Paramagnetic Insulator Research Articles

Local lattice distortions and electronic phases in perovskite manganite Pr0.5Sr0.5MnO3

We use variable temperature and magnetic field total x-ray scattering to study the crystal structure of the strongly correlated Pr0.5Sr0.5MnO3 perovskite, which is a paramagnetic insulator at room temperature, becomes a ferromagnetic metal at 272 K and, upon further decreasing the temperature, turns into an antiferromagnetic insulator at 105 K. We find that a model featuring a monoclinic symmetry captures the structure and its temperature and field evolution well, eliminating the need to evoke a phase segregation scenario as done in prior studies. It appears that coupled variations in Mn–oxygen bonding distances and angles from their values in an undistorted perovskite lattice, i.e., coupled local lattice distortions, assist the phase transitions in Pr0.5Sr0.5MnO3, contributing to its unique physical properties. Local structural distortions thus emerge as an important degree of freedom in strongly correlated systems, in particular perovskite manganates, and, therefore, they should be fully accounted for when their fascinating physics is considered.

Observation of Nagaoka polarons in a Fermi-Hubbard quantum simulator.

Quantum interference can deeply alter the nature of many-body phases of matter1. In the case of the Hubbard model, Nagaoka proved that introducing a single itinerant charge can transform a paramagnetic insulator into a ferromagnet through path interference2-4. However, amicroscopic observation of this kinetic magnetism induced by individually imaged dopants has been so far elusive. Here we demonstrate the emergence of Nagaoka polarons in a Hubbard system realized with strongly interacting fermions in a triangular optical lattice5,6. Using quantum gas microscopy, we image these polarons asextended ferromagnetic bubbles around particle dopants arising from the local interplay of coherent dopant motion and spin exchange. By contrast, kinetic frustration due to the triangular geometry promotes antiferromagnetic polarons around hole dopants7. Our work augurs the exploration of exotic quantum phases driven by charge motion in strongly correlated systems and over sizes that are challenging for numerical simulation8-10.

A study on the kinetic arrest of magnetic phases in nanostructured Nd0.6Sr0.4MnO3 thin films

The Nd0.6Sr0.4MnO3 (NSMO) manganite system exhibits a phase transition from paramagnetic insulating (PMI) to ferromagnetic metallic (FMM) state around its Curie temperature T C = 270 K (bulk). The morphology-driven changes in the kinetically arrested magnetic phases in NSMO thin films with granular and crossed-nano-rod-type morphology are studied. The manganite thin films at low temperatures possess a magnetic glassy state arising from the coexistence of the high-temperature PMI and the low-temperature FMM phases. The extent of kinetic arrest and its relaxation was studied using the ‘cooling and heating in unequal field (CHUF)’ protocol in magnetic and magnetotransport investigations. The sample with rod morphology showed a large extent of phase coexistence compared to the granular sample. Further, with a field-cooling protocol, time-evolution studies were carried out to understand the relaxation of arrested magnetic phases across these morphologically distinct thin films. The results on the devitrification of the arrested magnetic state are interpreted from the point of view of homogeneous and heterogeneous nucleation of the ferromagnetic phase in the paramagnetic matrix with respect to temperature.

Layered kagome compound Na2Ni3S4 with topological flat band

We report structural and electronic properties of Na2Ni3S4, a quasi-two-dimensional compound composed of alternating layers of [Ni3S4]2− and Na+. The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom. Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV. Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated d xz -orbital with C 3 + T symmetry in the presence of crystal-field splitting.

Underpinnings behind the magnetic order-to-disorder transition and property anomaly of disproportionated insulating samarium nickelate

Perovskite nickelates (RNiO3), as benchmark materials for exploring underlying physics of phase transitions in strong correlated systems, are well-known for their rich structural characteristics and physicochemical properties. Understanding phase transition from the spin-ordered antiferromagnetic (AFM) state to the spin-disordered paramagnetic (PM) state of RNiO3 requires accurate models at finite temperatures. Here, we investigate the underpinning physics behind the property anomaly and AFM-to-PM transition in samarium nickelate (SmNiO3) using a thermodynamic zentropy approach, which depicts the temperature-dependent total entropy for a system of interest via a nested formula through integration of quantum mechanics and statistical mechanics. The SmNiO3 displays the ground-state of AFM insulator as well as the PM metal and PM insulator at elevated temperatures. It turns out that the magnetic configurational entropy induced by competition among different spatially fluctuating polymorphous spin configurations, contributes to the specific-heat anomaly and the phase transition of SmNiO3 from AFM to PM state. The predicted Néel temperature of 241±25 K for SmNiO3 is in satisfactory agreement with the experimental value. Unlike empirical interpretations in literature, the present work indicates that the magnetic order-to-disorder transition in SmNiO3 is associated with the magnetic configurational entropy due to thermal mixture of multiple spin configurations at elevated temperatures, which can be quantitively captured by the first-principles based thermodynamic zentropy approach.

Highly Efficient Spin Transport in a Paramagnetic Insulator at Room Temperature

AbstractThe exploration of new materials exhibiting excellent spin transport properties, particularly those capable of efficiently transmitting pure spin currents at room temperature, is a crucial aspect of spintronics. This work reports the observation of room‐temperature spin transport in a paramagnetic insulator Gd3Ga5O12 (GGG). By measuring the longitudinal spin Seebeck voltage, spin Hall magnetoresistance, and anomalous Hall resistance in a Y3Fe5O12 (YIG)/GGG/Pt heterostructure at room temperature, the spin current is found to propagate in GGG up to a distance of 7 nm through the paramagnons, which are the collective excitations of the local spin within the paramagnet. Remarkably, this spin propagation phenomenon occurs at a small magnetic field and irrespective of whether the spin current injected into GGG is thermally induced from YIG or electrically generated through the spin Hall effect of Pt. Moreover, the inclusion of a thin GGG spacer layer increases the thermal spin current from YIG to Pt by 31%. Calculations based on the diffusive magnon transport model indicate high spin conductance at the GGG/Pt interface. These findings highlight the potential of a wide range of paramagnetic materials in facilitating spin transport and advancing the development of spintronic devices.

Detection of paramagnetic to charge-ordered insulator phase transitions in phase separated (La[formula omitted]Pr[formula omitted])0.67Ca0.33MnO3 thin films

Hole-doped manganese oxides (manganites) exhibit phase separation which allows for the manipulation of phase transitions with the application of external stimuli. The three competing phases of the manganite (La1−yPry)1−xCaxMnO3 (LPCMO) are paramagnetic insulating (PMI), charge-ordered insulating (COI), and ferromagnetic metallic (FMM). In LPCMO thin films, the substrate induced strain often makes it difficult to distinguish between the two insulating phases (PMI and COI) using resistivity and/or magnetization measurements. Here we report a method for determining the PMI-COI transition temperature of LPCMO thin films by fitting resistance vs. temperature (R−T) data to the Arrhenius equation. This method also yields the activation energy in the COI phase which agrees with direct measurements using scanning tunneling microscopy. Analysis of the R−T data suggests that adiabatic polaron hopping is the transport mechanism in both the PMI and COI phases while variable range hopping (VRH) is the dominant mechanism in the mixed FMM/COI state.

Positive and Negative Pressure Regimes in Anisotropically Strained V2O3 Films

AbstractThe metal‐insulator phase transitions in V2O3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V2O3 films are explored with nearly the same contraction along the c‐axis, but different degrees of ab‐plane expansion. With small ab‐plane expansion, the films behave similar to bulk V2O3 under hydrostatic pressure. However, with large ab‐plane expansion, the films are driven into the “negative pressure” regime, similar to that of Cr‐doped V2O3, exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180–500 K. This shows that c‐axis contraction alone, or an increase in c/a ratio is insufficient for inducing “negative pressure” effects. Actually, c‐axis contraction alone destabilizes the two insulating phases of V2O3, whereas a‐axis expansion tends to stabilize them. The effects of strain are modeled using density functional theory providing good agreement with experimental results. The findings show that chemical pressure alone cannot account for the phase diagram of (V1−xCrx)2O3. This work enables to manipulate a Mott transition above room temperature, thereby expanding the opportunities for applications of V2O3 in novel electronics.

Epitaxial growth of V2O3 thin films on Si(1 1 1) by molecular beam epitaxy

Vanadium sesquioxide (V2O3) is a strongly correlated electron material exhibiting two distinct metal–insulator transitions that can be tuned via strain, doping, or pressure, making it an interesting material for new-generation sensors or smart devices. For this purpose, it is required to achieve well-ordered epitaxial thin film growth with high-quality electrical and optical properties on technologically relevant substrates. We report the successful growth of epitaxial thin films of V2O3 via molecular beam epitaxy, in the paramagnetic insulating (PI) phase on the (111) plane of silicon, by tailoring the growth conditions. Extensive electrical, structural, and morphological characterization both in situ and ex situ has been performed on all samples. The structural analysis reveals that temperature plays a more impactful role in affecting the thin film microstructures than the oxygen partial pressure. When the epitaxy of V2O3 occurs on the unoxidized (111) plane of silicon, four equivalent epitaxial domains begin to form, leading to twin boundaries in the bulk of the film. The considerable lattice mismatch between silicon and V2O3 induces the growth of the corundum PI phase. Lastly, small deviations from stoichiometry due to different oxygen inflow during growth alter significantly the resistivity change upon cooling.

Rise and fall of Mott insulating gaps in YNiO3 paramagnets as a reflection of symmetry breaking and remaking

The ${\mathrm{YNiO}}_{3}$ nickelate is a paradigm $d$-electron oxide that manifests the intriguing temperature-mediated sequence of three phases transitions from (i) magnetically ordered insulator to (ii) paramagnetic (PM) insulator and then to (iii) PM metal. Such phenomena raised the question of the nature of the association of magnetism and structural symmetry breaking with the appearance in (i) and (ii) and disappearance in (iii) of insulating band gaps. It is demonstrated here that first-principles mean-field--like density-functional theory (DFT), driven by molecular dynamics temperature evolution, can describe not only the origin of the magnetically long-range ordered insulating phase (i), but also the creation of an insulating paramagnet (ii) that lacks spin- long-range order, and of a metallic paramagnet (iii) as temperature rises. This approach provides the patterns of structural and magnetic symmetry breaking at different temperatures, in parallel with band gaps obtained when the evolving geometries are used as input to DFT electronic band-structure calculations. This disentangles the complex interplay among spin, charge, and orbital degrees of freedom. Analysis shows that the success in describing the rise and fall of the insulating band gaps along the phase transition sequence is enabled by allowing sufficient flexibility in describing diverse local structural and magnetic motifs as input to DFT. This entails the use of sufficiently large supercells that allow expressing structural disproportionation of octahedra, as well as a description of PM phases as a distribution of local magnetic moments (rather than using a single averaged moment). It appears that the historic dismissal of mean-field--like DFT as being unable to describe such Mott-like transitions was premature, as it was based on consideration of averaged crystallographic unit cells, a description that washes out local symmetry-breaking motifs. The magnetically ordered insulating ${\mathrm{YNiO}}_{3}$ phase (i) and the PM insulating phase (ii) result in DFT from allowing symmetry breaking, evident already by considering the athermal internal energy. In contrast, the PM metallic phase (iii) is formed thermally by smearing out thus weakening symmetry breaking. Analysis of snapshots of the different forms of structural vs magnetic symmetry breaking shows that only the loss of the polymorphous distribution of magnetic moments existing in (ii) causes the fall of the band gap, resulting in the metallic state in (iii). The interesting conclusion is that such a description of the rise [in phases (i) and (ii)] and fall [in phase (iii)] of the insulating gap does not rely on the traditional Mott-like strong correlation understanding, but on breaking and remaking of magnetic and structural symmetries reflected in energy lowering.

Continuous Mott Transition in Moiré Semiconductors: Role of Long-Wavelength Inhomogeneities.

Recent experiments in moiré transition metal dichalcogenide materials have reported the observation of a continuous bandwidth-tuned transition from a metal to a paramagnetic Mott insulator at a fixed filling of one electron per moiré unit cell. The electrical transport measurements reveal a number of puzzling features that are seemingly at odds with the theoretical expectations of an interaction-induced, but disorder-free, bandwidth-tuned metal-insulator transition. In this Letter, we include the effects of long-wavelength inhomogeneities, building on the results for a continuous metal-insulator transition at fixed filling in the clean limit. We examine the effects of mesoscale inhomogeneities near the critical point on transport using the framework of random resistor networks, highlighting the salient differences from a simple percolation-based picture. We place our results in the context of recent and ongoing experiments.

Energy relaxation in (La0.6Pr0.4)0.7Ca0.3MnO3 films across the metal-insulator transition

The colossal magnetoresistive manganite ${({\mathrm{La}}_{0.6}{\mathrm{Pr}}_{0.4})}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (LPCMO) undergoes a ferromagnetic (FM) metal to paramagnetic insulator phase transition at around 195 K, which develops via an intermediate polaronic state strongly susceptible to external magnetic fields. Transient reflectivity studies of LPCMO on timescales from sub-picoseconds to nanoseconds reveal that the overall system dynamics are strongly influenced by the phase transition with a sharp and strong decrease in the relaxation time while crossing from the ferromagnetic metallic phase into the paramagnetic insulating phase. We show that the long relaxation times of the reflectivity after nonequilibrium excitation within the FM phase are caused by the slow recovery of the ordered FM state after laser-induced demagnetization. Control of the nonequilibrium dynamics close to the phase transition is demonstrated by applying moderate magnetic fields below 1 T.

Mechanism of paramagnetic spin Seebeck effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

Spin-excitons in high magnetic fields

Spin-Excitons are sharp branches of dispersive triplet excitations found with energies within the gap of paramagnetic Kondo Insulators, which exist for a restricted range of wave-vectors around Q̲a=(π,π,π). The application of a high applied magnetic field is expected to drive the non-magnetic Kondo Insulating State to become unstable. The instability occurs through different routes, which depend on the strength of the quasi-particle interactions. One route towards instability, which occurs for strong quasi-particle interactions, is by a transition to a field-induced antiferromagnetic state. The second mode of instability, which occurs for weak interactions, is simply due to the magnetic field closing the hybridization gap. In this route, the non-magnetic Kondo insulator becomes unstable to a spin-polarized metallic state. Here we examine the effects of an applied magnetic field on spin-excitons. In the presence of a weak magnetic field, the excitations split into three branches, where the splittings of the dispersion relation are proportional to the applied field. The three branches exist for different ranges of Q̲ and have different intensities. For strong interactions, the lowest-energy branch, which has the weakest intensity and exists for the smallest Q̲ range, completely softens at the transition to the antiferromagnetic phase. This lower branch of excitations is found to disappear at the boundary separating the two modes of instability, leaving the upper two branches intact.

Spin-orbit excitons and electronic configuration of the 5d4 insulator Sr3Ir2O7F2

Here, we report on the low-energy excitations within the paramagnetic spin-orbit insulator ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}{\mathrm{F}}_{2}$ studied via resonant inelastic x-ray scattering, ab initio quantum chemical calculations, and model Hamiltonian simulations. This material is a unique ${d}^{4} {\mathrm{Ir}}^{5+}$ analog of ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ that forms when F ions are intercalated within the SrO layers spacing the square lattice ${\mathrm{IrO}}_{6}$ bilayers of ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. Due to the large distortions about the ${\mathrm{Ir}}^{5+}$ ions, our computations demonstrate that a large single-ion anisotropy yields an $S=1$ ($L\ensuremath{\approx}1, J\ensuremath{\approx}0$) ground-state wave function. Weakly coupled, excitonic modes out of the ${S}_{z}=0$ ground state are observed and are well described by a phenomenological spin-orbit exciton model previously developed for $3d$ and $4d$ transition metal ions. The implications of our results regarding the interpretation of previous studies of hole-doped iridates close to ${d}^{4}$ fillings are discussed.

High resolution ferromagnetic and antiferromagnetic correlations in La0.7Ca0.3MnO3

La0.7Ca0.3MnO3 (LCMO30) is a perovskite manganite with a phase transition (TC = 238 K) between a paramagnetic insulator and a ferromagnetic metal. We use single-crystal, inelastic neutron scattering to refine previous work on the low energy (ℏω < 10 meV) magnetic excitations. At 300 K the spectrum consists of a diffusive ferromagnetic component at the pseudo-cubic Brillouin zone center coexisting with a quasielastic excitation located at the pseudo-cubic Brillouin zone edge. Both components are anti-correlated with ferromagnetic long-range order, showing weakened intensity at 215 K in the ferromagnetic phase. Polarized neutron scattering shows both features to be purely magnetic in origin. This indicates the simultaneous presence of ferromagnetic and antiferromagnetic correlations (located, respectively, at the zone center and zone edge) in a three-dimensional perovskite, while this coexistence had been previously observed only in a bilayer compound.

Adjusting the K-doping of La1-xKxMnO3 (0.1 ≤ x ≤ 0.35) films to obtain high TCR and LFMR at room-temperature

Compared with conventional thermal and magnetic sensing materials, perovskite manganese oxide films with a rapid shift from ferromagnetic metals to paramagnetic insulators can be more suitable for the fabrication of uncooled infrared bolometers and magnetic sensors. However, the main challenge is to obtain perovskite manganese oxide films with high temperature coefficient of resistivity (TCR) and low field magnetoresistance (LFMR) at room temperature. In this study, La1-xKxMnO3 (LKMO, 0.10 ≤ x ≤ 0.35) films with different K-dopant amounts were prepared on SrTiO3 substrates using sol–gel spin coating method. A comprehensive analysis of their structure, morphology, and ionic valence enabled one to conclude that varying K-dopant content from 0.10 to 0.30 caused a decrease in Mn-O bond lengths and an increase in Mn-O-Mn bond angles, as well as caused the grain growth and the rise in Mn4+ content in LKMO films. All these changes resulted in a significant alteration in the resistivity of LKMO films, thereby providing the larger TCR and LFMR values at room temperature. According to the results, La0.7K0.3MnO3 with 13.13 % K−1TCR and La0.85K0.15MnO3 with 46.39% LFMR (under an external magnetic field of 1 T) are promising materials for the manufacturing of uncooled infrared bolometers and magnetic sensors, respectively.

Phase diagram of the Hubbard model on a honeycomb lattice: A cluster slave-spin study

The cluster slave-spin method is implemented to research the ground state properties of the honeycomb lattice Hubbard model with doping $\delta$ and coupling $U$ being its parameters. At half-filling, a single direct and continuous phase transition between the semi-metal and antiferromagnetic (AFM) insulator is found at $U_{\text{AFM}}=2.43t$ that is in the Gross-Neveu-Yukawa universality class, where a relation between the staggered magnetization $M$ and the AFM energy gap $\Delta_{\text{AFM}}$ is established as $M \propto \Delta_{\text{AFM}}$, compared to $M \propto \Delta_{\text{AFM}} ( \ln{\Delta_{\text{AFM}}})^2$ in the square lattice case. A first-order semi-metal to the underlying paramagnetic (PM) insulator Mott transition is corroborated at $U_{\text{Mott}}=8.36t$, which is responsible for a broad crossover around $U_{c} = 5.4t$ between the weak- and strong-coupling regimes in the AFM state that increases with $\delta$, in contrast to the square lattice case. In the doped system, the compressibility $\kappa$ near the van Hove singularity at $\delta=1/4$ is suppressed substantially by the interaction before the semi-metal to AFM transition occurs, whereas $\kappa$ near the Dirac points is very close to the noninteracting one, indicating that the Dirac cone structure of the energy dispersion is rather robust. An overall phase diagram in the $U$-$\delta$ plane is presented, consisting of four regimes: the AFM insulator at $\delta=0$ for $U> U_{\text{AFM}}$, the AFM metal with compressibility $\kappa>0$ or $\kappa<0$, and the PM semi-metal, and the AFM metal with $\kappa<0$ only exists in an extremely small area near the phase boundary between the AFM and PM state.

Penta-[formula omitted] revisited: Superior stability, synthesis condition exploration, negative Poisson’s ratio and quasi-flat bands

Pentagon-based two-dimensional (2D) materials have received tremendous attention owing to their unique atomic configurations, extraordinary physical and chemical properties, and their potential for broad applications. As one member of 2D penta-based materials, penta-CN2 has been proposed to be a high energy density material. Here, based on an in-depth thermodynamic and ab initio molecular dynamics analysis, we reveal that penta-CN2 has superior stability with an ultrahigh melting point up to 2500K. Moreover, it remains an amorphous C-N network under the melting state, which makes penta-CN2 suitable for coating material in aviation area. Encouraged by the prospect of excellent applications, we further propose an experimental condition for synthesizing the penta-CN2. Specifically, the Nb(100) surface would be a potential substrate, the CH4 and N2H4 could be used as carbon and nitrogen sources under T = 1600K with pressure p(H2)=p(N2H4)=4×10−4 bar, p(CH4)=(10∼100)p(H2). Further calculation of electro-mechanical properties shows that penta-CN2 has intriguing properties, which includes large-range negative Poisson’s ratio and paramagnetic insulator to ferromagnetic half-metal transition induced by hole doping. Our work is expected to give a profound view of penta-CN2 and guide the future experiment works.

Bivalent Sr2+ doping to improve room-temperature TCR of La0.8-xSrxAg0.2MnO3 polycrystalline ceramics

Rare earth manganites have attractive wide attentions due to their promising applications in infrared bolometers and magnetic information storage. Their temperature coefficient of resistivity (TCR) need further improved so as to achieve its applications at room temperature. In this work, La0.8-xSrxAg0.2MnO3 (LSAMO, x = 0.00, 0.10, 0.15, 0.175, and 0.25) ceramics are fabricated by sol-gel method. Their structure and morphology as well as the optical, electrical and magnetic properties are studied. The structural distortion of Sr doped specimens is identified by the shift of the X-ray diffraction peaks toward the low angles and the red-shift of their Fourier transform infrared spectra. Meanwhile, the Mn4+ ion concentration increases, indicating that the double exchange effect enhances in the ceramics. Furthermore, the TCR values are obviously influenced by the doped Sr content. It enhances from 2.5% K−1 at 211.5 K for La0.8Ag0.2MnO3 to 16.9% K−1 at 299.8 K for La0.625Sr0.175Ag0.2MnO3. The origin of this improvement is attributed to the transformation of rare earth manganites from ferromagnetic metal to paramagnetic insulator and the distortion of MnO6 octahedra. These results uncover the effect of Sr2+ doping on the room-temperature TCR values of ceramics, making it suitable for advanced uncooling device applications.