Antiferromagnetic Mott Insulator Research Articles

A microscopic view of checkerboard and striped charge orders through doping antiferromagnetic Mott insulator

The emergence of charge order in doped Mott insulators has received considerable attention due to its relevance to a variety of realistic materials and experiments. To investigate the interplay between magnetic and charge order, we have studied the semiclassical Kondo lattice model, which includes both electronic and magnetic degrees of freedom. By combining Langevin dynamical simulations with the kernel polynomial method, our results reveal the presence of charged stripe order, checkerboard order, and non-uniform charge disorder in the near-half-filling regime. Importantly, our simulations show that both the doping level and the strength of the s-d exchange coupling play a crucial role in facilitating charge order formation. These phases give rise to distinct electronic structures as well as excitations in the magnetic dynamics, providing insights into the underlying mechanism of charge ordering phenomena.

Scattering theory of mesons in doped antiferromagnetic Mott insulators: Multichannel perspective and Feshbach resonance

Scattering theory of mesons in doped antiferromagnetic Mott insulators: Multichannel perspective and Feshbach resonance

A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator

A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator

Antiferromagnetic spintronics: From metals to functional oxides

Antiferromagnetic spintronics exploits unique properties of antiferromagnetic materials to create new and improved functionalities in future spintronic applications. Here, we briefly review the experimental efforts in our group to unravel spin transport properties in antiferromagnetic materials. Our investigations were initially focused on metallic antiferromagnets, where the first evidence of antiferromagnetic spin-transfer torque was discovered. Because of the lack of metallic antiferromagnets, we then shifted towards antiferromagnetic Mott insulators, where a plethora of transport phenomena was found. For instance, we observed a very large anisotropic magnetoresistance, which can be used to detect the magnetic state of an antiferromagnet. We also observed reversible resistive switching and now provide unequivocal evidence that the resistive switching is associated with structural distortions driven by an electric field. Our findings support the potential of electrically controlled functional oxides for various memory technologies.

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Ab initio materials design of superconductivity in d9 nickelates

Motivated by the recent theoretical materials design of superconducting d9 nickelates for which the charge transfer from the NiO2 layer to the block layer is completely suppressed [M. Hirayama et al., Phys. Rev. B 101, 075107 (2020)], we perform a calculation based on the dynamical vertex approximation and obtain the phase diagram of RbCa2NiO3 and A2NiO2Br2, where A is a cation with a valence of 2.5+. We show that the phase diagram of these nickelates exhibits the same essential features as those found in cuprates. Namely, superconductivity appears upon hole-doping into an antiferromagnetic Mott insulator, and the superconducting transition temperature shows a dome-like shape. This demonstrates that the electron correlations play an essential role in nickelate superconductors, and we can control them by changing block layers.

Ultrafast signatures of spin and orbital order in antiferromagnetic α-Sr2CrO4

The antiferromagnetic Mott insulator α-Sr2CrO4 possesses multiple spin and orbital ordered phases, but their unique interplay is still relatively unexplored. Here, we used femtosecond optical spectroscopy to study ultrafast spin and orbital ordering dynamics in α-Sr2CrO4 through their non-equilibrium response to photoexcitation. By varying the pump photon energy, we selectively drove inter-site spin hopping between neighboring Cr t2g orbitals and charge transfer-type transitions between oxygen 2p and Cr eg orbitals. The resulting transient reflectivity dynamics revealed temperature-dependent anomalies across the Néel temperature for spin ordering as well as the transition temperatures linked to different types of orbital order. Our results reveal distinct relaxation timescales for spin and orbital orders in α-Sr2CrO4 and provide experimental evidence for the phase transition at TO, possibly related to antiferro-type orbital ordering.

Continuous collapse of antiferromagnetic order toward a quantum critical point in a single-component molecular material

We uncover the pressure variation of electronic states of the single-component molecular material $\mathrm{Pd}{(\mathrm{tmdt})}_{2}$ that hosts the antiferromagnetic (AFM) Mott insulator at ambient pressure, using $^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) spectroscopy. Under pressures up to 8.5 kbar, the NMR spectral broadening and the peak structure in the temperature dependence of the spin-lattice relaxation rate $1\text{/}{T}_{1}$ are observed and indicate that the AFM transition persists in this pressure range. However, the Neel temperature and the AFM magnetic moment are continuously decreased toward a critical pressure of $\ensuremath{\sim}9$ kbar. At 10 kbar, these AFM features in the spectra and $1\text{/}{T}_{1}$ are suppressed, and instead $1\text{/}{T}_{1}T$ exhibits a critical increase toward zero temperature, complying with the self-consistent renormalization (SCR) theory in two-dimensional AFM itinerant magnets [$1\text{/}{T}_{1}T$ $\ensuremath{\sim}$ (${T+\ensuremath{\theta})}^{\ensuremath{-}1}$], which also explains the systematic change of $1\text{/}{T}_{1}T$ at higher pressures. The present results indicate that $\mathrm{Pd}{(\mathrm{tmdt})}_{2}$ shows a pressure-induced quantum phase transition from the AFM Mott insulator to the paramagnetic metal at the quantum critical point of $\ensuremath{\sim}9$ kbar.

Spin singlet and quasiparticle excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AF) Mott insulator to a superconducting (SC) metal in the Bi$_2$Sr$_2$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) cuprate using the electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation originate from the AF Mott insulator, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition, can be interpreted as the blocking of charge carriers by the enhancement of the antiferromagnetic correlations as the temperature drops. We find that the energy scale of the pseudogap, $\Delta_{\textrm{pg}}(p)$, closely follows the one of the spin singlet excitation, $\Delta_{\textrm{sse}}(p)$, with doping $p$. The quasiparticles lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap, $\Delta_{\textrm{sc}}^{\textrm{max}}$ and the SC transition temperature \Tc are linked in an extended range of doping such as $\Delta_{\textrm{sc}}^{\textrm{max}}(p) \propto \Delta_{\textrm{sse}}(p)\, T_c(p)$. This relation suggests that the AF correlations play a key role in the mechanism of superconductivity.

Giant stress response of terahertz magnons in a spin-orbit Mott insulator

Magnonic devices operating at terahertz frequencies offer intriguing prospects for high-speed electronics with minimal energy dissipation However, guiding and manipulating terahertz magnons via external parameters present formidable challenges. Here we report the results of magnetic Raman scattering experiments on the antiferromagnetic spin-orbit Mott insulator Sr2IrO4 under uniaxial stress. We find that the energies of zone-center magnons are extremely stress sensitive: lattice strain of 0.1% increases the magnon energy by 40%. The magnon response is symmetric with respect to the sign of the applied stress (tensile or compressive), but depends strongly on its direction in the IrO2 planes. A theory based on coupling of the spin-orbit-entangled iridium magnetic moments to lattice distortions provides a quantitative explanation of the Raman data and a comprehensive framework for the description of magnon-lattice interactions in magnets with strong spin-orbit coupling. The possibility to efficiently manipulate the propagation of terahertz magnons via external stress opens up multifold design options for reconfigurable magnonic devices.

Modulation of the NiOx bandgap by controlling oxygen stoichiometry

Transition metal oxides are a class of functional materials widely used in optoelectronics, spintronics, and memory technology. The oxygen stoichiometry of these oxides plays a vital role in determining their electronic, optical, and thermal properties. Post-growth annealing in ozone has shown to be effective in modifying these properties. Here, we choose NiO, an antiferromagnetic Mott insulator in perfect stoichiometry, as an example to show that its stoichiometry can be tuned continuously in a broad range by the control of the oxidation power during growth or a post-growth topotactic reduction process. The bandgap of the as-processed NiOx films was modulated in accordance with their resistivity, lattice constant, and Ni chemical valence. This method can be readily applied to other transition metal oxides for the optimization of their properties.

Dirac fermions with plaquette interactions. III. SU(N) phase diagram with Gross-Neveu criticality and first-order phase transition

Inspired by our recent works[1, 2] of SU(2) and SU(4) Dirac fermions subjected to plaquette interactions on square lattice, here we extend the large-scale quantum Monte Carlo investigations to the phase digram of correlated Dirac fermions with SU(6) and SU(8) symmetries subjected to the plaquette interaction on the same lattice. From SU(2) to SU(8), the rich phase diagram exhibits a plethora of emerging quantum phases such as the Dirac semimetal, the antiferromagnetic Mott insulator, valence bond solid (VBS) and the Dirac spin liquid and phase transitions including the Gross-Neveu chiral transitions with emergent continuous symmetry, the deconfined quantum criticality and the first order transition between interaction-driven columnar VBS and plaquette VBS. These rich phenomena coming from the simple-looking lattice models, firmly convey the message that the interplay between the $SU(N)$ Dirac fermions -- with enhanced internal symmetries -- and extended plaquette interactions -- beyond the on-site Hubbard type -- is the new playground to synthesise novel highly entangled quantum matter both at the model level and with experimental feasibilities.

First principles investigation of spin-orbit coupling driven magnetism of the 3d−5d double-perovskite (Sr/Ca)2FeIrO6

The interplay of spin-orbit coupling (SOC) with the other energy scales gives rise to various novel and interesting quantum phenomena in the Ir based double perovskites. In recent findings the double-perovskites ${\mathrm{Sr}}_{2}{\mathrm{FeIrO}}_{6}$ (SFIO) and ${\mathrm{Ca}}_{2}{\mathrm{FeIrO}}_{6}$ (CFIO) have been reported to fall under the category of a SOC driven antiferromagnetic (AFM) Mott insulator. Using density functional theory based first principles calculations, we have performed a comparative electronic structure investigation of these compounds. We reveal the microscopic origin of the difference in the AFM transition temperature observed in the experiments through the calculated exchange interactions and the Wannier functions. We addressed the role of SOC in dictating the ground state properties of these two compounds. Although both of the compounds are isoelectronic and isovalent, however, the effective strength and the impact of SOC is very different and nonmonotonic in these compounds, due to the delicate balance with the other energy scales. Our study advances the understanding of the nontrivial role of SOC in driving ground state magnetic properties of the Ir based oxides in general and explores the opportunities for further designing of the SOC driven quantum phases.

Quasi-Two-Dimensional Anomalous Hall Mott Insulator of Topologically Engineered Jeff=1/2 Electrons

We investigate an experimental toy-model system of a pseudospin-half square-lattice Hubbard Hamiltonian in [(SrIrO3)1/(CaTiO3)1] to include both nontrivial complex hopping and moderate electronic correlation. While the former induces electronic Berry phases as anticipated from the weak-coupling limit, the later stabilizes an antiferromagnetic (AFM) Mott insulator ground state in analogous to the strong-coupling limit. Their combined results in the real system are found to be an anomalous Hall effect with a non-monotonic temperature dependence due to the self-competition of the electron-hole pairing in the Mott state, and an exceptionally large Ising anisotropy that is captured as a giant magnon gap beyond the superexchange approach. The unusual phenomena highlight the rich interplay of electronic topology and electronic correlation in the intermediate-coupling regime that is largely unexplored and challenging in theoretical modelling.

Charge-transfer effect in Fe 2p core-level x-ray photoemission spectra of trivalent Fe oxides: LDA + DMFT study

Motivated by recent hard x-ray photoemission spectroscopy (XPS) experiment for trivalent Fe oxides Sr2FeMoO6 (ferrimagnetic correlated metal) and LaFeO3 (antiferromagnetic Mott insulator) (Phuyal et al 2021 J. Phys. Chem. C 125 11249–56), we present a theoretical analysis of the Fe 2p core-level spectra using a computational method based on local density approximation combined with dynamical mean-field theory. We find that a nonlocal screening (NLS) effect in the XPS final states is crucial for interpreting the experimental XPS result of both the Fe oxides. A close relationship between the NLS feature in core-level spectra and a long-range magnetic ordering is emphasized.

Dynamical signatures of thermal spin-charge deconfinement in the doped Ising model

The mechanism underlying charge transport in strongly correlated quantum systems, such as doped antiferromagnetic Mott insulators, remains poorly understood. Here we study the expansion dynamics of an initially localized hole inside a two-dimensional (2D) Ising antiferromagnet at variable temperature. Using a combination of classical Monte Carlo and a truncated basis method, we reveal two dynamically distinct regimes: A spin-charge confined region below a critical temperature $T^*$, characterized by slow spreading, and a spin-charge deconfined region above $T^*$, characterized by an unbounded diffusive expansion. The deconfinement temperature $T^*\approx 0.65 J_z$ we find is around the N\'eel temperature $T_{\rm N} = 0.567 J_z$ of the Ising background in 2D, but we expect $T^* < T_{\rm N}$ in higher dimensions. In both regimes we find that the mobile hole does not thermalize with the Ising spin background on the considered time scales, indicating weak effective coupling of spin- and charge degrees of freedom. Our results can be qualitatively understood by an effective parton model, and can be tested experimentally in state-of-the-art quantum gas microscopes.

Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating NiS2 single crystals

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${\mathrm{NiS}}_{2}$. Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an $\mathit{electrochemical}$ gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional $\mathit{electrostatic}$ gate effect previously seen in pyrite ${\mathrm{FeS}}_{2}$. We attribute this stark difference in electrochemical vs electrostatic gating response in ${\mathrm{NiS}}_{2}$ and ${\mathrm{FeS}}_{2}$ to the much larger S diffusion coefficient in ${\mathrm{NiS}}_{2}$. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric ${\mathrm{H}}_{2}\mathrm{O}$ provides an O reservoir. The present study of ${\mathrm{NiS}}_{2}$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.

Strong pairing in mixed-dimensional bilayer antiferromagnetic Mott insulators

Interacting many-body systems in reduced-dimensional settings, such as ladders and few-layer systems, are characterized by enhanced quantum fluctuations. Recently, two-dimensional bilayer systems have sparked considerable interest because they can host unusual phases, including unconventional superconductivity. Here we present a theoretical proposal for realizing high-temperature pairing of fermions in a class of bilayer Hubbard models. We introduce a general and highly efficient pairing mechanism for mobile charge carriers in doped antiferromagnetic Mott insulators. The pairing is caused by the energy that one charge gains when it follows the path created by another charge. We show that this mechanism leads to the formation of highly mobile but tightly bound pairs in the case of mixed-dimensional Fermi–Hubbard bilayer systems. This setting is closely related to the Fermi–Hubbard model believed to capture the physics of copper oxides, and can be realized in currently available ultracold atom experiments.

Stripes and spin-density waves in the doped two-dimensional Hubbard model: Ground state phase diagram

We determine the spin and charge orders in the ground state of the doped two-dimensional (2D) Hubbard model in its simplest form, namely with only nearest-neighbor hopping and on-site repulsion. At half-filling, the ground state is known to be an antiferromagnetic Mott insulator. Doping Mott insulators is believed to be relevant to the superconductivity observed in cuprates. A variety of candidates have been proposed for the ground state of the doped 2D Hubbard model. A recent work employing a combination of several state-of-the-art numerical many-body methods established the stripe order as the ground state near $1/8$ doping at strong interactions. In this paper, we apply one of these methods, the cutting-edge constrained-path auxiliary field quantum Monte Carlo (AFQMC) method with self-consistently optimized gauge constraints, to systematically study the model as a function of doping and interaction strength. With careful finite size scaling based on large-scale computations, we map out the ground state phase diagram in terms of its spin and charge order. We find that modulated antiferromagnetic order persists from near half-filling to about $1/5$ doping. At lower interaction strengths or larger doping, these ordered states are best described as spin-density waves, with essentially delocalized holes and modest oscillations in charge correlations. When the charge correlations are stronger (large interaction or small doping), they are best described as stripe states, with the holes more localized near the node in the antiferromagnetic spin order. In both cases, we find that the wavelength in the charge correlations is consistent with so-called filled stripes.

Nonequilibrium thermal state of a voltage-biased Mott insulator

We establish the nonequilibrium thermal phases of a voltage driven antiferromagnetic Mott insulator in three dimensions, realized at steady state under a voltage bias. Starting from the Keldysh action for the half-filled Hubbard model, we derive an effective Langevin equation for the ``slow'' magnetic variables. The coupling of electrons to these degrees of freedom determine the transport properties. At low temperature we find a voltage-driven discontinuous insulator-metal transition, along with hysteresis. We map the suppression of the N\'eel temperature ${T}_{N}$ and pseudogap temperature ${T}_{\text{pg}}$ with increasing voltage, and discover that the biased Mott insulator has a finite temperature insulator-metal transition. The low temperature results resolve an experimental puzzle about hysteresis, and the thermal results make testable predictions on spectra and nonlinear transport.