We derive a new microscopic spin Hamiltonian for Rashba-coupled double exchange metals. The Hamiltonian consists of anisotropic interactions of the Dzyaloshinskii-Moriya (DM) and Kitaev form, in addition to the standard isotropic term. We validate the spin Hamiltonian by comparing results with those on the exact spin-fermion model, and present its phase diagram using large scale Monte Carlo simulations. In addition to ferromagnetic, planar spiral and flux states, the model hosts skyrmion crystal and classical spin-liquid states characterized, respectively, by multiple peaks and a diffuse ring pattern in the spin structure factor. The filamentary domain wall structures in the spin-liquid state are in remarkable agreement with experimental data on thin films of MnSi-type B20 metals and transition metals and their alloys.
Mott insulating behavior is induced by strong electron correlation and can lead to exotic states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of novel Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunneling microscopy and angle-resolved photoemission. Our combined experimental and theoretical study indicates that electron corr…
Twisted van der Waals bilayers provide an ideal platform to study the electron correlation in solids. Of particular interest is the 30 degree twisted bilayer honeycomb lattice system, which possesses an incommensurate moire pattern and uncommon electronic behaviors may appear due to the absence of phase coherence. Such system is extremely sensitive to further twist and many intriguing phenomena will occur. In this work, based on first-principles calculations we show that, for further twist near 30 degree, there could induce dramatically different dielectric behaviors of electron between left a…
We report recent analytical progress in the quest for spin models realising exotic phases. We focus on the question of `reverse-engineering' a local, SU(2) invariant S=1/2 Hamiltonian to exhibit phases predicted on the basis of effective models, such as large-N or quantum dimer models. This aim is to provide a point-of-principle demonstration of the possibility of constructing such microscopic lattice Hamiltonians, as well as to complement and guide numerical (and experimental) approaches to the same question. In particular, we demonstrate how to utilise peturbed Klein Hamiltonians to generate…
We present a microscopic electronic description of how antiskyrmions can be stabilized in a Dresselhaus spin-orbit coupled magnetic metal. Furthermore, we show that the antiskyrmions can be tuned into Bloch skyrmions via a change in sign of hopping integral. The results are based on the state of the art hybrid Monte Carlo simulations. Origin of such topological textures is understood via an effective spin-only model. Our results uncover a novel connection between two very distinct topological spin textures of immense current interest, and present a microscopic explanation of skyrmion formation…
Strong geometrical frustration in magnets leads to exotic states, such as spin liquids, spin supersolids and complex magnetic textures. SrCu2(BO3)2, a spin-1/2 Heisenberg antiferromagnet in the archetypical Shastry-Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. The structure of these plateaus is still highly controversial due to the intrinsic complexity associated with frustration and competing length scales. We reveal new magnetic textures in SrCu2(BO3)2 via magnetostriction and magnetocaloric measurements in fields …
Topological insulator is a new state of matter which exhibits exotic surface electronic properties. Determining the spin texture of this class of material is of paramount importance for understanding its topological order and can lead to potential applications in spintronics. Here, we have investigated the nature of the surface state of the topological insulator with hexagonal warping subjected to an off-resonant circularly polarized light. The resulting electronic ground state exhibits a novel feature of spin texture breaking the conventional spin-momentum locking present on a topological ins…
We derive and study a spin one-half Hamiltonian on a honeycomb lattice describing the exchange interactions between Ir$^{4+}$ ions in a family of layered iridates $A_2$IrO$_3$ ($A$=Li,Na). Depending on the microscopic parameters, the Hamiltonian interpolates between the Heisenberg and exactly solvable Kitaev models. Exact diagonalization and a complementary spin-wave analysis reveal the presence of an extended spin-liquid phase near the Kitaev limit and a conventional Néel state close to the Heisenberg limit. The two phases are separated by an unusual stripy antiferromagnetic state, which is t…
A periodic array of topological spin textures, such as skyrmions and hedgehogs, is called the multiple-$Q$ spin texture, as it is represented by a superposition of multiple spin density waves. Depending on the way of superposition, not only the magnetic but also the topological properties are modified, leading to a variety of quantum transport and optical phenomena caused by the emergent electromagnetic fields through the Berry phase. Among others, the phase degree of freedom of the superposed waves is potentially important for such modifications, but its effect has not been fully elucidated t…
The nature of an electric ferro-axial moment characterizing a time-reversal-even axial-vector quantity is theoretically investigated under magnetic orderings. We clarify that a vortex spin texture results in the emergence of the ferro-axial moment depending on the helicity. By introducing a multipole description, we show that the ferro-axial nature appears when two types of odd-parity magnetic multipoles become active under the vortex spin texture: One is the magnetic monopole and the other is the magnetic toroidal dipole. We present all the magnetic point groups to possess the ferro-axial mom…
Antiferromagnetic spintronics is an emerging area of quantum technologies that leverage the coupling between spin and orbital degrees of freedom in exotic materials. Spin-orbit interactions allow spin or angular momentum to be injected via electrical stimuli to manipulate the spin texture of a material, enabling the storage of information and energy. In general, the physical process is intrinsically local: spin is carried by an electrical current, imparted into the magnetic system, and the spin texture then rotates. The collective excitations of complex spin textures have rarely been utilized …
Twisted structures of chiral cubic ferromagnetics MnSi and Cu$_2$OSeO$_3$ can be described both in the frame of the phenomenological Ginzburg-Landau theory and using the microscopical Heisenberg formalism with a chirality brought in by the Dzyaloshinskii-Moriya (DM) interaction. Recent progress in quantum first-principal methods allows to calculate interatomic bond parameters of the Heisenberg model, namely, isotropic exchange constants $J_{ij}$ and DM vectors $\mathbf{D}_{ij}$, which can be used for simulations of observed magnetic textures and comparison of their calculated characteristics, …
Microscopic Emergence of Ancilla Lattice Physics in the Emery Model
Institute of Theoretical Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院理论物理研究所）；Physics Department, TUM School of Natural Sciences, Technical University of Munich（慕尼黑工业大学自然科学学院物理系）
High tunability of two dimensional magnetic materials (by strain, gating, heterostructuring or otherwise) provides unique conditions for studying versatile magnetic properties and controlling emergent magnetic phases. Expanding the scope of achievable magnetic phenomena in such materials is important for both fundamental and technological advances. Here we perform atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of atomic monolayers in the limit of suppressed first-neighbors exchange interaction. We report the rich phase diagram of exotic magnetic configurations, obt…
We present a ^{115}In NMR study of the quasi two-dimensional heavy-fermion superconductor CeCoIn_5 believed to host a Fulde-Ferrel-Larkin-Ovchinnkov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab-plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR…
Spin textures with nontrivial topology, such as vortices and skyrmions, have attracted attention as a source of unconventional magnetic, transport, and optical phenomena. Recently, a new generation of topological spin textures has been extensively studied in itinerant magnets; in contrast to the conventional ones induced, e.g., by the Dzyaloshinskii-Moriya interaction in noncentrosymmetric systems, they are characterized by extremely short magnetic periods and stable even in centrosymmetric systems. Here we review such new types of topological spin textures with particular emphasis on their st…
We present a microscopic theory of skyrmions in the monolayer quantum Hall ferromagnet. It is a peculiar feature of the system that the number density and the spin density are entangled intrinsically as dictated by the W$%_{\infty}$ algebra. The skyrmion and antiskyrmion states are constructed as W$_{\infty }$-rotated states of the hole-excited and electron-excited states, respectively. They are spin textures accompanied with density modulation that decreases the Coulomb energy. We calculate their excitation energy as a function of the Zeeman gap and compared the result with experimental data.
A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many opto-spintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi$_2$Se$_3$ using a laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto un-observed three-dimensiona…
Skyrmion and Meron Crystals in Intermetallic Gd$_3$Ru$_4$Al$_{12}$: Microscopic Model Insights into Chiral Phases
University of Science and Technology of China（中国科学技术大学）；University of Tennessee（田纳西大学）；Oak Ridge National Laboratory（橡树岭国家实验室）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；The University of Tokyo（东京大学）
Magnetic and electronic states of an Ising-spin Kondo lattice model on a kagome lattice are investigated by a Monte Carlo simulation. In addition to the conventional ferromagnetic and ferrimagnetic orders, we show that this model exhibits several thermally-induced phases, such as partially disordered, Kosteritz-Thouless-like, and loop-liquid states. In the partially disordered state, we show that the magnetic transition is associated with the charge-gap formation. We find that the density of state shows characteristic peaks reflecting the underlying spin texture. In the loop-liquid state, we a…
The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in which antiferromagnetically-interacting S=1/2 spins are located on a nearly equilateral triangular lattice attracts considerable interest both from experimental and theoretical aspects, because a simple antiferromagnetic order may be inhibited because of the geometrical frustration and hence an exotic ground state is expected. Furthermore, recent two reports on the ground state of this system have made it further intriguing by showing completely controversial results; one indicates the gapless state and the other gapped. …
Understanding and control of spin degrees of freedom on the surfaces of topological materials are key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal (TR) symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi2Se3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies, which directly demonstrate the mechani…
We investigate the interplay between topological effects and Mott physics in two dimensions on a graphene-like lattice, via a tight-binding model containing an anisotropic spin-orbit coupling on the next-nearest-neighbour links and the Hubbard interaction. We thoroughly analyze the resulting phases, namely a topological band insulator phase or anisotropic quantum Spin Hall phase until moderate interactions, a Néel and Spiral phase at large interactions in the Mott regime, as well as the formation of a spin-orbital texture in the bulk at the Mott transition. The emergent magnetic orders at larg…
A heavy-fermion superconductor UPt$_3$ is a unique spin-triplet superconductor with multiple superconducting phases. Here we provide the first report on the first-principles analysis of the microscopic superconducting gap structure. We find that the promising gap structure is an unprecedented $E_{2u}$ state, which is completely different from the previous phenomenological $E_{2u}$ models. Our obtained $E_{2u}$ state has in-plane twofold vertical line nodes on small Fermi surfaces and point nodes with linear dispersion on a large Fermi surface. These peculiar features cannot be explained in the…
The spin-gap system with a bond-disorder (CH3)2CHNH3-Cu(ClxBr1-x)3 is reported by experiments on macroscopic quantities of the uniform susceptibility and the specific heat to show a magnetic order at low temperatures when the value of x is within the limited region between 0.44 and 0.87, and otherwise a gapped behavior. We have investigated microscopically the ground state of the sample with x=0.35 by muon spin relaxation (mSR) to observe that the frequency spectrum of the spin fluctuation tends to concentrate around zero energy as lowering temperatures. This behavior was interpreted as the so…
We have performed field-swept 59Co NMR measurements on the 'exotic' spin-chain material, Ca3Co2O6, crystallizing in K4CdCl6-type rhombohedral structure, believed to exhibit two magnetic transition temperatures (around 24 and 12 K). This is the first NMR investigation of this family of compounds. We are able to detect the NMR signal below about 15 K, which is a conclusive proof for the existence of non-magnetic Co ions (Co3+ - low-spin 3d6, in addition to magnetic Co ions) at least in a narrow temperature range around 10 K, thereby resolving a controversy on this issue in the literature. In add…
The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe$_2$As$_2$, which provides a foundation for further studies. We show that the energy of the spin density wave (SDW) in BaFe$_2$As$_2$ is lowered through exotic exchange splitting of the band structure, rather than Fermi surface nesting of itinerant electrons. This clearly demonstrates that a metallic SDW state could be so…
An important feature of strong correlated electron systems is the tunability between interesting ground states such as unconventional superconductivity and exotic magnetism. Pressure is a clean, continuous and systematic tuning parameter. However, due to the restricted accessibility introduced by high-pressure devices, compatible magnetic field sensors with sufficient sensitivity are rare. This greatly limits the detections and detailed studies of pressure-induced phenomena. Here, we utilize nitrogen vacancy (NV) centers in diamond as a powerful, spatially-resolved vector field sensor for mate…
We use the microscopic Hartree-Fock approximation to investigate various quantum phase transitions associated with possible spontaneous symmetry breaking induced by a tilted magnetic field in the integral quantum Hall regime of wide parabolic wells and zero width double well (bilayer) systems. We propose a general class of variational wavefunctions that describe several types of parity, spin, and translational symmetry breaking, including spin and charge density wave phases. Zero temperature quantum phase diagrams for these systems are calculated in the parameter regime of experimental interes…
Two-dimensional electron gases (2DEGs) in SrTiO$_3$ have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the $d$-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orb…
The recently discovered three dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice via odd number of band inversions, driving the system through a topological quantum phase transition. By directly measuring the topological invariants (for the method to directly measure Fu-Kane {$ν_0$}, see Hsieh \textit{et.al.,} Science 323, 919 (2009) at http://www.sciencemag.org/content/323/5916/919.abstract) we report the obse…
