Anomalous spin-waves and the commensurate-incommensurate magnetic phase transition in LiNiPO4
T. B. S. Jensen, N. B. Christensen, M. Kenzelmann, H. M. Rønnow, C. Niedermayer, N. H. Andersen, K. Lefmann, M. Jiménez-Ruiz, F. Demmel, J. Li, J. L. Zarestky, D. Vaknin
备注：
4 pages, 4 figures, accepted for publication in Phys. Rev. B
Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase with ordering temperature 20.8 K. An anomalous low-energy mode is observed at the modulation vector of the incommensurate (IC) AF phase appearing above the 20.8 K. A linear spin-wave model based on Heisenberg exchange couplings and single ion anisotropies accounts for all the observed spin-wave dispersions and intensities. Along the b axis an unusually strong next-nearest-neighbor AF coupling competes with the dominant nearest-neighbor AF exchange interaction and causes the IC structure.
A scheme for making ab-initio calculations of the dynamic paramagnetic spin susceptibilities of solids at finite temperatures is described. It is based on Time-Dependent Density Functional Theory and employs an electronic multiple scattering formalism. Incommensurate and commensurate anti-ferromagnetic spin fluctuations in paramagnetic Cr and compositionally disordered Cr95V5 and Cr95Re5 alloys are studied together with the connection with the nesting of their Fermi surfaces. We find that the spin fluctuations can be described rather simply in terms of an overdamped oscillator model. Good agre…
Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li2Co(WO4)2 samples. Under zero magnetic field, two successive magnetic transitions at TN1 ~ 9.4 K and TN2 ~ 7.4 K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li2Co(WO4)2 enters an incommensurate magnetic state with a temperature dependent k between TN1 and TN2. The magnetic propagation vector locks-in to a commensurate value k = (1/2, 1/4, 1/4) below TN2. The antiferromagnetic struct…
We have synthesized single crystalline sample of Mn$_2$V$_2$O$_7$ using floating zone technique and investigated the ground state using magnetic susceptibility, heat capacity and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1} =$ 19 K and $T_{N2} =$ 11.8 K indicating two distinct magnetically ordered phases. The single crystal neutron diffraction study shows that in the temperature ($T$) range 11.8 K $\le T \le$ 19 K the magnetic structure is commensurate with propagation vector $k_1 = (0, 0.5, 0)$, while upon lowering te…
In this work, we study strongly interacting spinor atoms in a lattice subject to a 2 dimensional (2d) anisotropic Rashba type of spin orbital coupling (SOC) and an Zeeman field. We find the interplay between the Zeeman field and the SOC provides a new platform to host rich and novel classes of quantum commensurate and in-commensurate phases, excitations and phase transitions. These commensurate phases include two collinear states at low and high Zeeman field, two co-planar canted states at Mirror reflected SOC parameters respectively. Most importantly, there are non-coplanar incommensurate Sky…
Linearly polarized spectra of far-infrared (IR) transmission in HoMn2O5 multiferroic single crystals have been studied in the frequency range between 8.5 and 105 cm-1 and for temperatures between 5 K and 300 K. Polarization of IR-active excitations depends on the crystallographic directions in HoMn2O5 and is sensitive to the magnetic phase transitions. We attribute some of the infrared-active excitations to electric-dipole transitions between ligand-field split states of Ho3+ ions. For light polarization along crystalline b-axis, the oscillator strength of electric dipoles at low frequencies (…
Electronic Coherence Evolution at the Nearly Commensurate Incommensurate CDW Boundary of 1T-TaS2
Xiamen University Malaysia（厦门大学马来西亚分校）；University of Connecticut（康涅狄格大学）；Institute of Materials Science, Storrs, University of Connecticut（康涅狄格大学斯托尔斯材料科学研究所）；Center for Functional Nanomaterials, Brookhaven National Lab（布鲁克海文国家实验室功能纳米材料中心）；Synchrotron Light Research Institute (Public Organization), Ministry of Higher Education Science Research and Innovation（高等教育、科学研究与创新部同步光研究所）；National Synchrotron Light Source II, Brookhaven National Lab（布鲁克海文国家实验室国家同步光源二号设施）；Xiamen University（厦门大学）
We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b-axis of single crystals of YMn$^{4+}$(Mn$_{1-x}$Ga$_{x}$)$^{3+}$O$_{5}$ with $x=0.047$ and 0.12, in which nonmagnetic Ga-ions dilute Mn$^{3+}$ spins. The $x=0.047$ sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the $x=0.12$ sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak-ferro…
We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li$_{2}$NiW$_{2}$O$_{8}$ showing successive transitions at $T_{\rm N1}\simeq 18$ K and $T_{\rm N2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $\mathbf k=(\frac12,0,\frac12)$ below $T_{\rm N2}$. The ordered moment of 1.8 $μ_B$ at 1.5 K is directed along $[0.89(9),-0.10(5),-0.49(6)]$ and matches the magnetic easy axis of spin-1 Ni$^{2+}$ ions, which is det…
Neutron diffraction measurements under high magnetic fields have been performed for the multiferroic compound HoMn$_{2}$O$_{5}$. At zero field, high-temperature incommensurate magnetic (HT-ICM) -- commensurate magnetic (CM) -- low-temperature incommensurate magnetic (LT-ICM) orders occur with decreasing temperature, where ferroelectric polarization arises only in the CM phase. Upon applying a magnetic field, the LT-ICM phase completely disappears and the CM phase is induced at the lowest temperature. This field-induced CM state is completely associated with the field-induced electric polarizat…
When a crystal is subjected to a periodic potential, under certain circumstances (such as when the period of the potential is close to the crystal periodicity; the potential is strong enough, etc.) it might adjust itself to follow the periodicity of the potential, resulting in a, so called, commensurate state. Such commensurate-incommensurate transitions are ubiquitous phenomena in many areas of condensed matter physics: from magnetism and dislocations in crystals, to vortices in superconductors, and atomic layers adsorbed on a crystalline surface. Of particular interest might be the propertie…
Structural Evolution during Reversible Halogen Intercalation into WTe2: Commensurate-Incommensurate WTe2I and Multistage WTe2Brx (x = 0.5, 1.0 and 1.25)
Department of Materials, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague（布拉格捷克理工大学核科学与物理工程学院材料系）
The interlayer couplings in commensurate and incommensurate bilayer structures of transition metal dichalcogenides are investigated with perturbative treatment. The interlayer coupling in \pm K valleys can be decomposed into a series of hopping terms with distinct phase factors. In H-type and R-type commensurate bilayers, the interference between the three main terms leads to a sensitive dependence of the interlayer coupling strength on the translation. The magnitudes of the main hopping terms are found to some dozen of meV by fitting to the ab initio results. The interlayer couplings in Γvall…
We study the phase diagram and the commensurate-incommensurate transitions in a phase field model of a two-dimensional crystal lattice in the presence of an external pinning potential. The model allows for both elastic and plastic deformations and provides a continuum description of lattice systems, such as for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a mode expansion analysis is used to determine the ground states and the commensurate-incommensurate transitions in the model as a function of the strength of the pinning potential and the lattice mismatch paramete…
Path-integral Monte Carlo calculations have been performed to study the $^4$He adsorption on $γ$-graphyne, a planar network of benzene rings connected by acetylene bonds. Assuming the $^4$He-substrate interaction described by a pairwise sum of empirical $^4$He-carbon interatomic potentials, we find that unlike $α$-graphyne, a single sheet of $γ$-graphyne is not permeable to $^4$He atoms in spite of its large surface area. One-dimensional density distributions computed as a function of the distance from the graphyne surface reveal a layer-by-layer growth of $^4$He atoms. A partially-filled $^4$…
Commensurate-Incommensurate Transition in Submonolayer $^3$He on Graphite
University of Hyogo（兵库大学）；National Institute of Advanced Industrial Science and Technology (AIST)（产业技术综合研究所）；The University of Osaka（大阪大学）；The University of Tokyo（东京大学）
We study the existence of incommensurate phases in the phase diagram of the two orbital double exchange model coupled with Jahn-Teller phonons and with superexchange interactions. In agreement with experimental results, we find that undoped manganites $RMnO_3$ ($R$ being some rare earth element) show temperature induced commensurate-incommensurate phase transitions. In the incommensurate phase the magnetic wave vector varies with temperature. The incommensurate phase arises from the competition between the short range antiferromagnetic superexchange interaction and the long range ferromagnetic…
We have performed elastic neutron scattering studies of the quantum critical antiferromagnet Cr$_{0.963}$V$_{0.037}$. We have found that unlike pure Cr, which orders at two incommensurate wavevectors, Cr$_{0.963}$V$_{0.037}$ orders at four incommensurate and one commensurate wavevectors. We have found strong temperature dependent scattering at the commensurate and incommensurate wavevectors below 250 K. Results indicate that the primary effect of V doping on Cr is the modification of the nesting conditions of the Fermi surface and not the decreasing of the Neel temperature.
We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.
Spin density wave (SDW) states of a quasi-one-dimensional system with an incommensurate wave vector perpendicular to the chain have been studied in the presence of two kinds of commensurate potentials, which originate in a quarter-filled band and dimerization along the chain. In terms of a phase variable of the SDW order parameter, we treat classically the two-dimensional Hamiltonian, which includes both acoustic excitations with long wave length and a vortex excitation with short wave length. A phase diagram on the plane of temperature and chemical potential (where the latter corresponds to t…
Starting from the basic Landau model for the incommensurate-commensurate materials of the class II, we derive the spectrum of collective modes for all (meta)stable states from the corresponding phase diagram. It is shown that all incommensurate states posses Goldstone modes with acoustic dispersions. The representation in terms of collective modes is also used in the calculation and discussion of static dielectric response for systems with the commensurate wave number in the center of the Brillouin zone.
Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 K and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to the Gamma point. The C-CDW transition temperatures as a function of film thickness are determined from the evolution of these new Raman peaks and they are found to decrease from 473K to …
We study the elastic properties of incommensurate solid 4He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (C11, C12, C13, C33, and C44), longitudinal and transversal speeds of sound, and the T = 0 Debye temperature of incommensurate and commensurate hcp 4He using the diffusion Monte Carlo method. Our results show that under compression the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e. P ~ 25 bars) some of the elastic constants accounting for strain deformations o…
The basic Landau model for uniaxial systems of the II class is nonintegrable, and allows for various stable and metastable periodic configurations, beside that representing the uniform (or dimerized) ordering. In the present paper we complete the analysis of this model by performing the second order variational procedure, and formulating the combined Floquet-Bloch approach to the ensuing nonstandard linear eigenvalue problem. This approach enables an analytic derivation of some general conclusions on the stability of particular states, and on the nature of accompanied collective excitations. F…
We present inelastic neutron scattering experiments on a single-domain crystal of lightly-doped La1.96Sr0.04CuO4. We find that the magnetic excitation spectrum in this insulating phase with a diagonal incommensurate spin modulation is remarkably similar to that in the superconducting regime, where the spin modulation is bond parallel. In particular, we find that the dispersion slope at low energy is essentially independent of doping and temperature over a significant range. The energy at which the excitations cross the commensurate antiferromagnetic wave vector increases roughly linearly with …
Multiple charge density wave (CDW) phases in 2H-TaSe2 are investigated by high-resolution synchrotron x-ray diffraction. In a narrow temperature range immediately above the commensurate CDW transition, we observe a multi-q superstructure with coexisting commensurate and incommensurate order parameters, clearly distinct from the fully incommensurate state at higher temperatures. This multi-q ordered phase, characterized by a temperature hysteresis, is found both during warming and cooling, in contrast to previous reports. In the normal state, the incommensurate superstructure reflection gives w…
At high pressure, bismuth acquires a complex incommensurate host-guest structure, only recently discovered. Characterizing the structure and properties of this incommensurate phase from first principles is challenging owing to its non-periodic nature. In this study we use large scale DFT calculations to model commensurate approximants to the Bi-III phase, and in particular to describe the atomic modulations with respect to their ideal positions, shown here to strongly affect the electronic structure of the lattice and its stability. The equation of state and range of stability of Bi-III are re…
Understanding the appearance of commensurate and incommensurate modulations in perovskite antiferroelectrics (AFEs) is of great importance for material design and engineering. The dielectric and elastic properties of the AFE domain boundaries are lack of investigation. In this work, a novel Landau theory is proposed to understand the transformation of AFE commensurate and incommensurate phases, by considering the coupling between the oxygen octahedral tilt mode and the polar mode. The derived relationship between the modulation periodicity and temperature is in good agreement with the experime…
We propose a new but simpler explanation of the phases of a Frenkel-Kontorova chain of atoms, and we demonstrate it by examples. Combined with this, we present a criticism of the theory of so-called commensurate and incommensurate states, especially for finite chains. We reject the putative observation of an Aubry-phase transition in a finite chain.
The triangular spin lattice of NiBr$_{2}$ is a canonical example of a frustrated helimagnet that shows a temperature-driven phase transition from a collinear commensurate antiferromagnetic structure to an incommensurate spin helix on cooling. Employing neutron diffraction, bulk magnetization, and magnetic susceptibility measurements, we have studied the f\hspace*{.5pt}ield-induced magnetic states of the NiBr$_{2}$ single crystal. Experimental f\hspace*{.5pt}indings enable us to recapitalize the driving forces of the spin spiral ordering in the triangular spin-lattice systems, in general. Neutr…
The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magneto-elastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature-dependence of the spontaneous polarization, in particular its sign reversal at the commensurate-incommensurate transition.
We propose the Landau model for lock-in phase transitions in uniaxially modulated improper ferroelectric incommensurate-commensurate systems of class I. It includes Umklapp terms of third and fourth order and secondary order parameter representing the local polarization. The corresponding phase diagram has the structure of harmless staircase, with the allowed wave numbers obeying the Farey tree algorithm. Among the stable commensurate phases only those with periods equal to odd number of lattice constants have finite macroscopic polarizations. These results are in excellent agreement with expe…
We present the results of a theoretical analysis of two-dimensionally modulated incommensurate phases in crystals and define the space distribution of magnetization and polarization vector in the incommensurate phase and demonstrate that their averaging in the incommensurate phase period equals zero. We apply symmetry arguments and show that despite the fact that the phase transition in the system is described by reducible representation of the space group high symmetry phase the superspace symmetry group of the in-commensurate phase contains all symmetry point groups of the high symmetry phas…
The phase diagram of the first layer of H$_2$ adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo (DMC) simulations. We have found that, as in the case of $^4$He, the ground state of molecular hydrogen is a $\sqrt3 \times \sqrt3$ commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered, and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.
Pinning phenomena in an incommensurate two-chain model of friction are studied numerically. The pinning effect due to the breaking of analyticity exists in the present model. The pinning behavior is, however, quite different from that for the breaking of analyticity state of the Frenkel-Kontorova model. When the elasticity of chains or the strength of interchain interaction is changed, pinning force and maximum static frictional force show anomalously complicated behavior accompanied by a successive phase transition and they vanish completely under certain conditions.
The scaling of friction with the contact size $A$ and (in)commensurabilty of nanoscopic and mesoscopic crystals on a regular substrate are investigated analytically for triangular nanocrystals on hexagonal substrates. The crystals are assumed to be stiff, but not completely rigid. Commensurate and incommensurate configurations are identified systematically. It is shown that three distinct friction branches coexist, an incommensurate one that does not scale with the contact size ($A^0$) and two commensurate ones which scale differently (with $A^{1/2}$ and $A$) and are associated with various co…
We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the…
We report on electrical gating of the charge-density-wave phases and current in h-BN capped three-terminal 1T-TaS$_2$ heterostructure devices. It is demonstrated that the application of a gate bias can shift the source-drain current-voltage hysteresis associated with the transition between the nearly commensurate and incommensurate charge-density wave phases. The evolution of the hysteresis and the presence of abrupt spikes in the current while sweeping the gate voltage suggest that the effect is electrical rather than self-heating. We attribute the gating to an electric-field effect on the co…
