fluctuations toward a Kitaev-type spin liquid would have the wrong sign to explain the data. However, a slight variant of a previously proposed model predicts a large ${\kappa }_{xy}$, demonstrating that the low-temperature thermal Hall effect could be generated exclusively by the Berry curvature of the magnon bands. The experimental data of ${\kappa }_{xy}$ can therefore serve as another method to constrain a proposed effective Hamiltonian.

We show that stable skyrmions with diameters of a few nanometers can emerge in atomic Pd/Fe bilayers on the Rh(111) surface. Based on density functional theory we calculate the exchange and the Dzyaloshinskii-Moriya interaction as well as the magnetocrystalline anisotropy energy. The latter two terms are driven by spin-orbit coupling and significantly reduced compared to Pd/Fe bilayers on Ir(111) as expected since Rh and Ir are isoelectronic $4d$ and $5d$ transition metals. However, there is still a spin spiral ground state at zero magnetic field. Atomistic spin dynamics simulations show that a skyrmion phase occurs at small magnetic fields of $\sim 1$ T. Skyrmion diameters amount to 2–8 nm and skyrmion lifetimes are up to 1 h at temperatures of 25–45 K.

For almost 50 years, the formation of stripe domains in magnetic thin films has been considered as an abrupt magnetization phase transition driven by an interplay of the intrinsic perpendicular anisotropy and the shape anisotropy. This allows the determination of a critical film thickness for a given value of the intrinsic perpendicular anisotropy. The present experimental and theoretical analysis shows that micromagnetic structures such as domain walls and their junctions considerably soften a magnetic thin film. The consequence is a significant reduction of the critical film thickness with respect to the originally derived value.

Quantum entanglement, the essential resource for quantum information processing, has rich dynamics under different environments. Probing different entanglement dynamics typically requires exquisite control of complicated system-environment coupling in real experimental systems. Here, by a simple control of the effective solid-state spin bath in a diamond sample, we observe rich entanglement dynamics, including the conventional asymptotic decay as well as the entanglement sudden death, a term coined for the phenomenon of complete disappearance of entanglement after a short finite time interval. Furthermore, we observe counterintuitive entanglement rebirth after its sudden death in the same diamond sample by tuning an experimental parameter, demonstrating that we can conveniently control the non-Markovianity of the system-environment coupling through a natural experimental knob. Further tuning of this experimental knob can make the entanglement dynamics completely coherent under the same environmental coupling. Probing of entanglement dynamics, apart from its fundamental interest, may find applications in quantum information processing through control of the environmental coupling.

We study the effect of a thermal environment on the quantum annealing dynamics of a transverse-field Ising chain. The environment is modeled as a single Ohmic bath of quantum harmonic oscillators weakly interacting with the total transverse magnetization of the chain in a translationally invariant manner. We show that the density of defects generated at the end of the annealing process displays a minimum as a function of the annealing time, the so-called optimal working point, only in rather special regions of the bath temperature and coupling strength plane. We discuss the relevance of our results for current and future experimental implementations with quantum annealing hardware.

The critical properties and magnetocaloric effect of semiconducting ferrimagnet ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ single crystals have been investigated by bulk magnetization and heat capacity around $T_c$. Critical exponents $\beta =0.41\pm 0.01$ with a critical temperature $T_c=74.18\pm 0.08\mathrm{K}$ and $\gamma =1.21\pm 0.02$ with $T_c=74.35\pm 0.05\mathrm{K}$ are deduced by the Kouvel-Fisher plot, whereas $\delta =4.29\pm 0.05(3.40\pm 0.02)$ is obtained by a critical isotherm analysis at $T=74\left(75\right)\mathrm{K}$. The magnetic exchange distance is found to decay as $J\left(r\right)\approx r^{-4.79}$, which lies between the mean-field and 3D Heisenberg models. Moreover, the magnetic entropy change $-\mathrm{\Delta }{S}_M$ features a maximum at $T_c$, i.e., $-\mathrm{\Delta }{S}_M^{\max }\sim 2.53\left(1.67\right)\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ with in-plane (out-of-plane) field change of 5 T, confirming large magnetic anisotropy. The heat capacity measurement further gives $-\mathrm{\Delta }{S}_M^{\max }\sim 2.94\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ and the corresponding adiabatic temperature change $\mathrm{\Delta }{T}_{ad}\sim 1.14\mathrm{K}$ with out-of-plane field change of 9 T.

In this paper, we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite ${\mathrm{YbFeO}}_3$ at temperatures below and above the spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6\mathrm{K}$, in magnetic fields applied along the $a,b$, and $c$ axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of $\sim 60\mathrm{meV}$, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim 1\mathrm{meV}$ within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$ drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinonlike fluctuations. At $T<T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at $T>T_{\mathrm{SR}}$ only a gapped broad spinonlike continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem $J_{\text{Yb-Yb}}$, mainly neglected in previous studies, creates unusual quantum spin dynamics on the low-energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be “hidden” by a dominating interaction.

In the quantum anomalous Hall effect, quantized Hall resistance and vanishing longitudinal resistivity are predicted to result from the presence of dissipationless, chiral edge states and an insulating two-dimensional bulk, without requiring an external magnetic field. Here, we explore the potential of this effect in magnetic topological insulator thin films for metrological applications. Using a cryogenic current comparator system, we measure quantization of the Hall resistance to within one part per million and, at lower current bias, longitudinal resistivity under 10 $\mathrm{m}\mathrm{\Omega }$ at zero magnetic field. Increasing the current density past a critical value leads to a breakdown of the quantized, low-dissipation state, which we attribute to electron heating in bulk current flow. We further investigate the prebreakdown regime by measuring transport dependence on temperature, current, and geometry, and find evidence for bulk dissipation, including thermal activation and possible variable-range hopping.

Based on first-principles calculations and analysis of crystal symmetries, we propose a kind of hourglasslike nodal net (HNN) semimetal in centrosymmetric ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ that is constructed by two hourglasslike nodal chains at mutually orthogonal planes in the extended Brillouin zone (BZ) when the weak spin-orbit coupling (SOC) mainly from the $6s$ orbital of Bi atoms is ignored. The joint point in the nodal net structure is a special double Dirac point located at the BZ corner. Different from previous HNNs [T. Bzdušek et al., Nature (London) 538, 75 (2016)] where the SOC and double group nonsymmorphic symmetries are necessary and also different from the accidental nodal net, this HNN structure is inevitably formed and guaranteed by spinless nonsymmorphic symmetries and thus robust against any symmetry-remaining perturbations. The Fermi surface in ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ consisting of a toruslike electron pocket and a toruslike hole pocket may lead to unusual transport properties. A simple four-band tight-binding model is built to reproduce the HNN structure. For a semi-infinite ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$, the “drumhead”-like surface states with nearly flat dispersions are demonstrated on (001) and (100) surfaces, respectively. If such a weak-SOC effect is taken into consideration, this HNN structure will be slightly broken, leaving a pair of hourglasslike Dirac points at the twofold screw axis. This type of hourglasslike Dirac semimetal is symmetry enforced and does not need band inversion anymore. Our discovery provides a platform to study novel topological semimetal states from nonsymmorphic symmetries.