Metallic LiOsO$_3$ undergoes a continuous ferroelectric-like structural phase transition below $T_c$ = 140 K to realize a polar metal. To understand the microscopic interactions that drive this transition, we study its critical behavior above $T_c$ via electromechanical coupling - distortions of the lattice induced by short-range dipole-dipole correlations arising from Li off-center displacements. By mapping the full angular distribution of second harmonic electric-quadrupole radiation from LiOsO$_3$ and performing a simplified hyper-polarizable bond model analysis, we uncover subtle symmetry-preserving lattice distortions over a broad temperature range extending from $T_c$ up to around 230 K, characterized by non-uniform changes in the short and long Li-O bond lengths. Such an extended region of critical fluctuations may explain anomalous features reported in specific heat and Raman scattering data, and suggests the presence of competing interactions that are not accounted for in existing theoretical treatments. More broadly, our results showcase how electromechanical effects serve as a probe of critical behavior near inversion symmetry breaking transitions in metals.
LiOsO3 is the first experimentally confirmed polar metal. Previous works suggested that the ground state of LiOsO$_3$ is just close to the critical point of metal-insulator transition. In this work the electronic state of LiOsO$_3$ is tuned by epitaxial biaxial strain, which undergoes the Slater-type metal-insulator transition under tensile strain, i.e., the G-type antiferromagnetism emerges. The underlying mechanism of bandwidth tuning can be extended to its sister compound NaOsO$_3$, which shows an opposite transition from a antiferromagnetic insulator to a nonmagnetic metal under hydrostati…
We examine the electronic properties of newly discovered "ferroelectric" metal LiOsO$_3$ combining density-functional and dynamical mean-field theories. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in 1/1 superlattice of LiOsO$_3$ and LiNbO$_3$. We use electronic structure calculations to predict that the (LiOsO$_3$)$_1$/(LiNbO$_3$)$_1$ superlattice is a type-I multiferroic material with a ferrolectric polarization of 41.2~$μ$C cm$^{-2}$, Curie temperature of 927\,K, and Néel temperature of 671\,K. O…
We present a neutron and x-ray scattering investigation of Ca$_3$LiOsO$_6$, a material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions, an interaction mechanism that has undergone relatively little investigation. This contrasts with the ubiquitous superexchange interaction mechanism involving a single anion that has well defined and long standing rules. Despite the apparent 1D nature and triangular units of magnetic osmium ions the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We e…
Over 50 years ago, Anderson and Blount proposed that ferroelectric-like structural phase transitions may occur in metals, despite the expected screening of the Coulomb interactions that often drive polar transitions. Recently, theoretical treatments have suggested that such transitions require the itinerant electrons be decoupled from the soft transverse optical phonons responsible for polar order. However, this decoupled electron mechanism (DEM) has yet to be experimentally observed. Here we utilize ultrafast spectroscopy to uncover evidence of the DEM in LiOsO$_3$, the first known band metal…
Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4$d^3$ and 5$d^3$ double pervoskite antiferromagnets La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$. These reveal the development of an inelastic spin gap in La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ of $\sim$ 1.8(8) meV and 6(1) meV, below their respective ordering temperatures, $T_N$, $\sim$ 23.8 K and 30 K. The bandwidth of the spin excitations is shown to be $\sim$ 5.7(9) to 12(1) meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t$_{2g}$ levels of Ru$^{5+}$…
The possibility of reconciliation between seemingly mutually exclusive properties in one system can not only lead to theoretical breakthroughs but also potential novel applications. The research on the coexistence of two purportedly contra-indicated properties, ferroelectricity/polarity and conductivity, proposed by Anderson and Blount over 50 years ago was recently revitalized by the discovery of the first unambiguous polar metal LiOsO3 and further fueled by the demonstration of the first switchable ferroelectric metal WTe2. In this review, we first discuss the reasons why the coexistence of …
Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distorti…
It is of great interest to design and make materials in which ferroelectric polarisation is coupled to other order parameters such as lattice, magnetic and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realised. Here we report detailed crystallog…
Electronic reconstruction at the polar interface LaMnO$_3$/SrMnO$_3$ (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and super exchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO$_3$/SrTiO$_3$ interface. In addition, the Mn e$_g$ electrons leak…
Density-functional calculations are carried out to investigate incommensurate magnetic structures and ferroelectric polarization in newly discovered multiferroic material MnI$_2$. The exchange interactions among local moments on Mn are parameterized by mapping the mean-field Heisenberg model on to total energy difference of several magnetic ordering states. The experimentally observed noncollinear magnetic states are well reproduced by using this model and exchange interaction parameters. The direction of polarization experimentally measured is also consistent with the result derived from the …
We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and polar phonons in the 1:1 SrMnO$_3$/LaMnO$_3$ superlattice from first principles. Magnetic phase transitions of the superlattice induced by epitaxial strain are observed, consistent with previous reports. We find that oxygen octahedral rotations lower the ground state energy but do not destroy the magnetic phase transitions induced by strain. We compute zone center phonon frequencies and eigenvectors as functions of epitaxial strain and magnetic ordering. A substantial increase of the …
Based on a two-dimensional extended Hubbard model with electron-phonon interaction, we have studied the effect of polaron formation on the charge ordering (CO) transition. It is found that for fully ferromagnetically ordered spins the CO state may go through a process of appearance, collapse and reappearance with decreasing temperature. This is entirely due to a emperature-dependent polaron bandwidth. On the other hand, when a paramagnetic spin state is considered, only a simple reentrant behavior of the CO transition is found, which is only partly due to polaron effect. This model is proposed…
We investigated the metal-ordered manganite system YBaMn2O6 using electron spin resonance (ESR) in the paramagnetic regime across the charge-ordering and structural phase transition at T_CO = 480 K and T_t = 520 K, respectively. All ESR parameters exhibit jump-like changes at T_t while the charge-ordering at T_CO manifests itself only as a weak and broad anomaly. Above T_t the ESR spin susceptibility is reduced with respect to the dc-susceptibility, indicating that only the t_2g-core spins of Mn ions contribute to the resonance absorption. The contribution of the e_g-spins is suppressed by the…
Neutron scattering has been used to study the nature of the ferromagnetic transition in a single crystal of the perovskite La0.7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Both the scattering due to the polaron correlations and the anomalous quasielastic component in the magnetic fluctuation spectrum maximize very close to T_C, in a manner remarkably similar to the resistivity, indicating that they have a common origin.
Ferroelectric spiral magnets DyMnO3 and TbMnO3 show similar behavior of electric polarization in applied magnetic fields. Studies of the field dependence of lattice modulations on the contrary show a completely different picture. Whereas in TbMnO3 the polarization flop from P||c to P||a is accompanied by a sudden change from incommensurate to commensurate wave vector modulation, in DyMnO3 the wave vector varies continuously through the flop transition. This smooth behavior may be related to the giant magnetocapacitive effect observed in DyMnO3.
We apply dynamical mean field theory to study a prototypical model that describes charge ordering in the presence of both electron-lattice interactions and intersite electrostatic repulsion between electrons. We calculate the optical and d.c. conductivity, and derive approximate formulas valid in the limiting electron-lattice coupling regimes. In the weak coupling regime, we recover the usual behavior of charge density waves, characterized by a transfer of spectral weight due to the opening of a gap in the excitation spectrum. In the opposite limit of very strong electron-lattice coupling, ins…
Interesting emergent behavior in quantum materials arises when the interaction of electrons with the lattice leads to partial localization and ordering of charge at low temperatures. The triangular lattice of some transition metal dichalcogenides additionally presents an interesting case, where spin order is frustrated, leading to an additional complex interplay of interactions involving spin, charge and lattice degrees of freedom. Here we present a study of local symmetry breaking of the lattice structure in the layered dichalcogenide material 1T-TaS$_{2}$ using x-ray pair-distribution functi…
Using density-functional theory calculations and experimental investigations on structural, magnetic and dielectric properties, we have elucidated a unique tetragonal ground state for MnTi2O4, a Ti^{3+} (3d^1)-ion containing spinel-oxide. With lowering of temperature around 164 K, cubic MnTi2O4 undergoes a structural transition into a polar P4_1 tetragonal structure and at further lower temperatures, around 45 K, the system undergoes a paramagnetic to ferrimagnetic transition. Magnetic superexchange interactions involving Mn and Ti spins and minimization of strain energy associated with co-ope…
Using spectral ellipsometry, we have measured the dielectric function of a Na$_{0.82(2)}$CoO$_2$ crystal that exhibits bulk antiferromagnetism with T$_{N}$=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and the lattice response that are indicative of charge ordering in the CoO$_{2}$ layers. The second transition coincides with T$_{N}$=19.8 K and reveals a sizeable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magneto-polarons due to a charge-induced spin-sta…
The properties of a dilute electron gas, coupled to the lattice degrees of freedom, are studied and compared with the properties of an electron gas at half-filling, where spinless fermions with two orbitals per lattice site are considered. The simplest model which includes both the local electron-lattice interaction of the Jahn-Teller type and the electronic correlations is the $E\otimesβ$-Jahn-Teller-Hubbard model. We analyze the formation and stability of Jahn-Teller polarons and bipolarons, respectively. Our approach is based on a hopping expansion in the strong-coupling regime. The results…
A model of chemical bonding between ions in manganites involving covalent one-electron $σ$ bonding is suggested. The covalent one-electron $σ$ bonding gives rise to a strongly correlated state of electrons resulting from the exchange interaction between electrons when they are simultaneously at cation and anion orbitals. The manifestation of the correlatred state is the spin and spatial ordering of the electrons resulting in the formation of a spin- ordered electron lattice. The conductivity of manganites in this model is the consequence of displacement of the electron lattice (or its part) fr…
We consider bosonic dipolar molecules in an optical lattice prepared in a mixture of different rotational states. The 1/r^3 interaction between molecules for this system is produced by exchanging a quantum of angular momentum between two molecules. We show that the Mott states of such systems have a large variety of non-trivial spin orderings including a state with ordering wave vector that can be changed by tilting the lattice. As the Mott insulating phase is melted, we also describe several exotic superfluid phases that will occur.
Orthorhombic HoMnO3 is a multiferroic in which Mn antiferromagnetic order induces ferroelectricity. A second transition occurs within the multiferroic phase, in which a strong enhancement of the ferroelectric polarization occurs concomitantly to antiferromagnetic ordering of Ho 4f magnetic moments. Using the element selectivity of resonant X-ray diffraction, we study the magnetic order of the Mn 3d and Ho 4f moments. We explicitly show that the Mn magnetic order is affected by the Ho 4f magnetic ordering transition. Based on the azimuthal dependence of the (0 q 0) and (0 1-q 0) magnetic reflec…
The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin and orbital ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high TC of 108 K over a wide temperature range including different spin-orbital ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic C-type spin ordering transition taking place at 108 K, which indicates that DyVO3 belongs to type II mu…
Ferroelectric and ferromagnetic materials possess spontaneous electric and magnetic order, respectively, which can be switched by the corresponding applied electric and magnetic fields. Multiferroics combine these properties in a single material, providing an avenue for controlling electric polarization with a magnetic field and magnetism with an electric field. These materials have been intensively studied in recent years, both for their fundamental scientific interest as well as their potential applications in a broad range of magnetoelectric devices [1, 2, 3, 4]. However, the microscopic or…
We study effects of an external magnetic field on charge ordering in the one-dimensional extended Hubbard model at quarter filling by means of the quantum Monte Carlo method. We find that the Zeeman coupling enhances the charge order correlation, which is prominent when the system is located in the critical regime near the charge ordering transition at zero magnetic field. This behavior is interpreted in terms of the crossover to the fully spin-polarized limit where the model is exactly solvable. Furthermore, by incorporating the interchain Coulomb repulsion, we show that the charge-ordering t…
Temperature dependent optical spectra are reported for beta-Na0.33V2O5. The sodium ordering transition at T_Na = 240 K, and in particular the charge ordering transition at T_MI = 136 K strongly influence the optical spectra. The metal-insulator transition at \tmi leads to the opening of a psuedogap (\hbarω= 1700 cm-1), and to the appearance of a large number of optical phonons. These observations, and the presence of a mid-infrared band (typical for low dimensional metals) strongly suggests that the charge carriers in beta-Na0.33V2O5 are small polarons.
Soft x-ray resonant scattering at the Ni L2,3 edges is used to test models of magnetic and orbital-ordering below the metal-insulator transition in NdNiO3. The large branching ratio of the L3 to L2 intensities of the (1/2,0,1/2) reflection and the observed azimuthal angle and polarization dependence originates from a non collinear magnetic structure. The absence of an orbital signal and the non collinear magnetic structure show that the nickelates are materials for which orbital ordering is absent at the metal-insulator transition.
We report on a comprehensive study of the electronic and magnetic structure of the striped nickelate La5/3Sr1/3NiO4. The investigation is carried out using band structure calculations based on density functional theory. A magnetic structure compatible with experiment is obtained from spin-polarized calculations within the generalized gradient approximation (GGA), whereas inclusion of a local Coulomb interaction in the LDA+U framework results in a different ground state. The influence of the various interaction parameters is discussed in detail.
We demonstrate with LSDA+U calculations that x-ray scattering at the K edge of Mn is sensitive to orbital ordering in one energy range and Jahn-Teller distortion in another. Contrary to what is suggested by atomic or cluster models used to date we show that band structure effects rather than local Coulomb interactions dominate the polarization dependence of the K edge scattering and therefore it is sensitive to nearest neighbour bond length distortions and next nearest neighbour orbital occupation. Based on this we propose a new mechanism for K edge x-ray scattering in the manganates which we …
We present the measurements of the polarized optical spectra of NaV2O5 and LiV2O5. In an energy range from 0.5 to 5.5 eV we observe similar peaks in the E parallel a spectra of LiV2O5 and NaV2O5, which suggests similar electronic structure along the a axis in both materials. On the other hand, we find an almost complete suppression of the peaks in sigma_b of LiV2O5 around 1 and 5 eV. We attribute this suppression to the charge localization originating from the existence of double-chain charge-ordering patterin in LiV2O5.
