Vanadium Sesquioxide (V$_2$O$_3$) is an antiferromagnetic insulator below $T_{\mathrm N}\approx$ 155 K. The magnetic order is not of C- or G-type as one would infer from the bipartite character of the hexagonal basal plane in the high-temperature corundum structure. In fact, the Néel transition is accompanied by a monoclinic distortion that makes one bond of the honeycomb plane inequivalent from the other two, thus justifying a magnetic structure with one ferromagnetic bond and two antiferromagnetic ones. We show here that the magnetic ordering, the accompanying monoclinic structural distortion, the magnetic anisotropy and also the recently discovered high-pressure monoclinic phase, can all be accurately described by conventional electronic structure calculations within GGA and GGA+U. Our results are in line with DMFT calculations for the paramagnetic phase, which predict that the insulating character is driven by a correlation-enhanced crystal field splitting between $e^π_g$ and $a_{1g}$ orbitals that pushes the latter above the chemical potential. We find that the $a_{1g}$ orbital, although almost empty in the insulating phase, is actually responsible for the unusual magnetic order as it leads to magnetic frustration whose effect is similar to a next-nearest-neighbor exchange in a Heisenberg model on a honeycomb lattice.
Vanadium sesquioxide, V2O3, boasts a rich phase diagram whose description necessitates the accounting for many-body Coulomb correlations. Spectral properties of this compound have been successfully addressed within dynamical mean field theory to an extent that results of recent angle resolved photoemission experiments have been correctly predicted. While photoemission spectroscopy probes the occupied part of the one-particle spectrum, optical experiments measure transitions into empty states and thus provide complementary information. In this work, we focus on the optical properties of V2O3 in…
Topotactic phase transformation in correlated vanadium dioxide through oxygen vacancy ordering
Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education & School of Chemistry and Materials Science, Shanxi Normal University（山西师范大学化学与材料科学学院、教育部磁性分子与磁性信息材料重点实验室）；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）
Two-dimensional (2D) Van der Waals ferromagnets carry the promise of ultimately miniature spintronics and information storage devices. Among the newly discovered 2D ferromagnets all inherit the magnetic ordering from their bulk ancestors. Here we report a new 2D ferromagnetic semiconductor at room temperature, 2H phase vanadium diselenide (VSe2) which show ferromagnetic at 2D form only. This unique 2D ferromagnetic semiconductor manifests an enhanced magnetic ordering owing to structural anisotropy at 2D limit.
We study quantum phase transitions induced by the on-site spin-orbit interaction lambda(L.S) in a toy model of vanadium chains. In the lambda->0 limit, the decoupled spin and orbital sectors are described by a Haldane and an Ising chain, respectively. The gapped ground state is composed of a ferro-orbital order and a spin liquid with finite correlation lengths. In the opposite limit, strong spin-orbital entanglement results in a simultaneous spin and orbital-moment ordering, which can be viewed as an orbital liquid. Using a combination of analytical arguments and density-matrix renormalization…
We present an analytical model of orbital ordering in vanadium spinel MnV2O4. The model is based on recent first-principles calculation indicating a strong trigonal distortion at the vanadium sites of this compound [Phys. Rev. Lett. 102, 216405 (2009)]. At the single-ion level, the trigonal crystal field leaves a doubly degenerate atomic ground state and breaks the approximate rotational symmetry of t2g orbitals. We find that the effective interaction between the low-energy doublets is described by a quantum antiferromagnetic 120-degree model on the pyrochlore lattice. We obtain the classical …
Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydro…
The debate about whether the insulating phases of vanadium dioxide (VO2) can be described by band theory or must be described by a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases, but have not yet successfully accounted for the observed magnetic orderings. Strongly-correlated theories have had limited quantitative success. Here we report that, by using hard pseudopotentials and an optimized hy…
To understand the magnetism of VSe$_2$, whose monolayer form has recently been reported to be a room temperature ferromagnet, Here, the combined method of conventional density functional theory with dynamical mean-field theory has been adopted. This higher-level computation method enables us to resolve many of existing controversies and contradictions in between theory and experiment. First of all, this new approach is shown to give the correct magnetic properties of both bulk and two-dimensional limit of VSe$_2$ which demonstrates its superiority to the conventional methods. The results demon…
Among the various alkali oxides the sesquioxides Rb4O6 and Cs4O6 are of special interest. Electronic structure calculations using the local spin-density approximation predicted that Rb4O6 should be a half-metallic ferromagnet, which was later contradicted when an experimental investigation of the temperature dependent magnetization of Rb4O6 showed a low-temperature magnetic transition and differences between zero-field-cooled (ZFC) and field-cooled (FC) measurements. Such behavior is known from spin glasses and frustrated systems. Rb4O6 and Cs4O6 comprise two different types of dioxygen anions…
Vanadium sesquioxide, V2O3, is a prototypical metal-to-insulator system where, in temperature-dependent studies, the transition always coincides with a corundum-to-monoclinic structural transition. As a function of pressure, V2O3 follows the expected behavior of increased metallicity due to a larger bandwidth for pressures up to 12.5 GPa. Surprisingly, for higher pressures when the structure becomes unstable, the resistance starts to increase. Around 32.5 GPa at 300 K, we observe a novel pressure-induced corundum-to-monoclinic transition between two metallic phases, showing that the structural…
A simple physical conception explaining the experimentally observed temperature dependence of the local mode frequencies in the ordering metal hydrides is proposed. Calculations are performed for bcc vanadium deuterides. The model can be applied to other physical phenomena which are related to the hydrogen motion in metal lattices.
Crystal structure and magnetic properties of the layered vanadium phosphate PbZnVO(PO4)2 are studied using x-ray powder diffraction, magnetization and specific heat measurements, as well as band structure calculations. The compound resembles AA'VO(PO4)2 vanadium phosphates and fits to the extended frustrated square lattice model with the couplings J(1), J(1)' between nearest-neighbors and J(2), J(2)' between next-nearest-neighbors. The temperature dependence of the magnetization yields estimates of averaged nearest-neighbor and next-nearest-neighbor couplings, J(1) ~ -5.2 K and J(2) ~ 10.0 K, …
We elucidate the thermodynamics of sodium (Na) intercalation into the sodium super-ionic conductor (NaSICON)-type electrode, Na$_x$V$_2$(PO$_4$)$_3$, for promising Na-ion batteries with high-power density. This is the first report of a computational temperature-composition phase diagram of the NaSICON-type electrode Na$_x$V$_2$(PO$_4$)$_3$. We identify two thermodynamically stable phases at the compositions Na$_2$V$_2$(PO$_4$)$_3$ and Na$_{3.5}$V$_2$(PO$_4$)$_3$, and their structural features are described for the first time based on our computational analysis. We unveil the crystal-structure …
$R$V${_6}$Sn${_6}$ ($R$ = Y, Gd - Tm, Lu) single crystals are synthesized by Sn-flux method and their physical properties are characterized by magnetization, resistivity, and specific heat measurements. Powder X-ray diffraction patterns of all samples can be well indexed with the hexagonal HfFe$_6$Ge$_6$-type structure, where rare-earth atoms form hexagonal layers and vanadium atoms form Kagome layers. At high temperatures, magnetic susceptibility measurements of moment bearing rare-earths ($R$ = Gd - Tm) follow Curie-Weiss behavior. Effective moments estimated from the polycrystalline average…
Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V[TCNE]$_{x}$ with glass and epoxy extends the film lifetime from an hour to a few weeks, what is limiting its lifetime remains poorly understood. Here we characterize encapsulated V[TCNE]$_{x}$ films using confocal microscopy, Raman spectroscopy, ferromagnetic …
Anomalous x-ray diffraction studies show that the charge ordering in $α^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.
The electronic structure of the quasi-lowdimensional vanadium sulfide \bavs3 is investigated for the different phases above the magnetic ordering temperature. By means of density functional theory and its combination with dynamical-mean field theory, we follow the evolution of the relevant low-energy electronic states on cooling. Hence we go in the metallic regime from the room temperature hexagonal phase to the orthorhombic phase after the first structural transition, and close with the monoclinic insulating phase below the metal-insulator transition. Due to the low symmetry and expected inte…
The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, Néel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarization and that excess spin-driven…
We found anomalies in the temperature dependence of the dielectric and the magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency ranges. The anomalies occur at the phase transition temperature T_c, at which the spin gap opens. The real parts of the dielectric constants epsilon_a and epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow region close to T_c) is proportional to the intensity of the x-ray reflection appearing at T_c. The dielectric constant anomaly can be explained by the zigzag charge ordering in the ab-plane appearing below T_c. …
The structure and properties of the 1:1 superlattice of LaVO$_{3}$ and SrVO$_{3}$ are investigated with a first-principles density-functional-theory-plus-$U$ (DFT+$U$) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered cation ordering to produce a polar structure with nonzero spontaneous polarization normal to the interfaces. This polarization is produced by electron transfer between the V$^{3+}$ and V$^{4+}$ layers, and is comparable to that of conventional ferroelec…
We present a detailed investigation of the magnetic properties of complex vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization, specific heat, 31P NMR measurements, and band structure calculations. Experimental data evidence the presence of ferro- and antiferromagnetic interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss temperature θ_{CW} < 1 K that contrasts with the maximum of magnetic susceptibility at 3 K. Specific heat and NMR measurements also reveal weak exchange couplings with the thermodynamic energy scale J_c = 10-15 K. Additionally, the reduce…
Crystal structure, electronic structure, and magnetic behavior of the spin-1/2 quantum magnet Na1.5VOPO4F0.5 are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The high-temperature second-order alpha <--> beta transition at 500 K is of the order-disorder type, whereas the low-temperature beta <--> gamma+gamma' transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range (gamma) and short-range (gamma') order of Na. Despite the…
We present a Density Functional Theory (DFT) study of the structural and electronic properties of bare rutile VO$_{2}$(110) surfaces and its oxygen-rich terminations. We discuss the performance of various DFT functionals, including PBE, PBE+U (U = 2 eV), SCAN and SCAN+rVV functionals with non-magnetic and ferromagnetic spin ordering. We predict the presence of a ring-like termination that is electronically and structurally related to a V$_{2}$O$_{5}$(001) monolayer and shows a higher stability than pure oxygen adsorption phases. Despite the fact that the calculated phase stabilities depend on …
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below about 14 K. In addition to this orbitally driven ferroelectricity, lacunar spinels are an interesting material class, as the vanadium ions form V4 clusters representing stable molecular entities with a common electron distribution and a well-defined level scheme of molecular states resulting in a unique spin state per V4 molecule. Here we report detailed x-ray, magnetic susceptibility, electrical resistivity, heat capacity, thermal expansio…
A new fcc oxygen defect pyrochlore structure system Lu2V2O_{7-x} with x = 0.40 to 0.65 was synthesized from the known fcc ferromagnetic semiconductor pyrochlore compound Lu2V2O7 which can be written as Lu2V2O6O' with two inequivalent oxygen sites O and O'. Rietveld x-ray diffraction refinements showed significant Lu-V antisite disorder for x >= 0.5. The lattice parameter versus x (including x = 0) shows a distinct maximum at x ~ 0.4. We propose that these observations can be explained if the oxygen defects are on the O' sublattice of the structure. The magnetic susceptibility versus temperatur…
Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from first-principles…
Magnetic field-induced momentum-dependent symmetry breaking in a kagome superconductor
Rice University（莱斯大学）；Weizmann Institute of Science（魏茨曼科学研究所）；University of Washington（华盛顿大学）；Yale University（耶鲁大学）；University of West Bohemia in Pilsen（西波希米亚大学）；Boston College（波士顿学院）；The Pennsylvania State University（宾夕法尼亚州立大学）；University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）
To explore the possibility of quantum phase transitions and even quantum criticality in LaCrSb$_3$ based compounds, we performed measurements under pressure as well as a vanadium substitution study. The Curie temperature of LaCrSb$_3$ was found to be invariant under pressure. Although pressure was not able to suppress the ferromagnetism, chemical substitution was used as another parameter to tune the magnetism. We grew La(V$_x$Cr$_{1-x}$)Sb$_3$ (\textit{x} = 0 -- 1.0) single crystals, and studied the series by measurements of temperature and field dependent magnetic susceptibility, magnetizati…
Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM…
