Correlated electron systems, particularly iron-based superconductors, are extremely sensitive to strain, which inevitably occurs in the crystal growth process. Built-in strain of this type has been proposed as a possible explanation for experiments where nematic order has been observed at high temperatures corresponding to the nominally tetragonal phase of iron-based superconductors. Strain is assumed to produce linear defect structures, e.g. dislocations, which are quite similar to O vacancy chainlets in the underdoped cuprate superconductor YBCO. Here we investigate a simple microscopic model of dislocations in the presence of electronic correlations, which create defect states that can drive magnetic anisotropy of this kind, if spin orbit interaction is present. We estimate the contribution of these dislocations to magnetic anisotropy as detected by current torque magnetometry experiments in both cuprates and Fe-based systems.
We consider the role of potential scatterers in the nematic phase of Fe-based superconductors above the transition temperature to the (pi,0) magnetic state but below the orthorhombic structural transition. The anisotropic spin fluctuations in this region can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials which scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. We calculate the…
Magnetic order at room temperature induced by atomic lattice defects, like vacancies, interstitials or pairs of them, has been observed in a large number of different nonmagnetic hosts, such as pure graphite, oxides and silicon-based materials. High Curie temperatures and time independent magnetic response at room temperature indicate the extraordinary robustness of this new phenomenon in solid state magnetism. In this work, we review experimental and theoretical results in pure TiO$_2$ (anatase), which magnetic order can be triggered by low-energy ion irradiation. In particular, we discuss th…
In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetic…
We show that the strain-induced resistivity anisotropy in the tetragonal state of the representative underdoped Fe-arsenides BaFe$_2$As$_2$, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ is independent of disorder over a wide range of defect and impurity concentrations. This result demonstrates that the anisotropy in the in-plane resistivity in the paramagnetic orthorhombic state of this material is not due to elastic scattering from anisotropic defects, and is most easily understood in terms of an intrinsic anisotropy in the electronic structure.
Excited state optimization for strongly correlated quantum defects using ensemble variational Monte Carlo
Anthony J. Leggett Institute for Condensed Matter Physics, Department of Physics, Grainger College of Engineering, University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格雷格工程学院物理系安东尼·J·雷格特凝聚态物理研究所）
A longstanding puzzle for the lowest Landau level crystal phase has been an order of magnitude discrepancy between the theoretically calculated energy of the defects and the measured activation gap. We perform an extensive study of various kinds of defects in the correlated composite fermion crystal and find that the lowest energy defect is a six-fold symmetric "hyper-correlated bubble interstitial," in which an interstitial particle forms a strongly correlated bound state with a particle of the crystal. The energy of the bubble defect is a factor of $\sim$3 smaller than that of the lowest ene…
A photonic realization of Bloch oscillations (BOs) of two correlated electrons that move on a one-dimensional periodic lattice, based on spatial light transport in a square waveguide array with a defect line, is theoretically proposed. The signature of correlated BOs, such as frequency doubling of the oscillation frequency induced by particle interaction, can be simply visualized by monitoring the spatial path followed by an optical beam that excites the array near the defect line.
Using an effective one-band Hubbard model with disorder, we consider magnetic states of the correlated oxide interfaces, where effective hole self-doping and a magnetially ordered state emerge due to electronic and ionic reconstructions. By employing the coherent potential approximation, we analyze the effect of random oxygen vacancies on the two-dimensional magnetism. We find that the random vacancies enhance the ferromagnetically ordered state and stabilize a robust magnetization above a critical vacancy concentration of about c=0.1. In the strong-correlated regime, we also obtain a nonmonot…
Quantum Monte Carlo assessment of embedding for for strongly correlated defects: interplay between mean-field starting point and interactions
University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；University of California, Santa Barbara（加州大学圣塔芭芭拉分校）；Flatiron Institute（平顿研究所）；Bielefeld University（比勒费尔德大学）；Stony Brook University（石溪大学）
Magnetic anisotropy from interligand hopping in strongly correlated insulators: application to the magnon spectrum of CrI$_3$
RIKEN Center for Emergent Matter Science (CEMS)（RIKEN 新兴物质科学中心）；Zernike Institute for Advanced Materials, University of Groningen（格罗宁根大学泽尼克先进材料研究所）；CNR-SPIN Institute for Superconducting and other Innovative Materials and Devices（CNR-SPIN 超导及其他创新材料与器件研究所）
We propose that the dissipative dynamics of topological defects in a spiral state is responsible for the transport properties in the spin-glass phase of cuprates. Using the collective-coordinate method, we show that topological defects are coupled to a bath of magnetic excitations. By integrating out the bath degrees of freedom, we find that the dynamical properties of the topological defects are dissipative. The calculated damping matrix is related to the in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data.
Complexity in materials often arises from competing interactions at the atomic length scale. One such example are the strongly correlated heavy-fermion materials where the competition between Kondo screening and antiferromagnetic ordering is believed to be the origin of their puzzling non-Fermi-liquid properties. Insight into such complex physical behavior in strongly correlated electron systems can be gained by impurity doping. Here, we develop a microscopic theoretical framework to demonstrate that defects implanted in heavy-fermion materials provide an opportunity for unveiling competing in…
We study interference patterns and Friedel oscillations (FO) due to scattering from two or more localized impurities and scattering from extended inhomogeneities in the two-dimensional lattice systems of interacting fermions. Correlations between particles are accounted for by using an approximate method based on the real-space dynamical mean-field theory and a homogeneous self-energy approximation (HSEA), where the site-dependent part of the self-energy is neglected. We find that the interference maxima and minima change systematically as we vary the relative distance between the two impuriti…
The electron transport in a 1D conductor with an isolated local defect such as an impurity or a non-adiabatic contact is studied theoretically. New regime of conduction in correlated 1D systems is predicted beyond the well-known regime of tunneling resulting in the power-law I-V-curves. In this regime a quantum wire becomes "opened" at voltage biases above the threshold value $V_T$, giving rise to a rapid increase of the dc current, $\bar I$, accompanied by ac oscillations of frequency $f = \bar I/e$. The effect of ac generation is related to sliding of the Friedel oscillations of electronic d…
Previous work has demonstrated the existence of soliton defect states with charges +/- e/2 in the limits of zero and of infinite on-site Coulomb interactions in the one-dimensional (1D) quarter-filled band. For large but finite on-site Coulomb interaction, the low temperature 2k_F bond distortion that occurs within the 4k_F bond-distorted phase is accompanied by charge-ordering on the sites. We show that a ``re-integerization'' of the defect charge occurs in this bond-charge density wave (BCDW) state due to a ``binding'' of the fractional charges. We indicate briefly possible implications of t…
Transformation of Coulomb interaction terms to the pseudo-orbital basis constituted by $J=1/2$ and $3/2$ states arising from spin-orbit coupling provides a versatile tool. This formalism is applied to investigate magnetic anisotropy effects on low-energy spin-wave excitations as well as high-energy spin-orbit exciton modes in $\rm Sr_2 Ir O_4$. The Hund's coupling term explictly yields easy-plane anisotropy, resulting in gapless (in-plane) and gapped (out-of-plane) modes, in agreement with recent resonant inelastic x-ray scattering (RIXS) measurements. The collective mode of inter-orbital, spi…
At the interface between two distinct materials desirable properties, such as superconductivity, can be greatly enhanced, or entirely new functionalities may emerge. Similar to in artificially engineered heterostructures, clean functional interfaces alternatively exist in electronically textured bulk materials. Electronic textures emerge spontaneously due to competing atomic-scale interactions, the control of which, would enable a top-down approach for designing tunable intrinsic heterostructures. This is particularly attractive for correlated electron materials, where spontaneous heterostruct…
We perform ab-initio calculations on Bi$_\mathrm{Se}$ antisite defects in the surface of Bi$_2$Se$_3$, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptional large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments becomes more localized, and no energy gap is present.
We have studied the magnetic torque in uranium monosulfide (US) single crystals to explore the magnetic anisotropy in this material. Uranium monosulfide crystallizes in cubic, NaCl-type of crystal structure and exhibits the largest magneto-crystalline anisotropy observed in cubic systems. By performing detailed torque measurements we observe a strongly anisotropic behavior in the paramagnetic phase due to crystal defects and quadrupolar pair interactions. Our studies also confirm the presence of a large anisotropy in the ferromagnetic state of the US system with the <100>, <111>, and <110> dir…
Unveiling the critical role of interfacial strain in adjusting electronic phase transitions in correlated vanadium dioxide
Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education & School of Materials Science and Engineering, Shanxi Normal University（山西师范大学 磁性分子与磁信息材料教育部重点实验室 & 材料科学与工程学院）；Research Institute of Materials Science, Shanxi Key Laboratory of Advanced Magnetic Materials and Devices, Shanxi Normal University（山西师范大学 材料科学研究所 & 山西省先进磁性材料与器件重点实验室）
In a common paradigm, the electronic structure of condensed matter is divided into weakly and strongly correlated compounds. While conventional band theory usually works well for the former class, many-body effects are essential for the latter. Materials like the familiar SrTiO$_3$ compound that bridge or even abandon this characterization scheme are highly interesting. Here it is shown by means of combining density functional theory with dynamical-mean field theory that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly-correlated $t_{2g}$ low-energy quasiparticles a…
