Band Jahn-Teller Research Articles

Magnetic properties of the Co-doped MnNiGe system in high magnetic field

High-field magnetic properties of the Co-doped MnNiGe system, Mn0.9Co0.1NiGe, which undergoes a ferromagnetic and structural phase transition at Tt= 215 K, were investigated. Additionally, first-principles calculations were performed on Mn0.875Co0.125NiGe. The magnetic field, μ0H, dependence of magnetization, M(μ0H), of Mn0.9Co0.1NiGe saturates at ∼73 A⋅m2⋅kg−1 below Tt and shows significant hysteresis in the range of 215 ≤T≤ 230 K due to the magnetic-field-induced structural phase transition. The isothermal magnetic entropy change, ΔS, peaks around Tt, reaches -79.7 J⋅K−1⋅kg−1 for Δμ0H = 30 T. The density of states (DOS) calculated for Mn0.875Co0.125NiGe in the paramagnetic-hexagonal phase (Tt<T) exhibits a peak at the Fermi energy, EF, with significant contributions from the Mn 3d orbitals. In the ferromagnetic-orthorhombic phase, DOS at EF is suppressed, indicating that the structural phase transition at Tt in this system is primarily driven by the band Jahn-Teller effect.

Intrinsic instability to martensite phases in ferromagnetic shape memory alloy Ni2MnGa : Quasiparticle self-consistent GW investigation

We investigated the electronic structure of a ferromagnetic shape memory alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ utilizing an advanced approach, quasiparticle self-consistent $GW$, which takes account of electron localization effects without empirical parameters. The Ni ${e}_{g}$ orbitals in the cubic phase, which lead to martensite phase transition, were found to locate on the Fermi level, implying a clear definitive origin of band Jahn-Teller (JT) effect in comparison with the results obtained by the density functional approach of generalized gradient approximation. From the analysis of generalized susceptibility in the cubic phase, the instabilities responsible for the modulated structures of 10M, 14M, and 6M were found to be an intrinsic property in the electronic states. These states may stabilize the modulated one, accompanied by tetragonal local JT distortions. Their property of Fermi surface nesting sensitively depends on a subtle change in the magnetic moment, corresponding to the experimental fact that the modulated structure appears depending on temperature and the composition of the magnetic element. The secondary nesting vector along [110] direction was discussed in relation to a modulation alignment of the nanotwin boundary.

Electromotive power generated by pressure gradient in metals with the band Jahn-Teller effect

In band Jahn-Teller (BJT) metals, some lattice distortions with their symmetries different from that of the bulk structure can linearly couple to the BJT electrons, so pressures to produce or enhance such distortions can affect the BJT electrons near the Fermi level. This pressure-electric effect in BJT metals is studied theoretically on the basis of the ${\mathrm{\ensuremath{\Gamma}}}_{12}$ subband model for the structural transition in A15 compounds. The applied pressure is assumed to consist of a spatially uniform pressure and a local pressure. The former induces or enhances a uniform distortion, which contributes to the redistribution of electrons between the ${\mathrm{\ensuremath{\Gamma}}}_{12}$ subbands, while the latter produces a local distortion with its gradient, which causes flows of band electrons between different regions in space. The steady electronic state under the uniform and local pressures is obtained by use of the Boltzmann equation and the relaxation-time approximation. It is clarified that the local pressure with its gradient can generate an electromotive power when the degeneracy of the BJT subbands is lifted spontaneously and/or by the external uniform pressure. The obtained pressure-gradient electric effect is predicted to be a probe into BJT effects which may work in some metals with structural transitions.

Second-harmonic generation in atomically thin 1T−TiSe2 and its possible origin from charge density wave transitions

Optical second-harmonic generation (SHG) can only occur in noncentrosymmetric crystals in the leading electric-dipole approximation. Transition metal dichalco genides with the $1T$ octahedral coordination is centrosymmetric, hence precluding SHG. Here we report the surprising observation of SHG in atomically thin $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$, a prototypical charge density wave (CDW) material. Its intensity peaks in the trilayer, reaching 2% of that in monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$, a two-dimensional crystal featuring pronounced nonlinear optical effect. The SHG signal exhibits a sixfold polarization dependence characteristic of a lattice with threefold rotational symmetry. It monotonically decreases with increasing temperature and persists at room temperature. Raman spectroscopy demonstrates that the CDW order is robust in atomically thin samples, with the transition temperature slightly lower than in the bulk. The SHG can be explained by the lattice distortion associated with the CDW as well as its fluctuation above the transition temperature. These results challenge the exciton insulator scenario and the chiral nature of the CDW, but support the band Jahn-Teller mechanism. Our work demonstrates SHG as a sensitive probe of the stacking order of the CDW in $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$ and enriches the material base for nonlinear optical effects.

The mechanism of magnetostructural transition in Heusler alloy Co2V1.5Ga0.5

Recently, magnetic field induced low magnetization tetragonal martensite to high magnetization cubic austenite transformation has been realized in Heusler alloy Co50V34Ga16 (Applied Physics Letters112, 211903 (2018)). In this paper, first-principles calculations have been performed on Heusler alloy Co2V1.5Ga0.5 to reveal the mechanism for magnetostructural transition. It has been found that the alloy prefers to crystallize into ferromagnetic L21 type structure at austenite, with Co atoms tending to occupy the Wyckoff sites A (0, 0, 0) and C (0.5, 0.5, 0.5), V atoms occupying at site B (0.25, 0.25, 0.25), Ga and the extra V atoms entering D (0.75, 0.75, 0.75) site. Moments of Co and V atoms parallel to each other and a total formula moment of 2.95 μB is achieved. A potential of tetragonal distortion from ferromagnetic cubic structure to non-magnetic tetragonal structure has been predicted from the view of energetically favorable state. The stability of tetragonal Co2V1.5Ga0.5 is further confirmed by the phonon spectrum. The peaks of dx2-y2 and dz2 states for Co and V(D) 3d states near the Fermi level for the cubic structure split at the tetragonal structure, implying the structural transition is mainly attributed to the band Jahn-Teller effect. The hybridization between the Co 3d states and the 3d states of V at D site plays an important role in the martensitic transformation. A volume contraction of 1.3% is obtained accompanying with the magnetostructural transition.

Search for multipolar instability in URu2Si2 studied by ultrasonic measurements under pulsed magnetic field

The elastic properties of URu$_2$Si$_2$ in the high-magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5$f$-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, so called `hidden order'(HO) and associated hybridization of conduction and $f$-electron ($c$-$f$ hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the $\Gamma_4$(B$_{\rm 2g}$) and $\Gamma_5$(E$_{\rm g}$) symmetry modes $C_{66}$ and $C_{44}$ show elastic softening that is enhanced above 30 T, while the characteristic softening of the $\Gamma_3$(B$_{\rm 1g}$) symmetry mode $(C_{11}-C_{12})/2$ is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven $\Gamma_3$(B$_{\rm 1g}$) lattice instability in URu$_2$Si$_2$. However, the results from this work cannot be explained by using existing crystalline-electric field (CEF) schemes applied to the quadrupolar susceptibility in a local $5f^2$ configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

Band Jahn Teller Like Distortion In TiPd Shape Memory Alloy On Addition Of Magnetic Impurity Co

In this paper, we have reported the electronic structure and magnetic properties of TiPd alloyed with two different concentrations of magnetic impurity Co in the framework of density functional theory using the full potential linearized augmented plane wave method. The prominent high temperature shape memory alloy (HTSMA) - TiPd, undergoes martensitic transformation from the parent B2 (cubic) phase to the product B19 (orthorhombic) phase. Among the two configurations [Ti(Pd,Co) and (Ti,Co)Pd] considered, Co is found to prefer Ti sublattice more than the Pd sublattice from the formation energy calculations for both B2 and B19 phases. The addition of Co induced magnetic behaviour in the compounds investigated. The densities of states (total, site and angular momentum decomposed) are studied in detail. Band Jahn Teller (BJT) effects are used to understand structural instability in solids. In BJT, the peak in the vicinity of Fermi level, in the spin down channel of the high symmetry phase splits or broadens on deformation. A band Jahn Teller like broadening is observed in the density of states curve of Ti25Pd50Co25. This broadening is accompanied by a decrease in electronic energy and number of states at Fermi level. BJT effect is an accompanying feature of martensitic transformation.

The structural, electronic and magnetic properties for the transition process between nonmagnetic and magnetic states in CoFe1+xTi1−xAl

We investigated the evolution process of equilibrium lattice parameter (ELP), magnetic properties and electronic structure from nonmagnetic semiconductive CoFeTiAl to ferrimagnetic CoFeFeAl in theory and experiment. The theoretical results show that CoFe1+xTi1−xAl compounds have a diluted magnetism with a 100% spin-polarized ratio at the Fermi level when X is less than 30%. When X is in the range of 30%–60%, CoFe1+xTi1−xAl compounds have also a high spin-polarized ratio. The ELP and magnetization have a gradual and linear change with the increasing Fe level in the ranges of X = 0–50% and 70%–100%. An unexpected sharp jump in ELP and magnetization occurs near the composition point of X = 65%, which was attributed to band Jahn-Teller effect. In experiment, CoFe1+xTi1−xAl (X = 0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%) compounds with Heusler structure were successfully synthesized. The experimental results are in agreement with the theoretical results when X is less than 30% or higher than 70%. The sharp jump in ELP and magnetization are experimentally observed near the composition point of 65%. The deviation of the experimental magnetization from theoretical results in the range of 50%–70% is attributed to the theoretical underestimation to ELP. We predict that by the same mechanism of the band Jahn-Teller effect, the sharp change in the magnetization and lattice parameter should also be possible to occur with the changing temperature for a certain composition of CoFe1+XTi1−XAl compounds. A new type of ferromagnetic shape memory alloys based on a martensitic transformation with an invariant crystallographic symmetry is proposed.

On the tetragonality of martensites in ferrous shape memory alloy Fe3Pt: A first-principles study

First-principles calculations have been performed to study the effects of point defects on the tetragonality of martensites in ferrous shape memory alloy Fe3Pt. By computing the Bain path, I show that by considering point defects, all the face-centered cubic (fct) and body-centered cubic (bct) phases with c/a values close to the experimental observations are predicted. Atomic structure analysis gives evidence about the proposed “antiferrodistortive transformation” which was previously derived from the softened phonon mode of Fe3Pt. We discuss the dependence of the Bain path on the concentration of defects and show that the observed bct structures are closely related to the particular shape of the L12 Bain path. Magnetism is identified as the origin of the extremely flat Bain path. The role of the enhanced band Jahn-Teller effect in stabilizing the fct phases is discussed.

Fermi surfaces and orbital polarization in superconductingCeO0.5F0.5BiS2revealed by angle-resolved photoemission spectroscopy

We have investigated the electronic structure of ${\mathrm{BiS}}_{2}$-based ${\mathrm{CeO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around the $X$ point, the straight portion parallel to the ${k}_{y}$ direction is dominated by Bi $6{p}_{x}$ character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the ${\mathrm{BiS}}_{2}$ system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

Spectroscopic evidence of band Jahn-Teller distortion upon martensitic phase transition in Heusler-type Ni-Fe(Co)-Ga ferromagnetic shape-memory alloy films

The temperature evolution of the electronic structure of a Ni-Fe(Co)-Ga/MgO(100), Heusler-type, ferromagnetic shape-memory alloy thin film has been followed by a bulk-sensitive hard x-ray photoelectron spectroscopy, element-selective soft x-ray magnetic circular dichroism, and first-principles calculation. The reversible changes of the electronic states near the Fermi energy show a hysteresis associated with the martensitic phase transition (MPT), where the pseudogap opens on cooling and closes again on warming. In addition, the Ni $3d$ spin magnetic moment increases by approximately two times across the MPT, whereas the change of Fe $3d$ moment is moderate. By comparing the experimental results with the calculated spin-resolved density of states, we conclude that the band Jahn-Teller effect of Ni $3d$ and Fe $3d$ orbitals is responsible for MPT.

Linear response theory for shear modulus C66 and Raman quadrupole susceptibility: evidence for nematic orbital fluctuations in Fe-based superconductors.

The emergence of the nematic order and fluctuations has been discussed as a central issue in Fe-based superconductors. To clarify the origin of the nematicity, we focus on the shear modulus C(66) and the Raman quadrupole susceptibility χ(x)(2)-y(2))(Raman). Because of the Aslamazov-Larkin vertex correction, the nematic-type orbital fluctuations are induced, and they enhance both 1/C(66) and χ(x(2)-y(2))(Raman) strongly. However, χ(x)(2)-y(2))(Raman) remains finite even at the structure transition temperature T(S), because of the absence of the band Jahn-Teller effect and the Pauli (intraband) contribution, as proved in terms of the linear response theory. The present study clarifies that the origin of the nematicity in Fe-based superconductors is the nematic orbital order and fluctuations.

Important Roles of Te 5p and Ir 5d Spin–Orbit Interactions on the Multi-band Electronic Structure of Triangular Lattice Superconductor Ir1−xPtxTe2

We report an angle-resolved photoemission spectroscopy (ARPES) study on a triangular lattice superconductor Ir$_{1-x}$Pt$_{x}$Te$_2$ in which the Ir-Ir or Te-Te bond formation, the band Jahn-Teller effect, and the spin-orbit interaction are cooperating and competing with one another. The Fermi surfaces of the substituted system are qualitatively similar to the band structure calculations for the undistorted IrTe$_2$ with an upward chemical potential shift due to electron doping. A combination of the ARPES and the band structure calculations indicates that the Te $5p$ spin-orbit interaction removes the $p_x/p_y$ orbital degeneracy and induces $p_x \pm ip_y$ type spin-orbit coupling near the A point. The inner and outer Fermi surfaces are entangled by the Te $5p$ and Ir $5d$ spin-orbit interactions which may provide exotic superconductivity with singlet-triplet mixing.

Electronic structure of the austenitic and martensitic state of magnetocaloric Ni-Mn-In Heusler alloy films

Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys doped with Co are investigated by experiment and theory. The nonstoichiometric Ni${}_{48}$Co${}_{5}$Mn${}_{35}$In${}_{12}$ epitaxial film undergoes a transition from a weakly magnetic martensitic phase below ${T}_{m}=350$ K to a ferromagnetic austenitic phase above ${T}_{m}$. Element-specific magnetic moments and the unoccupied density of states function is investigated using x-ray magnetic circular dichroism. We find an antiparallel alignment of Mn and Ni/Co magnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach considering experimental results on the atomic disorder in the sample. The good agreement of experimental and theoretically simulated x-ray absorption spectra in both phases confirms that a band Jahn-Teller like lifting of a degenerate $d$ state is the origin of the energetic preference of the martensitic phase at low temperatures.

Lattice instability of Ni-Mn-Ga ferromagnetic shape memory alloys probed by hard X-ray photoelectron spectroscopy

Lattice instability of off-stoichiometric Ni-Mn-Ga based ferromagnetic shape memory alloys with different valence electron concentrations (e/a’s) has been studied by utilizing hard X-ray photoelectron spectroscopy. Comparison of two alloys with different e/a’s suggests that a rigid band model is not applicable to explain the different martensitic transition temperatures. It is revealed that the specimen with larger e/a shows larger density of the Ni 3d eg states near the Fermi energy compared to specimen with lower e/a. The results can be reasonably explained by the band Jahn-Teller effect.

Band Jahn-Teller effects and Peierls Instability in IrTe2

We report a photoemission spectroscopy study on IrTe2 which exhibits an interesting lattice distortion below 270 K. Ir 4f core-level photoemission spectrum is broadened in going from 300K to 40K. This result indicates the charge moderation of Ir 5d electrons. Angle-resolved photoemission spectroscopy(ARPES) result at 300K shows multi-band Fermi surfaces with six-fold symmetry which are basically consistent with band structure calculations. At the distorted phase, the inner Fermi surfaces become quasi one-dimensional Fermi surface due to the lattice distortion.

Jahn-Teller effects in transition-metal compounds with small charge-transfer energy

We have studied Jahn-Teller effects in Cs2Au2Br6, ACu3Co4O12(A=Ca or Y), and IrTe2 in which the ligand p-to-transition-metal d charge-transfer energy is small or negative. The Au+/Au3+ charge disproportionation of Cs2Au2Br6 manifests in Au 4f photoemission spectra. In Cs2Au2Br6 with negative Δ and intermediate U, the charge disproportionation can be described using effective d orbitals constructed from the Au 5d and Br 4p orbitals and is stabilized by the Jahn-Teller distortion of the Au3+ site with low-spin d8 configuration. In ACu3Co4O12, Δs for Cu3+ and Co4+ are negative and Us are very large. The Zhang-Rice picture is valid to describe the electronic state, and the valence change from Cu2+/Co4+ to Cu3+/Co3+ can be viewed as the O 2p hole transfer from Co to Cu or d9 + d6L → d9L + d6. In IrTe2, both Δ and U are small and the Ir 5d and Te 5p electrons are itinerant to form the multi-band Fermi surfaces. The ideas of band Jahn-Teller transition and Peierls transition are useful to describe the structural instabilities.

Stoichiometry dependent phase transition in Mn-Co-Ga-based thin films: From cubic in-plane, soft magnetized to tetragonal perpendicular, hard magnetized

Epitaxial thin films of Mn3−xCoxGa were grown on MgO by magnetron co-sputtering with different Co content. Dependent on the Co content tetragonal or cubic structures are obtained. The composition dependence of saturation magnetization MS and uniaxial magnetic anisotropy Ku in the epitaxial films were investigated. A high magnetic anisotropy Ku of 1.2 MJ m−3 was achieved for the Mn2.6Co0.3Ga1.1 film with low magnetic moment of 0.84 μB. The valence band spectra of the films were investigated mainly by hard x-ray photoelectron spectroscopy. The evidence of sharp states in the cubic case, which are smeared out in the tetragonal case, proof the existence of a van Hove singularity that causes a band Jahn-Teller effect accompanied by a tetragonal distortion. These differences are in well agreement to the ab-initio calculations of the electronic structure.

Design Scheme of New Tetragonal Heusler Compounds for Spin‐Transfer Torque Applications and its Experimental Realization

Band Jahn-Teller type structural instabilities of cubic Mn(2)YZ Heusler compounds causing tetragonal distortions can be predicted by ab initio band-structure calculations. This allows for identification of new Heusler materials with tunable magnetic and structural properties that can satisfy the demands for spintronic applications, such as in spin-transfer torque-based devices.

Designing shape-memory Heusler alloys from first-principles

The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14%.