Ground State Changes Research Articles

Magnetism and associated exchange bias in Ni2−xCoxMn1.4Ga0.6

A series of Ni2−xCoxMn1.4Ga0.6 Heusler alloys have been systematically investigated by x-ray diffraction, dc magnetization, and ac susceptibility measurements. For all Co concentration, the alloys exhibit the L10 martensitic structure at room temperature. Interestingly, Co doping simultaneously causes a reduction in the ferromagnetic exchange interaction and enhancement of magnetic anisotropy in Ni2−xCoxMn1.4Ga0.6. Exchange bias effects under both zero field cooled and field cooled condition have been observed in all alloys for x<0.3. The ac susceptibility data show frequency dependence that changes with increasing Co concentration, indicating a change of ground state from spin glass to super spin glass. The experimental results are explained considering the atomic radii of Ni and Co and the fundamental magnetic interactions in Heusler alloys.

The Quantum Field Model of Experiences, Responses and Thoughts

Life is a series of experiences and responses woven in the chain of causality that builds the knowledge system in the universe, a complex system altogether. When we observe the subatomic system or the quantum field, the very act of measurement perturbs the quantum field within. The interaction produces change in the ground state of the quantum field. The mind field is treated as the quantum field as envisaged by various eminent scientists. The theory of perturbation helps us to compute the amount of energy change in the form of unitary time evolution operator followed by the Hamiltonian of the system. The Linear Response Theory helps us to compute not only the amount of energy responsible for the perturbation but to estimate the probable responses from such quantum field of the mind subjected to the perturbation. Thus the continuous series of measurements, perturbations and responses build the chain of causality within the mind field itself to perceive the continuity of the life. We will see in this attempt; the formulation of functionality of mind field part, identity of the observer, kind of observable, the process of realization of being experienced and null state of the mind.

Probing wavepacket dynamics using ultrafast x-ray spectroscopy

The advent of x-ray free electron lasers is providing new opportunities for probing the ultrafast excited state dynamics using structurally sensitive techniques. Herein we use excited state wavepacket dynamics of a prototypical Cu(I)-phenanthroline complex, [Cu(dmp)2]+ (dmp = 2, 9-dimethyl-1, 10-phenanthroline) to investigate how femtosecond vibrational and electronic relaxation is translated into transient x-ray absorption and emission. Using realistic experimental parameters we also derive the anticipated signal strengths for these transient features. This indicates that although recording a signal capturing the strongest transient (i.e. excited state–ground state) changes will be possible for all cases, only with x-ray absorption near-edge structure and extended x-ray absorption fine structure will it be possible to resolve the fine details associated with the wavepacket dynamics within realistic experimental acquisition times.

Discontinuity of maximum entropy inference and quantum phase transitions

In this paper, we discuss the connection between two genuinely quantum phenomena—the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit.

Structural and electronic response of U3Fe4Ge4 to high pressure

Structural, magnetic, and electrical properties have been studied on a U3Fe4Ge4 single crystal under hydrostatic pressure. The orthorhombic crystal structure is found to be stable up to 30 GPa, the highest applied pressure, but the compressibility is strongly anisotropic. Contrary to typical uranium intermetallics for which the softest lattice direction is along the shortest inter-uranium links, in U3Fe4Ge4 the lattice is compressed most in a perpendicular direction for the high pressure range. The elastic properties are modified considerably in the vicinity of 1 GPa when the b axis is transformed from least compressible to most compressible. The bulk modulus is found to be about 150 GPa. The anomalies in the elastic properties are reflected in the electronic properties that consistently indicate a change of the magnetic ground state from ferromagnetic to antiferromagnetic. Both types of order exhibit a gap in the magnon spectrum; however, it is twice as high for the ferromagnetic state. The magnetoresistance reveals field-induced transitions of different origins in the antiferromagnetic state along the easy and hard magnetization directions.

Phase diagram of CeRuPO under pressure investigated byP31-NMR: Comparison between CeRuPO under pressure and theCe(Ru1−xFex)POsystem

We have performed $^{31}$P-NMR measurements on single-crystalline CeRuPO under pressure in order to understand the variation in magnetic character against pressure. The NMR spectra for $H \perp c$ and $H \parallel c$ at 2.15GPa split below the ordered temperature, which is a microscopic evidence of the change in the magnetic ground state from the ferromagnetic (FM) state at ambient pressure to the antiferromagnetic (AFM) state under pressure. The analysis of NMR spectra suggests that the magnetic structure in AFM state is the stripe-type AFM state with the AFM moment $m_{\rm AFM} \perp c$-axis and changes by magnetic field perpendicular to $c$-axis. In addition, the dimensionality of magnetic correlations in the spin and the $k$ space is estimated. We reveal that three-dimensional magnetic correlations in CeRuPO are robust against pressure, which is quite different from the suppression of the magnetic correlations along the $c$-axis by Fe substitution in Ce(Ru$_{1-x}$Fe$_{x}$)PO.

Magnetic and electronic structure of ultrathinLa1−xSrxMnO3films at half doping

The magnetic, transport, and electronic properties of ultrathin epitaxial ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) films at near half doping ($x=0.47$, 0.50, and 0.55), grown under different misfit strains on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001) and ${\mathrm{La}}_{0.18}{\mathrm{Sr}}_{0.82}{\mathrm{Al}}_{0.59}{\mathrm{Ta}}_{0.41}{\mathrm{O}}_{3}$(001) (LSAT) substrates, are investigated. We find that all films exhibit metallic behavior below the magnetic critical temperature, while the magnetic properties change markedly with both doping and strain. However, while increased doping favors antiferromagnetic ordering, strain is the driving mechanism for the change in the magnetic properties, where with increasing tensile strain the magnetic ground state changes from ferromagnetic to antiferromagnetic at a critical lattice misfit threshold of about \ensuremath{-}1%. The bulk magnetometry data are confirmed by x-ray magnetic circular dichroism spectroscopy, while x-ray magnetic linear dichroism measured at room temperature demonstrates a progressive change in the orbital occupancy with increasing misfit strain from out of plane to in-plane, leading to a preferred antiferromagnetic metallic state at larger tensile strains.

Strain tuning of magnetism in Mn doped MoS2 monolayer

We study the strain tuning of magnetism in Mn doped MoS2 monolayer system. With the increase of the tensile strain, the magnetic ground state changes from a state with total magnetic moment Mtot =1.0 to another state with Mtot =3.0 for single doping in a 4 × 4 supercell. Physical mechanism is elucidated from the effects of the local bonding and geometry symmetries on orbital hybridization. In addition, we find the ferromagnetic coupling is favored for small distances between Mn atoms corresponding to the uniform doping concentration of 25%. More importantly, the ferromagnetic state is highly stable and robust to tensile strains. Therefore, diluted magnetic semiconductors can be obtained and the strain engineering should be a very promising approach to tune the magnetic moments.

Quantum-driven phase transition in ice described via an efficient Langevin approach

The phase transition from ice VII to ice X under extreme pressures is an example where quantum proton delocalization coexists with classical thermal fluctuations. We investigate this transition, including quantum effects on the nuclear motion through adapted Langevin dynamics. This approach, which allows us to follow the semiclassical trajectories of protons, provides excellent agreement with experimental vibrational spectra indicating a transition pressure of about 65 GPa. Furthermore, we map the full dynamical problem onto a pressure-dependent, one-dimensional mean-field potential for the proton. By solving exactly the corresponding Schr\"odinger equation, we disentangle tunneling and quantum delocalization from classical thermal effects and identify the transition through the topological changes of the proton ground state and its susceptibility. The process is dominated by quantum effects even at ambient temperature and can be considered to be a paradigmatic case of a quantum-driven phase transition.

Quantitative calculation of magnetic and electric properties in a d2–d3 mixed-valence vanadium dimer complex

The electronic structure and magnetic properties in the mixed-valence d2–d3 vanadium dimer complex [(PY5Me2)2V2(μ-5,6-dmbzim)]4+ under an electric field are investigated by using the density functional theory and the effective Hamiltonian method. With the aim of controlling the localization–delocalization of the excess electron, an electric field is applied in the direction along the two vanadium atoms. The ground state changes with an increase of the electric field from the highest spin S = 5/2 state to the intermediate spin S = 3/2 state, and then it changes to the lowest spin S = 1/2 state. Thus, in this realistic mixed-valence dimer system, the possibility of controlling the spin state by using the electric field is shown. Various magnetic and electric properties of this mixed-valence dimer complex are also calculated using our effective Hamiltonian. The methodology in this paper is quite general and is a useful approach for the calculation of electronic structures in various magnetic complexes.

Theoretical investigation of the structures, stabilities, and NLO responses of calcium-doped pyridazine: Alkaline-earth-based alkaline salt electrides

Currently, whether alkaline-earth-doped compounds with electride characteristics are novel candidates for high-performance nonlinear optical (NLO) materials is unknown. In this paper, using quantum chemical computations, we show that: when doping calcium atoms into a family of alkaline-substituted pyridazines, alkaline-earth-based alkaline salt electrides M-H3C4N2⋯Ca (MH, Li, and K) with distended excess electron clouds are formed. Interestingly, from the triplet to the singlet state, the chemical valence of calcium atom changes from +1 to 0, and the dipole moment direction (μ0) of the molecule reverses for each M-H3C4N2⋯Ca. Changing pyridazine from without (H4C4N2⋯Ca) to with one alkaline substituent (M-H3C4N2⋯Ca, MLi and K), the ground state changes from the triplet to the singlet state. The alkaline earth metal doping effect (electride effect) and alkaline salt effect on the static first hyperpolarizabilities (β0) demonstrates that (1) the β0 value is increased approximately 1371-fold from 2 (pyridazine, H4C4N2) to 2745au (Ca-doped pyridazine, H4C4N2⋯Ca), (2) the β0 value is increased approximately 1146-fold from 2 in pyridazine (H4C4N2) to 2294au in an Li-substituted pyridazine (Li-H3C4N2), and (3) the β0 value is increased 324-(MLi) and 106-(MK) fold from 826 (MLi) and 2294au (MK) to 268,679 (MLi) and 245,878au (MK), respectively, from the alkalized pyridazine (M-H3C4N2) to the Ca-doped pyridazine (M-H3C4N2⋯Ca). These results may provide a new means for designing high-performance NLO materials.

Magnetic instability induced by Rh doping in the Kondo semiconductor CeRu2Al10

The magnetic ground state of the Rh-doped Kondo semiconductor CeRu${}_{2}$Al${}_{10}$ [Ce(Ru${}_{1\ensuremath{-}x}$Rh${}_{x}$)${}_{2}$Al${}_{10}$] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the $x=0$ sample, while only one frequency is present in the $x=0.05$ and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the $x=0$ sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around $x=0.03$. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu${}_{2}$Al${}_{10}$ is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu${}_{2}$Al${}_{10}$ is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.

Energy levels of exciton in a gapped graphene sheet

A theory is presented for exciton formation in a graphene sheet using the center-of-mass approximation. The energy levels and wavefunctions of exciton are calculated analytically which show that the exciton can form if the band gap of graphene is not zero. We show that the energy gap of graphene plays the role of the mass which if not zero, leads to formation of the excitons. It is shown that the main quantum number of the exciton ground state changes with the graphene dielectric constant. Also, all of the states are found to be four-fold degenerate. The binding energy of exciton can reach as high as 1/4 of the energy gap of graphene which is notable among the conventional quasi-2D systems. This result can play an important rule in the photonics of graphene.

Change of magnetic ground state by light electron doping in CeOs2Al10

The effect of Ir substitution for Os in CeOs${}_{2}$Al${}_{10}$, with an unusually high N\'eel temperature of ${T}^{*}\ensuremath{\sim}28.5$ K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir ($\ensuremath{\sim}$$8%$) results in a pronounced change of the magnetic structure of the Ce sublattice. The induced magnetic ground state is controlled by single ion anisotropy and implies an antiferromagnetic arrangement of the Ce moments along the $a$ axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1)${\ensuremath{\mu}}_{B}$, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21 K. A comparison of the observed phenomena with the recently studied CeRu${}_{1.9}$Rh${}_{0.1}$Al${}_{10}$ system, exhibiting similar behavior [Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides additional ways for exploring the anomalous magnetic properties of the Ce(Ru/Os)${}_{2}$Al${}_{10}$ compounds.

Effects of charging and electric field on graphene functionalized with titanium

Titanium atoms are adsorbed to graphene with a significant binding energy and render diverse functionalities to it. Carrying out first-principles calculations, we investigated the effects of charging and static electric field on the physical and chemical properties of graphene covered by Ti adatoms. When uniformly Ti covered graphene is charged positively, its antiferromagnetic ground state changes to ferromagnetic metal and attains a permanent magnetic moment. Static electric field applied perpendicularly causes charge transfer between Ti and graphene, and can induce metal–insulator transition. While each Ti adatom adsorbed to graphene atom can hold four hydrogen molecules with a weak binding, these molecules can be released by charging or applying electric field perpendicularly. Hence, it is demonstrated that charging and applied static electric field induce quasi-continuous and side specific modifications in the charge distribution and potential energy of adatoms absorbed to single-layer nanostructures, resulting in fundamentally crucial effects on their physical and chemical properties.

Ground state of a spin-crossover molecule calculated by diffusion Monte Carlo

Spin crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge as one has to describe the spin ground state of a transition metal as the legand field changes. The problem is dominated by the interplay between strong electron correlation at the transition metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory we then assess the performances of density functional theory. We find that the low spin state is always over-stabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact-exchange.

Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered structures.

A slave rotor--Hartree-Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

Theoretical simulation of the photoassociation process for NaCs

We investigate the two-step association process of NaCs using the time-dependent wave packet method. Ground state atoms can be photoassociated to the low vibrational levels of the ground state for an NaCs molecule by the two-step association. The time-dependent Schrödinger equation of the association process is solved within a three-state model and the wave packet is propagated with the “split operator-Fourier transform" scheme and the rotating-wave approximation (RWA). The vibrational population distribution of the ground state can be obtained by projecting the wave packet to every vibrational level of the ground state. The results not only show that for NaCs achievement of photoassociation production is accompanied by the photodissociation of the higher vibrational molecules, but also show that the vibrational distribution in lower vibrational levels of the ground state changes with the laser parameters.

Bonding to antibonding transition for hole ground states in coupled InAs quantum wires

We calculate hole energies of the bonding and antibonding states in coupled quantum wires using 6-band kp model. It is shown that hole ground state changes character from a bonding to an antibonding orbital with increasing inter-wire distances. This effect is a consequence of the enhancement of light hole contribution to the ground state. The variations of g factors under external electric and magnetic fields demonstrate the antibonding character of hole ground states.

GENUINE CORRELATIONS IN FINITE-SIZE SPIN SYSTEMS

Genuine multipartite correlations in finite-size XY chains are studied as a function of the applied external magnetic field. We find that, for low temperatures, multipartite correlations are sensitive to the parity change in the Hamiltonian ground state, given that they exhibit a minimum every time that the ground state becomes degenerate. This implies that they can be used to detect the factorizing point, that is, the value of the external field such that, in the thermodynamical limit, the ground state becomes the tensor product of single-spin states.