Ground State Changes Research Articles

Spin Crossover in a Hexaamineiron(II) Complex: Experimental Confirmation of a Computational Prediction.

Single crystal structural analysis of [FeII(tame)2]Cl2⋅MeOH (tame=1,1,1‐tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high‐spin octahedral d 6 state and a low temperature low‐spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T 1/2=140 K. Single crystal, variable‐temperature optical spectroscopy of [FeII(tame)2]Cl2⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [FeII(tame)2]2+ during its de novo artificial evolution design as a spin‐crossover complex [Chem. Inf. Model. 2015, 55, 1844], offering the first experimental validation of a functional transition‐metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin‐passive structural components. A thermodynamic analysis based on an Ising‐like mean field model (Slichter–Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.

Investigation of electronic bound states of a radially modulated nanowire: Level anti-crossing and bound states in continuum

In this paper the electronic confinement energy states of a radially modulated cylindrical nanowire are numerically calculated, assuming that the modulated section has finite length and its radius is greater than the remaining section of the nanowire. The Schrodinger equation is solved within the effective mass approximation using the finite element method to specify the necessary conditions for emersion of 3-dimensionally confined states and to investigate size and magnetic field dependence of the confinement energies. Our results show that for any local increment of nanowire radius, there is at least one bound state for each channel of angular momentum. The energies of bound states of higher angular momentum are shown to lie in the energy continuum of lower angular momentums. In addition, there is some level crossing and anti-crossing in the size dependence of confinement energies which can be explained on the basis of symmetry. It is observed that in the vicinity of the level anti-crossing, the oscillator strengths associated to intersubband transitions from ground state change dramatically and vanish for some specific width of modulation.

Superconductor–Insulator Transition in NbTiN Films

Experimental results indicating a direct disorder-induced superconductor–insulator transition in NbTiN thin films have been reported. It has been shown that an increase in the resistance per square in the normal state is accompanied by the suppression of the critical temperature of the superconducting transition Tc according to the fermion mechanism of suppression of superconductivity by disorder. At the same time, the temperature of the Berezinskii–Kosterlitz–Thouless transition is completely suppressed at a nonzero critical temperature and, then, the ground state changes to insulating, which is characteristic of the boson model of suppression of superconductivity by disorder. It has been shown that the temperature dependences of the resistance of insulating films follow the Arrhenius activation law.

Glassy magnetic ground state and Kondo-like behaviour in Mn10FeGe8 alloy

We report a detailed investigation of the ground-state magnetic properties of newly synthesized alloy. The sample can be thought of being derived by substituting one Mn atom by Fe of the parent compound . Fe-substitution leads to a drastic change in the magnetic ground state as well as to the magneto-transport properties of the parent alloy. On cooling below 250 K, undergoes a transition from paramagnetic phase to a state having significant ferromagnetic correlations. The ground state is found to be canonical spin glass (CSG) type in nature as evident from the dc magnetization and ac susceptibility measurements. Interestingly, the resistivity data shows an upturn at low temperature below about 30 K, mimicking Kondo-like behaviour. turns out to be a rare example among 3d transition metal alloys, where a Kondo-like state coexists within a CSG phase.

Ground state in E ⊗ e Jahn-Teller and Renner-Teller systems: Account of nonadiabaticity.

The ground and lower excited states of an E⊗e Jahn-Teller system with linear and quadratic vibronic coupling are considered, taking nonadiabaticity into account. Our calculations confirm a common opinion that in the case of a weak quadratic coupling, the ground state is doubly degenerate and the first excited state is nondegenerate for any linear coupling. However, with increasing quadratic coupling for weak linear coupling, the nondegenerate state becomes the ground state. The values of vibronic parameters are found at which the ground state changes.

Topological Superconductivity in a Planar Josephson Junction

We consider a two-dimensional electron gas with strong spin-orbit coupling contacted by two superconducting leads, forming a Josephson junction. We show that in the presence of an in-plane Zeeman field the quasi-one-dimensional region between the two superconductors can support a topological superconducting phase hosting Majorana bound states at its ends. We study the phase diagram of the system as a function of the Zeeman field and the phase difference between the two superconductors (treated as an externally controlled parameter). Remarkably, at a phase difference of $\pi$, the topological phase is obtained for almost any value of the Zeeman field and chemical potential. In a setup where the phase is not controlled externally, we find that the system undergoes a first-order topological phase transition when the Zeeman field is varied. At the transition, the phase difference in the ground state changes abruptly from a value close to zero, at which the system is trivial, to a value close to $\pi$, at which the system is topological. The critical current through the junction exhibits a sharp minimum at the critical Zeeman field, and is therefore a natural diagnostic of the transition. We point out that in presence of a symmetry under a modified mirror reflection followed by time reversal, the system belongs to a higher symmetry class and the phase diagram as a function of the phase difference and the Zeeman field becomes richer.

Evolution of a magnetic order in the quasi-kagome lattice system CeRh1–xPdxSn (x ≤ 0.75)

The equiatomic compound CeRhSn with a quasi-kagome lattice of Ce atoms displays a non-Fermi liquid behavior at low temperatures. We have studied how the ground state changes with the substitution of Pd for Rh in CeRh1–xPdxSn (0 ≤ x ≤ 0.75) by measuring the specific heat C, magnetic susceptibility χ, and magnetization M. With increasing x, the lattice parameters a and c increase by 1.6%, the paramagnetic Curie temperature in χ(T) changes from –112 K to 16 K, and M(B = 5 T) at T = 1.8 K increases from 0.05μB/Ce to 1.4μB/Ce. A long-range magnetic order manifests in sharp peaks in C at 0.7, 1.0 and 1.5 K for x = 0.4, 0.5 and 0.65, respectively. An antiferromagnetic order for x = 0.75 is indicated by the observations of peaks in both χ(T) and C(T) at around 3.0 K and metamagnetic behavior in M(B) at B = 2 T. Our results point out that the doping of 4d electrons in the non-Fermi liquid system CeRhSn destroys the coherence of the quasi-kagome lattice and weakens the hybridization between the 4f state and conduction bands, leading to the evolution of antiferromagnetic order.

Experimental Investigation of Settlement in Sand Soil of Jacked Pile Subjected by Vertical Compressive Cyclic Loading

Repeated-variable external effects must be properly evaluated when solving many geotechnical engineering problems. Cyclic loads of different intensities is caused by technological and environmental effects, e.g. traffic, industrial machinery, wind, waves and snow. The influence to the investigated system (structure, ground) and usually is analyzed applying static or dynamic models. Low cycle load model is relevant to response of analyzed system, where inertia forces do not contribute essentially to it's behavior. In this case the effects of coupling plays the dominate role to the system response measures. The incremental deformation of ground (e.g. in terms of settlements of foundations) caused by cyclic nature of separate loads for buildings, where constant (weight) loads dominate versus variable loads is the case where dynamic response analysis is not necessary (low cycle case). Here static analysis is sufficient, but static response measures should involve parameters of loading process, consisting from constant and variable loads. The peculiarities of ground state changes (densification, yielding, etc.) prescribing foundation response range (elastic, elastic-plastic, etc.) should be taken into account.The experimental study for displacement (jacked) pile settlement in sand soil evaluation has been performed. The model pile has been tested for 5 different dead and variable load components. 50 load variation loading cycles have been applied, pile top displacements (id. est. ground settlements, as axial deformation of pile is negligible) versus number loading cycles measured. Peculiarities of settlement development versus different loading conditions have been reported.

Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1−xCox)2As2 single crystals

The magnetic ground state of the ${\mathrm{Eu}}^{2+}$ moments in a series of $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ is established. The ground-state magnetic structure of the ${\mathrm{Eu}}^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}=0.100(4)$, and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals with 0.$10\ensuremath{\le}\mathit{x}\ensuremath{\le}0.18$, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.

Geometrical and electronic structure of the molybdenum and tungsten halides MX3 and MX4 (M = Mo, W; X = F, Cl): Jahn-Teller effect and spin-orbit coupling

The ground and lower-lying excited electronic states of MX3 and MX4 (M = Mo, W; X = F, Cl) molecules were systematically studied by the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (XMCQDPT2) methods. Scalar-relativistic effects and spin-orbit coupling were taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator. The ground orbital state of MoF3 and MoCl3 complexes is quadruplet 4A2′ state in the equilibrium configuration with D3h symmetry. If the spin-orbit interaction is taken into account the calculations result in two 4E1/2 and 4E3/2 spin-orbit states with 11–14 cm−1 energy differences instead of one 4A2′ state. Spin-orbit interaction quenches the Jahn-Teller effect in 2E″ ground orbital state of WF3 complex in D3h configuration, and C2v configuration with (4E1/2 + 4E3/2) spin-mixed state corresponds to the minimum on the PES. The WCl3 complex reveals strong interaction of the spin-orbit states of different multiplicity. For WCl3 complex the configuration with D3h symmetry corresponds to the minimum on the PES for four lowest spin-mixed electronic states. The MoF4, MoCl4 and WCl4 complexes possess the tetrahedral equilibrium configuration with 3A2 orbital electronic state. If the spin-orbit interaction is taken into account the ground state changes to the threefold degenerated 3T2 spin-orbit state with weak Jahn-Teller effect. In the case of WF4 complex the strong Jahn-Teller effect has been discovered in the first excited 1E singlet electronic state of the tetrahedral structure, and D2d configuration with 1A1 electronic state possesses the lowest energy. The comparison with available experimental data has been performed. The tendencies in molecular parameters of complexes studied were analyzed.

Field-stepped broadband NMR in pulsed magnets and application to SrCu2(BO3)2 at 54 T

Pulsed magnets generate the highest magnetic fields as brief transients during which the observation of NMR is difficult, however, this is the only route to unique insight into material properties up to the regime of 100T. Here, it is shown how rather broad NMR spectra can be assembled in a pulsed magnet during a single field pulse by using the inherent time dependence of the field for the recording of field-stepped free induction decays that cover a broad frequency range. The technique is then applied to 11B NMR of the spin-dimer system SrCu2(BO3)2, a magnetic insulator known to undergo a series of field-driven changes of the magnetic ground state. At peak fields of about 54T at the Dresden High Magnetic Field Laboratory, 11B NMR spectra spanning a total of about 9MHz width are reconstructed. The results are in good accordance with a change from a high-temperature paramagnetic state to a low-temperature commensurate superstructure of field-induced spin-dimer triplets.

Evolution of magnetic properties in the solid solution [formula omitted

We have investigated the evolution of magnetic properties in pseudo-ternary UCo1−xPdxGe (x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) intermetallics by measurements of ac-magnetic susceptibility and dc-magnetization. The measured samples have been prepared by arc-melting and characterized by powder X-ray diffraction and X-ray energy-dispersive electron spectroscopy technique. Rietveld refinements of the X-ray patterns indicate that the pseudo-ternaries crystallize in the orthorhombic TiNiSi-type structure with Pnma space group as the parent UCoGe and UPdGe compounds do. The magnetic measurements reveal that in addition to an increase in 5f-electron localization degree with increasing Pd concentration, an interesting magnetic phase diagram, which is more complex than that expected from the gradual change in magnetic ground state due to the substitution. Three compositional ranges with different magnetic properties have been established; the coexistence of spin-glass and itinerant-electron ferromagnetic orderings for x≤0.3, antiferromagnetic order occurs in the range 0.4≤x≤0.7, and a complex ferromagnetic structure of more localized 5f moments takes place in x≥0.8. The evolution of magnetic ground states in UCo1−xPdxGe is discussed in terms of interplay between cooperative magnetic interactions. We consider also implications of increasing conduction electrons upon the Pd substitution in Ruderman-Kittel-Kasuya-Yosida interactions, which would give rise to the evolution of magnetic properties in UCo1−xPdxGe pseudo-ternaries.

Avoided ferromagnetic quantum critical point: Antiferromagnetic ground state in substituted CeFePO

AbstractWe have investigated single crystals of two substitution series Ce(Ru1−x Fex)PO and CeFe(As1−y Py)O in the vicinity to the quantum critical material CeFePO by means of magnetic‐susceptibility and specific‐heat measurements. We observe an antiferromagnetic ground state in the vicinity of the quantum critical point, with pronounced metamagnetic transitions for , which is the magnetically hard direction. Our results verify that a ferromagnetic quantum critical point is avoided in substituted CeFePO, because we clearly demonstrate that the ferromagnetic ground state changes into an antiferromagnetic one, when approaching the quantum critical point.Temperature–substitution phase diagram of the series Ce(Ru1−x Fex)PO and CeFe(As1−y Py)O.

Magnetocaloric properties of Eu1−xLaxTiO3 (0.01 ≤ x ≤ 0.2) for cryogenic magnetic cooling

We report magnetic and magnetocaloric (MCE) properties of polycrystalline Eu1−xLaxTiO3 samples over a wide composition range (0.01 ≤ x ≤ 0.20). It is found that the ground state changes from antiferromagnetic for x = 0.01 (TN = 5.2 K) to ferromagnetic for x ≥ 0.03 and the ferromagnetic Curie temperature increases from TC = 5.7 K for x = 0.03 to TC = 7.9 K for x = 0.20. The x = 0.01 sample shows a large reversible isothermal magnetic entropy change of −ΔSm = 23 (41.5) J/kg K and adiabatic temperature change of ΔTad = 9 (17.2) K around 6.7 K for a field change of μ0ΔH = 2 (5) Tesla. Although the peak value of −ΔSm decreases as La content increases, it is impressive in x = 0.2(−ΔSm = 31.41 J/kg K at T = 7.5 K for μ0ΔH = 5 T). The large value of MCE arises from suppression of the spin entropy associated with the localized moment (J = 7/2) of Eu2+:4f7 ions. This large MCE over a wide compositional range suggests that the Eu1−xLaxTiO3 series could be useful for magnetic cooling below 40 K.

Atomic spin-chain realization of a model for quantum criticality

The ability to manipulate single atoms has opened up the door to constructing interesting and useful quantum structures from the ground up. On the one hand, nanoscale arrangements of magnetic atoms are at the heart of future quantum computing and spintronic devices; on the other hand, they can be used as fundamental building blocks for the realization of textbook many-body quantum models, illustrating key concepts such as quantum phase transitions, topological order or frustration. Step-by-step assembly promises an interesting handle on the emergence of quantum collective behavior as one goes from one, to few, to many constituents. To achieve this, one must however maintain the ability to tune and measure local properties as the system size increases. Here, we use low-temperature scanning tunneling microscopy to construct arrays of magnetic atoms on a surface, designed to behave like spin-1/2 XXZ Heisenberg chains in a transverse field, for which a quantum phase transition from an antiferromagnetic to a paramagnetic phase is predicted in the thermodynamic limit. Site-resolved measurements on these finite size realizations reveal a number of sudden ground state changes when the field approaches the critical value, each corresponding to a new domain wall entering the chains. We observe that these state crossings become closer for longer chains, indicating the onset of critical behavior. Our results present opportunities for further studies on quantum behavior of many-body systems, as a function of their size and structural complexity.

Interplay between tensor force and deformation in even–even nuclei

In this work we study the effect of the nuclear tensor force on properties related with deformation. We focus on isotopes in the Mg, Si, S, Ar, Sr and Zr chains within the Hartree–Fock–Bogoliubov theory using the D1ST2a Gogny interaction. Contributions to the tensor energy in terms of saturated and unsaturated subshells are analyzed. Like–particle and proton–neutron parts of the tensor term are independently examinated. We found that the tensor term may considerably modify the potential energy landscapes and change the ground state shape. We analyze too how the pairing characteristics of the ground state change when the tensor force is included.

Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule.

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.

Structural phase transition in perovskite metal-formate frameworks: a Potts-type model with dipolar interactions.

We propose a combined experimental and numerical study to describe an order-disorder structural phase transition in perovskite-based [(CH3)2NH2][M(HCOO)3] (M = Zn(2+), Mn(2+), Fe(2+), Co(2+) and Ni(2+)) dense metal-organic frameworks (MOFs). The three-fold degenerate orientation of the molecular (CH3)2NH2(+) (DMA(+)) cation implies a selection of the statistical three-state model of the Potts type. It is constructed on a simple cubic lattice where each lattice point can be occupied by a DMA(+) cation in one of the available states. In our model the main interaction is the nearest-neighbor Potts-type interaction, which effectively accounts for the H-bonding between DMA(+) cations and M(HCOO)3(-) cages. The model is modified by accounting for the dipolar interactions which are evaluated for the real monoclinic lattice using density functional theory. We employ the Monte Carlo method to numerically study the model. The calculations are supplemented with the experimental measurements of electric polarization. The obtained results indicate that the three-state Potts model correctly describes the phase transition order in these MOFs, while dipolar interactions are necessary to obtain better agreement with the experimental polarization. We show that in our model with substantial dipolar interactions the ground state changes from uniform to the layers with alternating polarization directions.

Spin reorientation driven by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides

In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.

Magnetic properties of Czochralski-grown Ce2Pd2In single crystal

A Ce2Pd2In single crystal was synthesized employing the Czochralski growth method from a stoichiometric melt. It was found to crystallize in the tetragonal Mo2FeB2-type structure. The investigation by means of X-ray diffraction, magnetization, specific heat, and electrical resistivity measurements revealed two successive magnetic phase transitions at around 4.5 and 4.1K in the possibly slightly off-stoichiometric (Pd rich) crystal. Ferromagnetic ground state changes into the paramagnetic state through an intermediate antiferromagnetic phase. The measured magnetization and specific heat data were discussed in the view of first principles calculations of crystal field for Ce2Pd2In. Uniaxial magnetic anisotropy confining Ce moments into the c-axis has rather high energy exceeding 5meV. Full ordered Ce-4f1 moments 2.14μB do not indicate any Kondo-type interaction, no many body enhancement of the Sommerfeld coefficient can be deduced.