The current theoretical and experimental situations are reviewed for low-dimensional insulating systems with a low magnetic transition temperature TM and pronounced short-range magnetic order above this temperature. Both the standard and self-consistent spin-wave theories are shown to be insufficient to quantitatively describe the experimental data on these systems. Field-theoretical approaches taking into account the contribution of the spin-fluctuation excitations (neglected in the spin-wave theories) to the thermodynamic properties of ferro- and antiferromagnets are discussed.
The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which hybridization of itinerant 5d electrons with localized 4f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation methods. Below 6 K the Fe impurity induces simultaneous changes in the bulk local magnet…
We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+δ}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy {using samples of the latest generation. From the raw data we derive symmetry-resolved spectra.} The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. The spectra agree quantitatively with predictions by field theory [\onlinecite{Weidinger:2015}] facilitating the precise extraction of the Heisenberg coupling $J$.…
Magnetism in transition-metal compounds (TMCs) has traditionally been associated with spin degrees of freedom, because the orbital magnetic moments are typically largely quenched. On the other hand, magnetic order in 4f- and 5d-electron systems arises from spin and orbital moments that are rigidly tied together by the large intra-atomic spin-orbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4d-electron Mott insulator Ca$_2$RuO$_4$, we report a novel form of excitonic magnetism in the intermediate-strength regime of the SOC. The magnetic order is characterized by ``soft…
The nature of insulating phase in 5$d$ based Sr$_2$IrO$_4$ is quite debated as the theoretical as well as experimental investigations have put forward evidences in favor of both magnetically driven Slater-type and interaction driven Mott-type insulator. To understand this insulating behavior, we have investigated the nature of magnetic state in Sr$_2$IrO$_4$ through studying critical exponents, low temperature thermal demagnetization and magnetocaloric effect. The estimated critical exponents do not exactly match with any universality class, however, the values obey the scaling behavior. The e…
Quantum entanglement effects between the electronic spin and charge degrees of freedom are examined in an organic molecular solid, termed a dimer-Mott insulating system, in which molecular dimers are arranged in a crystal as fundamental units. A low energy effective model includes an antisymmetric exchange interaction, as one of the dominant magnetic interactions. This interaction favors a 90 degree spin configuration, and competes with the Heisenberg-type exchange interaction. Stabilities of the magnetic ordered phases are examined by using the spin-wave theory, as well as the Schwinger-boson…
Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V_2O_3
Effects of the spin-orbit coupling (SOC) and magnetic field on excitonic insulating (EI) states are investigated. We introduce the two-orbital Hubbard model with the crystalline field splitting, which is a minimal model for discussing the exciton condensation in strongly correlated electron systems, and analyze its effective Hamiltonian in the strong correlation limit by using the mean-field theory. In the absence of the SOC and magnetic field, the ground state changes from the nonmagnetic band-insulating state to the EI state by increasing the Hund coupling. In an applied magnetic field, the …
We investigate the necessary conditions for the emergence of complex, noncoplanar magnetic configurations in a Kondo lattice model with classical local moments on the geometrically frustrated Shastry-Sutherland lattice and their evolution in an external magnetic field. We demonstrate that topologically nontrivial spin textures, including a new canted flux state, with nonzero scalar chirality arise dynamically from realistic short-range interactions. Our results establish that a finite Dzyaloshinskii-Moriya (DM) interaction is necessary for the emergence of these novel magnetic states when the …
V2O3 famously features all four combinations of paramagnetic vs antiferromagnetic, and metallic vs insulating states of matter in response to %-level doping, pressure in the GPa range, and temperature below 300 K. Using time-of-flight neutron spectroscopy combined with density functional theory calculations of magnetic interactions, we have mapped and analyzed the inelastic magnetic neutron scattering cross section over a wide range of energy and momentum transfer in the chromium stabilized antiferromagnetic and paramagnetic insulating phases (AFI & PI). Our results reveal an important magneti…
The Mott insulating state of the Hubbard model at half-filling could be depicted as a spin liquid of singly occupied sites with holon-doublon quantum fluctuations localized in pairs. In one dimension the behavior is captured by a finite value of the charge parity string correlator, which fails to remain finite when generalized to higher dimensions. We recover a definition of parity brane correlator which may remain nonvanishing in presence of interchain coupling, by assigning an appropriate fractional phase to the parity breaking fluctuations. In case of Hubbard ladders at half-filling, we fin…
The magnetic structure of Ca$_2$MnReO$_6$ double perovskite is investigated by neutron powder diffraction and bulk magnetization, showing dominant non-collinear Mn magnetic moments [$4.35(7)$ $μ_B$] that are orthogonally aligned with the small Re moments [$0.22(4)$ $μ_B$]. $Ab$-initio electronic structure calculations show that the strong spin-orbit coupling for Re $5d$ electrons combined with a relatively modest on-site Coulomb repulsion ($U_{eff}^{Re} \gtrsim 0.6$ eV) is sufficient to render this material insulating. This is a rare example of spin-orbit assisted Mott insulator outside the re…
The resistivities of the dilute, strongly-interacting 2D electron systems in the insulating phase of a silicon MOSFET are the same for unpolarized electrons in the absence of magnetic field and for electrons that are fully spin polarized by the presence of an in-plane magnetic field. In both cases the resistivity obeys Efros-Shklovskii variable range hopping $ρ(T) = ρ_0 \mbox{exp}[(T_{ES}/T)^{1/2}]$, with $T_{ES}$ and $1/ρ_0$ mapping onto each other if one applies a shift of the critical density $n_c$ reported earlier. With and withoug magnetic field, the parameters $T_{ES}$ and $1/ρ_0 = σ_0$ …
Sr2IrO4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at TN=240 K. We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4. This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x=0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic st…
Ground State Properties of the Doped Kitaev-Heisenberg Chain: Topological Superconducting and Mott Insulating Phases Driven by Magnetic Frustration
University of Warsaw（华沙大学）；TU Wien（维也纳工业大学）；Technical University Dresden（德累斯顿工业大学）；Institute for Theoretical Solid State Physics, IFW Dresden（德累斯顿联邦材料研究所理论固态物理研究所）
Measurement and control of magnetic order and correlations in real time is a rapidly developing scientific area relevant for magnetic memory and spintronics [1,15]. In these experiments an ultra-short laser pulse (pump) is first absorbed by excitations carrying electric dipole moment. These then give their energy to the magnetic subsystem monitored by a time-resolved probe. A lot of progress has been made in investigations of ferromagnets but antiferromagnets are more challenging. Here we introduce time-resolved two-magnon Raman scattering as a novel real time probe of magnetic correlations es…
Rare-earth titanates are Mott insulators whose magnetic ground state -- antiferromagnetic (AFM) or ferromagnetic (FM) -- can be tuned by the radius of the rare-earth element. Here, we combine phenomenology and first-principles calculations to shed light on the generic magnetic phase diagram of a chemically substituted titanate on the rare-earth site that interpolates between an AFM and a FM state. Octahedral rotations present in these perovskites cause the AFM order to acquire a small FM component -- and vice-versa -- removing any multicritical point from the phase diagram. However, for a wide…
The large thermal Hall conductivity recently detected in Mott insulating cuprates has been attributed to chiral neutral spin excitations. A quantum spin liquid with Majorana excitations, Chern number +/-4 and large thermal Hall conductivity is found to be an excited state of a frustrated Heisenberg model on the square lattice. Using a Majorana mean-field theory and exact diagonalizations, we explore two possible routes to achieve this chiral quantum spin liquid, an orbital effect of an applied magnetic field and spin orbit couplings as present in cuprates. In particular, we show how only the o…
Neutron spectroscopy measurements reveal short-range spin correlations near and above the ferromagnetic-paramagnetic phase transition in manganite materials of the form La$_{1-x}A_{x}$MnO$_{3}$, including samples with an insulating ground state as well as colossal magnetoresistive samples with a metallic ground state. Quasielastic magnetic scattering is revealed that forms clear ridges running along the [100]-type directions in momentum space. A simple model consisting of a conduction electron hopping between spin polarized Mn ions that becomes self-trapped after a few hops captures the essent…
We study the two-dimensional square lattice Hubbard model for small to moderate interaction strengths $1\leq U/t\leq 4$ by means of the ladder dual fermion approach. The non-local correlations beyond dynamical mean-field theory lower the potential energy, lead to a maximum in the uniform susceptibility and induce a pseudogap in the density of states. While the self-energy exhibits precursors of a possible insulating phase linked to the appearance of long-range fluctuations, the metallic phase persists within the accessible temperature range. Finite-size effects affect results qualitatively. Up…
The spin-orbit coupling (SOC) interactions, electron correlation effects and Hund coupling cooperate and compete with each other, leading to novel properties, quantum phase and non-trivial topological electronic behavior in iridium oxides. Because of the well separated IrO6 octahedra approaching cubic crystal-field limit, the hexagonal iridates Sr3MIrO6 (M = Sr, Na and Li) serves as a canonical model system to investigate the underlying physical properties that arises from the novel Jeff state. Based on density functional theory calculations complemented by Green's function methods, we systema…
The unusual magnetic and electronic ground states of 5d iridates has been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here we present experimental and theoretical results on Sr$_4$RhO$_6$ that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic…
Non-magnetic insulating phase induced by Jahn-Teller effect in RNiO$_3$
Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）；Molecular Foundry, Lawrence Berkeley National Laboratory（分子发现中心，劳伦斯伯克利国家实验室）；Institute for Theoretical Physics, Clausthal University of Technology（理论物理研究所，克劳塔尔工业大学）；Institute for Theoretical Physics, Georg-August-Universität Göttingen（理论物理研究所，哥廷根大学）
In the quest for quantum spin liquids in three spatial dimensions (3D), we study the half-filled Hubbard model on the simple cubic lattice with hopping processes up to third neighbors. Employing the variational cluster approach (VCA), we determine the zero-temperature phase diagram: In addition to a paramagnetic metal at small interaction strength $U$ and various antiferromagnetic insulators at large $U$, we find an intermediate-$U$ antiferromagnetic metal. Most interestingly, we also identify a non-magnetic insulating region, extending from intermediate to strong $U$. Using VCA results in the…
We have explored the critical metal-insulator phenomena for pyrochlore-type $R_2$Ir$_2$O$_7$, in which electron correlation strength and magnetic configuration are systematically controlled by varying the average rare-earth ionic radius ($R$=Nd$_{1-x}$Pr$_{x}$ and Sm$_{y}$Nd$_{1-y}$), external pressure, and magnetic field. Metal-insulator transitions in bulk are caused by increasing $x$ or tuning external pressure, indicating that the effective electron correlation is responsible for the transition. The metallic state intervenes between the paramagnetic insulating and antiferromagnetically ord…
We investigate competing insulating phases in nearly metallic zigzag carbon nanotubes, under conditions where an applied magnetic flux approximately closes the single particle gap in one valley. Recent experiments have shown that an energy gap persists throughout magnetic field sweeps where the single-particle picture predicts that the gap should close and reopen. Using a bosonic low-energy effective theory to describe the interplay between electron-electron interactions, spin-orbit coupling, and magnetic field, we obtain a phase diagram consisting of several competing insulating phases that c…
We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie temperature of 40 K. We attribute this behavior to the existence of nanoscale Fe-rich ferromagnetic domains acting as superparamagnets. By fitting the magnetization curves using the Langevin function representing superparamagnetism plus the paramagnetic lin…
We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (~ 150 ps) to fast (< 70 ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat assisted magnetic recording, metallization can locally t…
