Suppression of superconductivity due to the proximity effect between a superconductor and a ferromagnet can be partially alleviated when a Cooper pair simultaneously samples different directions of the short-range exchange field. The superconductor's critical temperature, $T_C$, is therefore expected to partially recover when the ferromagnet is in a multidomain state, as opposed to a single-domain state. Here, we discuss series of experiments performed with ferromagnet(Pt/Co)/spacer(IrMn and Pt)/superconductor(NbN) heterostructures. By tuning the various parameters in play, e.g., superconducting coherence length-to-thicknesses ratio, and domain sizes, we obtained up to 10% recovery of the superconducting critical temperature $\mathrm{\Delta }{T}_C/T_C$. This large-scale recovery made investigations possible. In particular, from the spacer thickness dependence of $\mathrm{\Delta }{T}_C/T_C$, it was possible to deduce the characteristic length for Cooper pair penetration in an IrMn antiferromagnet. This information is crucial for electronic transport, and up to now has been difficult to access experimentally for antiferromagnets.

Spin-flop transition (SFT) consists in a jump-like reversal of antiferromagnetic (AF) lattice into a noncollinear state when the magnetic field increases above the critical value. Potentially the SFT can be utilized in many applications of a rapidly developing AF spintronics. However, the difficulty of using them in conventional antiferromagnets lies in (a) too large switching magnetic fields (b) the need for presence of a magnetic anisotropy, and (c) requirement to apply magnetic field along the correspondent anisotropy axis. In this work we propose to use artificial ferrimagnets (FEMs) in which the SFT occurs without anisotropy and the transition field can be lowered by adjusting exchange coupling in the structure. This is proved by experiment on artificial Fe-Gd FEMs where usage of Pd spacers allowed us to suppress the transition field by two orders of magnitude.

The nonsymmorphic ${\mathrm{Zr}}_2\mathrm{Ir}$ alloy is a possible topological semimetal candidate material and as such may be part of an exotic class of superconductors. ${\mathrm{Zr}}_2\mathrm{Ir}$ is a superconductor with a transition temperature of 7.4 K with critical fields of 19.6(3) mT and 3.79(3) T, as determined by heat capacity and magnetization. Zero-field muon spin relaxation measurements show that time-reversal symmetry is preserved in these materials. The specific heat and transverse field muon spin rotation measurements rule out any possibility to have a nodal or anisotropic superconducting gap, revealing a conventional $s$-wave nature in the superconducting ground state. Therefore this system is found to be a conventional nonsymmorphic superconductor, with time-reversal symmetry being preserved and an isotropic superconducting gap.

Using the model of classical Heisenberg exchange and Dzyaloshinskii-Moriya (DM) interaction, we show that the ground state of the B20 FeGe chiral magnet is a superposition of twisted helical spin-density waves formed by different sublattices of the crystal. Such twisted spin-density waves propagate in the same direction but with different phases and different directions of the rotation axes. We derive an advanced micromagnetic expression describing the exchange and DM interaction for such magnetic structures. In particular, we show that such magnetic order gives rise to new contributions to the micromagnetic energies of the exchange and DM interactions. By employing first-principles calculations based on density functional theory and using our micromagnetic model we show that the magnitude of the spin-spiral twist in B20 FeGe is of the same order as global spiraling. While the energy difference between the ground state of twisted spirals and the ferromagnetic state is in good agreement with the experimental results, for the spin spirals without a twist it is smaller by a factor of 3. In addition, we verify our results by employing spin-dynamics simulations. This calls for new experiments exploring the ground state properties of B20 chiral magnets.

A higher-order (HO) symmetry-protected topological (SPT) state can be realized in a plaquette-modulated square lattice antiferromagnet, which hosts a spin-$1/2$ degenerate mode on each corner of the lattice with open boundaries. In this work, we show with the field-theoretic analysis and quantum Monte Carlo simulations that the plaquette modulation can drive a direct topological quantum phase transition from the HOSPT to a trivial disordered phase across the deconfined quantum critical point (DQCP) between the antiferromagnetic (AF) order and the valence bond solid (VBS) order, thus the DQCP is a multicritical point bridging both the AF-VBS transition and the topological transition of the HOSPT phase. Our work thus reveals the ubiquitous duality between topological transitions of SPT phases and DQCPs.

In an attempt to theoretically investigate the quantum phase transition and criticality in topological models, we study a Kitaev chain with longer-range couplings (finite number of neighbors) as well as truly long-range couplings (infinite number of neighbors). We carry out an extensive topological characterization of the momentum space to explore the possibility of obtaining higher order winding numbers and analyze the nature of their stability in the model. The occurrences of phase transitions from even-to-even and odd-to-odd winding numbers are observed with decreasing longer-rangeness in the system. We derive topological quantum critical lines and study them to understand the behavior of criticality. A suppression of higher order winding numbers is observed with decreasing longer-rangeness in the model. We show that the mechanism behind such phenomena is due to the superposition and vanishing of the topological quantum critical lines associated with the higher winding number. Through the study of the Berry connection, we show the possible different behaviors of critical lines when they undergo superposition along with the corresponding critical exponents. We analyze the behavior of the long-range models through the momentum space characterization. We also provide the exact solution for the problem and discuss the experimental aspects of the work.

The dynamics of the coupled electron-nuclear spin system is studied in an ensemble of singly charged (In,Ga)As/GaAs quantum dots (QDs) using periodic optical excitation at 1 GHz repetition rate. In combination with the electron-nuclei interaction, the highly repetitive excitation allows us to lock the electron spins into magnetic resonance in a transverse external magnetic field. Sweeping the field to higher values, the locking leads to an effective “diamagnetic” response of significant strength due to dynamic nuclear polarization, which shields the QD electrons at least partly from the external field and can even keep the internal magnetic field constant up to 1.3 T field variation. We model the effect through a magnetic field-dependent polarization rate of the nuclei, from which we suggest a strategy for adjusting the nuclear polarization through the detuning between optical excitation and electronic transition, in addition to tuning the magnetic field.

Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields tilted from the normal to the 2D electron layer at different temperatures $T$. A decrease of MISO amplitude with temperature increase is observed. At moderate tilts, the temperature decrease of MISO amplitude is consistent with a decrease of the Dingle factor due to a reduction of the quantum electron lifetime at high temperatures. At large tilts, a different regime of strong MISO suppression with the temperature is observed. The proposed model relates this suppression to magnetic entanglement between subbands, leading to beating in oscillating density of states. The model yields corresponding temperature damping factor: $A_{\text{MISO}}\left(T\right)=X/sinh\left(X\right)$, where $X=2{\pi }^{2}kT\delta f$ and $\delta f$ is the difference frequency of oscillations of density of states in two subbands. This factor is in agreement with experiments. A Fermi liquid enhancement of MISO amplitude is observed.

Charge transfer effect can give rise to exotic two-dimensional phenomena at not only polar-nonpolar interfaces but also isopolar interfaces in complex oxides. Here, we investigate the valence state changes of both Ti and Fe at the $\mathrm{LaTi}{\mathrm{O}}_3/\mathrm{LaFe}{\mathrm{O}}_3$ interface by a combination of density functional theory calculations, molecular beam epitaxy, and aberration-corrected scanning transmission electron microscopy. We demonstrate that the charge transfer from Ti to Fe occurs near the interface. The majority of the transferred charge is located within 1 unit cell from the interface. Our results present a detailed understanding of the interfacial charge transfer effect upon the design of functional oxide heterostructures.

We theoretically study the de Haas–van Alphen (dHvA) oscillations in the system with changing the topology of the Fermi surface (the Lifshitz transition) by electron dopings. We employ the two-dimensional tight-binding model for $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ under pressure which has two Dirac points in the first Brillouin zone. When this system is slightly doped, there exists two closed Fermi surfaces with the same area and the dHvA oscillations become saw-ooth pattern or inversed sawtooth pattern for both cases of fixed electron filling ($\nu$) or fixed chemical potential ($\mu$) with respect to the magnetic field, respectively. By increasing dopings, the system approaches the Lifshitz transition where two closed Fermi surfaces are close each other. Then, we find that the pattern of the dHvA oscillations changes. A jump of the magnetization appears at the center of the fundamental period, and its magnitude increases in the case of the fixed electron filling, whereas a jump is separated into a pair of jumps and its separation becomes large in the case of the fixed chemical potential. This is due to the lifting of double degeneracy in the Landau levels. Since this lifting is seen in the two-dimensional Dirac fermion system with two Dirac points, the obtained results in this paper can be applied to not only $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$, but also other materials with closely located Dirac points, such as graphene under the uniaxial strain, in black phosphorus, twisted bilayer graphene, and so on.

We demonstrate how the quantum paraelectric ground state of ${\mathrm{SrTiO}}_3$ can be accessed via a microscopic ab initio approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in ${\mathrm{SrTiO}}_3$ are also well captured by the present microscopic framework.

Frustrated spin systems generically suffer from the negative sign problem inherent in Monte Carlo methods. Since the severity of this problem is formulation dependent, optimization strategies can be put forward. We introduce a phase pinning approach in the realm of the auxiliary field quantum Monte Carlo algorithm. If we can find an antiunitary operator that commutes with the one-body Hamiltonian coupled to the auxiliary field, then the phase of the action is pinned to 0 and $\pi$. For generalized Kitaev models, we can successfully apply this strategy and observe a remarkable improvement of the average sign. We use this method to study the thermodynamical and dynamical properties of the Kitaev-Heisenberg model down to temperatures corresponding to half of the exchange coupling constant. Our dynamical data reveal finite temperature properties of ordered and spin-liquid phases inherent in this model.

The boundaries of quantum materials can host a variety of exotic effects such as topologically robust edge states or anyonic quasiparticles. Here, we show that fermionic systems such as graphene that admit a low-energy Dirac description can exhibit counterintuitive relativistic effects at their boundaries. As an example, we consider carbon nanotubes and demonstrate that relativistic bulk spinor states can have nonzero charge density on the boundaries, in contrast to the sinusoidal distribution of nonrelativistic wave functions that are necessarily zero at the boundaries. This unusual property of relativistic spinors is complementary to the linear energy dispersion relation exhibited by Dirac materials and can influence their coupling to leads, transport properties, or their response to external fields.

The newly realized twisted graphene systems such as twisted bilayer graphene (TBG), twisted double bilayer graphene (TDBG), and twisted trilayer graphene (TTG) have attracted widespread attention. Therefore, a simple and exact model of the systems is urgent and vital for further study. Here, we construct the symmetry-adapted localized Wannier functions and the corresponding ab initio minimal two-valley four-band effective tight-binding models for generic twisted graphene systems with small twist angle. Such a two-valley model circumvents the Wannier obstruction caused by the fragile topology in the one-valley model. The real-space valley operator is introduced to explicitly describe the valley $U_v\left(1\right)$ symmetry. Each symmetry-adapted Wannier orbital shows a peculiar three-peak form with its maximum at AA spots and its center at AB or BA spots. An extended Hubbard model is also given and the related interaction parameters are presented explicitly. We provide an approach to systematically build the Wannier tight-binding model for generic twisted graphene systems. Our model provides a firm basis for further study of the many-body effects in these systems.

We propose an interpolation scheme to describe pair correlations in crystals with many-body interactions that requires only information on relative displacements for the nearest neighbours and in the long range. Using crystalline Ni as a test case, the scheme is shown to deliver the functional form for the radial distribution function at least as well as molecular dynamics simulations. The results provide a fast route for verification of interatomic potentials and study of many-body interactions using a combination of x-ray scattering and x-ray absorption spectroscopy.

A central problem in multicomponent lattice systems is to systematically quantify multipoint ordering. Ordering in such systems is often described in terms of pairs, even though this is not sufficient when three-point and higher-order interactions are included in the Hamiltonian. Current models and parameters for multipoint ordering are often only applicable for very specific cases or require approximating a subset of correlated occupational variables on a lattice as being uncorrelated. In this paper, cluster order parameters are introduced to systematically quantify arbitrary multipoint ordering motifs in substitutional systems through direct calculations of normalized cluster probabilities. These parameters can describe multipoint chemical ordering in crystal systems with multiple sublattices, multiple components, and systems with reduced symmetry. These are defined in this paper and applied to quantify four-point chemical ordering motifs in platinum/palladium alloy nanoparticles that are of practical interest to the synthesis of catalytic nanocages. Impacts of chemical ordering on nanocage stability are discussed. It is demonstrated that approximating four-point probabilities from superpositions of lower-order pair probabilities is not sufficient in cases where three- and four-body terms are included in the energy expression. Conclusions about the formation mechanisms of nanocages may change significantly when using common pair approximations.

The ${\mathrm{Pr}}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{CoO}}_3$ perovskite exhibits unique magnetostructural properties among the rest of ferromagnetic (FM)/metallic $L{n}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{CoO}}_3$ compounds. The sudden orthorhombic-tetragonal $(Imma\to I4/mcm)$ structural transition produces an unusual magnetic behavior vs temperature and external magnetic fields. The symmetry change is responsible for a spontaneous spin rotation in this metallic oxide. We have studied half-doped $L{n}_{0.50}{\left({\mathrm{Sr}}_{1-x}{A}_x\right)}_{0.50}{\mathrm{CoO}}_3$ cobaltites varying the ionic radius $r_A$ of A-site cations (divalent cations and lanthanides) to complete the T-$r_A$ phase diagram. The influence of the structural distortion and the A-cation size for the occurrence of a spontaneous spin reorientation in the metallic state has been investigated. As the magnetization reorientation is driven by the temperature-induced collapse of the orthorhombic distortion, a careful investigation of the structural symmetry is presented, varying the structural distortion of the Sr-rich half-doped cobaltites by means of both compositional and temperature changes. The region in the phase diagram of these FM/metallic cobaltites where $F{m}^{\prime }{m}^{\prime }m$ magnetic symmetry replaces $I{m}^{\prime }{m}^{\prime }a$ was determined. In that region, the magnetization direction has rotated 45 ° within the $a\text{−}b$ plane from the former to the latter.

Mutual interplay between the electronic degrees of freedom in solids, such as charge, spin, orbital, sublattice, and bond degrees of freedom, is a source of cross-correlated phenomena with unconventional electronic ordered states. Such degrees of freedom can be described by four types of multipoles (electric, magnetic, magnetic toroidal, and electric toroidal) in a unified way, which enable us to tightly connect the microscopic degrees of freedom with macroscopic physical responses in a transparent manner. We complete a classification of the multipoles in all 122 magnetic point groups based on group theory. The classification is useful to identify potentially active multipoles, not only in ordinary ferromagnetic and antiferromagnetic orderings but also in exotic orderings breaking time-reversal symmetry, e.g., a loop-current state. Moreover, the classification gives an insight into the microscopic origin of the multiferroic responses and quantum transports. By analyzing response functions up to the second order, we summarize the indispensable multipole moments for various responses, such as the linear magnetoelectric, piezoelectric, and elastic responses, and the nonlinear conductivity and Nernst coefficient. Our results highly promote a further discovery of functional multiferroic materials, guided by the bottom-up material design based on the symmetry-adapted multipoles.

The Meissner effect is one of the defining properties of superconductivity, with a conventional superconductor completely repelling an external magnetic field. In contrast to this diamagnetic behavior, odd-frequency superconducting pairing has often been seen to produce a paramagnetic Meissner effect, which instead makes the superconductor unstable due to the attraction of magnetic field. In this paper, we study how both even- and odd-frequency superconducting pairing contributes to the Meissner effect in a generic two-orbital superconductor with a tunable odd-frequency pairing component. By dividing the contributions to the Meissner effect into intra- and interband processes, we find that the odd-frequency pairing actually generates both dia- and paramagnetic Meissner responses, determined by the normal-state band structure. More specifically, for materials with two electronlike (holelike) low-energy bands, we find that the odd-frequency interband contribution is paramagnetic but nearly canceled by a diamagnetic odd-frequency intraband contribution. Combined with a diamagnetic even-frequency contribution, such superconductors thus always display a large diamagnetic Meissner response to an external magnetic field, even in the presence of large odd-frequency pairing. For materials with an inverted, or topological, band structure, we find the odd-frequency interband contribution to instead be diamagnetic and even the dominating contribution to the Meissner effect in the near-metallic regime. Taken together, our results show that odd-frequency pairing in multiorbital superconductors does not generate a destabilizing paramagnetic Meissner effect and can even generate a diamagnetic response in topological materials.

We find excellent agreement between tunneling and optical conductivity gap values in superconducting granular aluminum films, up to the metal-to-insulator transition. This behavior, in strong contrast with that recently reported in atomically disordered samples for which the optical gap becomes smaller than the tunneling gap, suggests that disorder is not at the origin of the transition. The large increase seen in the strong-coupling ratio and a finite value of the gap at $T_c$ near the metal-to-insulator transition are consistent with a BCS-to-BEC crossover.