Symmetry Of Ground State Research Articles

Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking.

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C≡C-Phenyl-X)2, where X = diphenylamino (DPA), NH2, OCH3, t-Bu, CH3, H, F, benzothiazole (BTH), CF3, CN, and NO2. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S1 state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S1 state. The values are small Δμ < 1.0 D for neutral substituents such as CH3, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO2. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation ( Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014 , 118 , 3749 - 3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.

Partition-DFT on the water dimer.

As is well known, the ground-state symmetry group of the water dimer switches from its equilibrium Cs-character to C2h-character as the distance between the two oxygen atoms of the dimer decreases below RO-O∼2.5 Å. For a range of RO-O between 1 and 5 Å, and for both symmetries, we apply Partition Density Functional Theory (PDFT) to find the unique monomer densities that sum to the correct dimer densities while minimizing the sum of the monomer energies. We calculate the work involved in deforming the isolated monomer densities and find that it is slightly larger for the Cs geometry for all RO-O. We discuss how the PDFT densities and the corresponding partition potentials support the orbital-interaction picture of hydrogen-bond formation.

Are There Signatures of Harmonic Oscillator Shells Far from Stability? First Spectroscopy of ^{110}Zr.

The first measurement of the low-lying states of the neutron-rich ^{110}Zr and ^{112}Mo was performed via in-beam γ-ray spectroscopy after one proton removal on hydrogen at ∼200 MeV/nucleon. The 2_{1}^{+} excitation energies were found at 185(11)keV in ^{110}Zr, and 235(7)keV in ^{112}Mo, while the R_{42}=E(4_{1}^{+})/E(2_{1}^{+}) ratios are 3.1(2), close to the rigid rotor value, and 2.7(1), respectively. These results are compared to modern energy density functional based configuration mixing models using Gogny and Skyrme effective interactions. We conclude that first levels of ^{110}Zr exhibit a rotational behavior, in agreement with previous observations of lighter zirconium isotopes as well as with the most advanced MonteCarlo shell model predictions. The data, therefore, do not support a harmonic oscillator shell stabilization scenario at Z=40 and N=70. The present data also invalidate predictions for a tetrahedral ground state symmetry in ^{110}Zr.

The quartet ground state in CeB6: An inelastic x-ray scattering study

We investigated the ground-state symmetry of the cubic hidden order compound CeB6 by means of core level non-resonant inelastic x-ray scattering (NIXS). The information is obtained from the directional dependence of the scattering function that arises from higher than dipole transitions. Our new method confirms that the ground state is well described using a localized crystal-field model assuming a Γ8 quartet ground state.

Structural, magnetic and vibrational properties of multiferroic GaFeO3 at high pressure

The crystal, magnetic structure and vibrational spectra of multiferroic GaFeO3 have been studied by means of neutron, X-ray powder diffraction and Raman spectroscopy at pressures up to 6.2 and 42 GPa, respectively. A presence of Fe/Ga antisite disorder leads to a formation of the ferrimagnetic ground state with the Néel temperature TN = 292 K at ambient pressure. Upon compression, the magnetic ground state symmetry remains the same and the Néel temperature increases with a pressure coefficient (1/TN)(dTN/dP) = 0.011(1) GPa−1. Application of high pressure above 21 GPa leads to a gradual structural phase transition from the polar orthorhombic Pc21n phase to nonpolar orthorhombic Pbnm phase. It is accompanied by anomalies in the pressure behaviour of several Raman modes. Pressure dependencies of lattice parameters and Raman modes frequencies in the observed structural phases were obtained.

Metal–Metal Bonding Stabilized Ground State Structure of Early Transition Metal Monoxide TM–MO (TM = Ti, Hf, V, Ta)

It is commonly believed that early transition metal monoxides (TM–MOs) crystallize in simple rock-salt structures (symmetry FM3M) for their ground states. Here, by combining structure-searching algorithm and first-principles calculations, we identified structures that are more stable than the ideal rock-salt for the early TM–MOs (TM = Ti, Hf, V, Ta). For TiO, HfO, and TaO, ground state symmetries of P62M), I41/AMD and P1 are obtained, respectively, which have distinct structural and electronic properties compared to the rock-salt structure. However, it is rather complex for the case of VO due to the existence of magnetic ordering. For VO, magnetic ordering behavior exists in the rock-salt and the predicted P1 structure according to the hybrid functional calculations. After relaxation, the magnetic ordering causes local distortion in the original rock-salt structure, leading to a R3M symmetry, which becomes more stable than the predicted P1 structure. Furthermore, the ionic TM–O bonding of the predic...

Unconventional superconducting states on doped Shastry–Sutherland lattice

By using a renormalized mean-field theory, we investigate the phase diagram of t–t′–J–J′ model on a two dimensional Shastry–Sutherland (SS) lattice which can be realized in a group of layered compounds such as SrCu2(BO3)2, Yb2Pt2Pb. We find that the symmetry of ground state depends on frustration parameter η=t′/t and doping concentration. For weak and intermediate |η|, dx2−y2-wave superconducting state is robust in a wide region. While for larger |η| > 1 cases, the superconducting ground state has s–s-wave symmetry. Around the most frustrated η=±1 point, dx2−y2-wave state, s–s-wave state as well as staggered-flux state appear, the energy favorable state is the ground state. The order of the phase transition between different states depends on the continuity of the mean field order parameters.

Polarized hard X-ray photoemission system with micro-positioning technique for probing ground-state symmetry of strongly correlated materials.

An angle-resolved linearly polarized hard X-ray photoemission spectroscopy (HAXPES) system has been developed to study the ground-state symmetry of strongly correlated materials. The linear polarization of the incoming X-ray beam is switched by a transmission-type phase retarder composed of two diamond (100) crystals. The best value of the degree of linear polarization was found to be -0.96, containing a vertical polarization component of 98%. A newly developed low-temperature two-axis manipulator enables easy polar and azimuthal rotations to select the detection direction of photoelectrons. The lowest temperature achieved was 9 K, offering the chance to access the ground state even for strongly correlated electron systems in cubic symmetry. A co-axial sample monitoring system with long-working-distance microscope enables the same region on the sample surface to be measured before and after rotation. Combining this sample monitoring system with a micro-focused X-ray beam by means of an ellipsoidal Kirkpatrick-Baez mirror (25 µm × 25 µm FWHM), polarized valence-band HAXPES has been performed on NiO for voltage application as resistive random access memory to demonstrate the micro-positioning technique and polarization switching.

The significant role of covalency in determining the ground state of cobalt phthalocyanines molecule

To shed some light on the metal 3d ground state configuration of cobalt phthalocyanines system, so far in debate, we present an investigation by X-ray absorption spectroscopy (XAS) at Co L2,3 edge and theoretical calculation. The density functional theory calculations reveal highly anisotropic covalent bond between central cobalt ion and nitrogen ligands, with the dominant σ donor accompanied by weak π-back acceptor interaction. Our combined experimental and theoretical study on the Co-L2,3 XAS spectra demonstrate a robust ground state of 2A1g symmetry that is built from 73% 3d7 character and 27% 3d8L¯ (L¯ denotes a ligand hole) components, as the first excited-state with 2Eg symmetry lies about 158 meV higher in energy. The effect of anisotropic and isotropic covalency on the ground state was also calculated and the results indicate that the ground state with 2A1g symmetry is robust in a large range of anisotropic covalent strength while a transition of ground state from 2A1g to 2Eg configuration when isotropic covalent strength increases to a certain extent. Here, we address a significant anisotropic covalent effect of short Co(II)-N bond on the ground state and suggest that it should be taken into account in determining the ground state of analogous cobalt complexes.

Persistent order due to transiently enhanced nesting in an electronically excited charge density wave.

Non-equilibrium conditions may lead to novel properties of materials with broken symmetry ground states not accessible in equilibrium as vividly demonstrated by non-linearly driven mid-infrared active phonon excitation. Potential energy surfaces of electronically excited states also allow to direct nuclear motion, but relaxation of the excess energy typically excites fluctuations leading to a reduced or even vanishing order parameter as characterized by an electronic energy gap. Here, using femtosecond time- and angle-resolved photoemission spectroscopy, we demonstrate a tendency towards transient stabilization of a charge density wave after near-infrared excitation, counteracting the suppression of order in the non-equilibrium state. Analysis of the dynamic electronic structure reveals a remaining energy gap in a highly excited transient state. Our observation can be explained by a competition between fluctuations in the electronically excited state, which tend to reduce order, and transiently enhanced Fermi surface nesting stabilizing the order.

Adiabatic cooling of bosons in lattices to magnetically ordered quantum states

We suggest and analyze a new scheme to adiabatically cool bosonic atoms to picokelvin temperatures which should allow the observation of magnetic ordering via superexchange in optical lattices. The starting point is a gapped phase called the spin Mott phase where each site is occupied by one spin-up and one spin-down atom. An adiabatic ramp leads to an xy-ferromagnetic phase. We show that the combination of time-dependent density matrix renormalization group methods with quantum trajectories can be used to fully address possible experimental limitations due to decoherence, and demonstrate that the magnetic correlations are robust for experimentally realizable ramp speeds. Using a microscopic master equation treatment of light scattering in the many-particle system, we test the robustness of adiabatic state preparation against decoherence. Due to different ground-state symmetries, we also find a metastable state with xy-ferromagnetic order if the ramp crosses to regimes where the ground state is a z-ferromagnet. The bosonic spin Mott phase as the initial gapped state for adiabatic cooling has many features in common with a fermionic band insulator, but the use of bosons should enable experiments with substantially lower initial entropies.

Tuning the Ground State Symmetry of Acetylenyl Radicals.

The lowest excited state of the acetylenyl radical, HCC, is a 2Π state, only 0.46 eV above the ground state, 2Σ+. The promotion of an electron from a π bond pair to a singly occupied σ hybrid orbital is all that is involved, and so we set out to tune those orbital energies, and with them the relative energetics of 2Π and 2Σ+ states. A strategy of varying ligand electronegativity, employed in a previous study on substituted carbynes, RC, was useful, but proved more difficult to apply for substituted acetylenyl radicals, RCC. However, π-donor/acceptor substitution is effective in modifying the state energies. We are able to design molecules with 2Π ground states (NaOCC, H2NCC (2A″), HCSi, FCSi, etc.) and vary the 2Σ+–2Π energy gap over a 4 eV range. We find an inconsistency between bond order and bond dissociation energy measures of the bond strength in the Si-containing molecules; we provide an explanation through an analysis of the relevant potential energy curves.

Evidence for Γ8 Ground-State Symmetry of Cubic YbB12 Probed by Linear Dichroism in Core-Level Photoemission

We have successfully observed linear dichroism in angle-resolved Yb3+ 3d5/2 core-level photoemission spectra for YbB12 in cubic symmetry. Its anisotropic 4f charge distribution due to the crystal-field splitting is responsible for the linear dichroism, which has been verified by spectral simulations using ionic calculations with the full multiplet theory for a single-site Yb3+ ion in cubic symmetry. The observed linear dichroism as well as the polarization-dependent spectra in two different photoelectron directions for YbB12 are quantitatively reproduced by theoretical analysis for the Gamma_8 ground state, indicating the Gamma_8 ground-state symmetry for the Yb3+ ions mixed with the Yb2+ state.

Ground and Excited States of the [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ Clusters: Insight into the Electronic Structure of the [Fe(H₂O)₆]²⁺-[Fe(H₂O)₆]³⁺ Complex.

We report the ground and low-lying electronically excited states of the [Fe(H2O)6](2+) and [Fe(H2O)6](3+) clusters using multiconfiguration electronic structure theory. In particular, we have constructed the potential energy curves (PECs) with respect to the iron-oxygen distance when removing all water ligands at the same time from the cluster minima and established their correlation to the long-range dissociation channels. Due to the fact that both the second and third ionization potentials of iron are larger than the one for water, the ground-state products asymptotically correlate with dissociation channels that are repulsive in nature at large separations, as they contain at least one H2O(+) fragment and a singly positively charged metal center (Fe(+)). The most stable equilibrium structures emanate, via intersections and/or avoided crossings, from the channels consisting of the lowest electronic states of Fe(2+)((5)D, 3d(6)) or Fe(3+)((6)S, 3d(5)) and six neutral water molecules. Upon hydration, the ground state of Fe(2+)(H2O)6 is a triply ((5)Tg) degenerate one, with the doubly ((5)Eg) degenerate state lying ∼20 kcal/mol higher in energy. Similarly, the Fe(3+)(H2O)6 cluster has a ground state of (6)Ag symmetry under Th symmetry, which is well-separated from the first excited state. We also examine a multitude of electronically excited states of many possible spin multiplicities and report the optimized geometries for several selected states. The PECs of those states exhibit a high density of states. Focusing on the ground and the first few excited states of the [Fe(H2O)6](2+) and [Fe(H2O)6](3+) clusters, we studied their mutual interaction in the gas phase. We obtained the optimal geometries of the Fe(2+)(H2O)6-Fe(3+)(H2O)6 gas-phase complex for different Fe-Fe distances. For distances shorter than 6.0 Å, the water molecules in the respective first solvation shells located between the two metal centers were found to interact via weak hydrogen bonds. We examined a total of 10 electronic states for this complex, including those corresponding to the electron transfer (ET) from the ferrous to the ferric ion. The ET process is discussed and a possible path via a quasi-symmetric transition state is suggested.

Electric dipole sheets inBaTiO3/BaZrO3superlattices

We investigate two-dimensional electric dipole sheets in the superlattice made of BaTiO$_{3}$ and BaZrO$_{3}$ using first-principles-based Monte-Carlo simulations and density functional calculations. Electric dipole domains and complex patterns are observed and the complex dipole structures with various symmetries (e.g. Pma2, Cmcm and Pmc2_{1}) are further confirmed by density functional calculations, which are found to be almost degenerate in energy with the ferroelectric ground state of the Amm2 symmetry, therefore strongly resembling magnetic sheets. More complex dipole patterns, including vortices and anti-vortices, are also observed, which may constitute the intermediate states that overcome the high energy barrier of different polarization orientations previously predicted by Lebedev\onlinecite{Lebedev2013}. We also show that such system possesses large electrostrictive effects that may be technologically important.

Mn2@Si15: the smallest triple ring tubular silicon cluster.

The smallest triple ring tubular silicon cluster Mn2@Si15 is reported for the first time. Theoretical structural identification shows that the Mn2@Si15 tubular structure whose triple ring is composed by three five-membered Si rings in anti-prism motif, is stable in high symmetry (D5h) and singlet ground state ((1)A1'). The dimer Mn2 is placed inside the tubular along the C5 axis, and the Mn dopant form single Si-Mn bonds with Si skeleton, whereas the Mn-Mn is characterized as a triple bond. The effect of Mn2 on the stability of the Si15 triple ring structure arises from strong orbital overlap of Mn2 with Si15.

Ground-state Properties and Symmetry Energy of Mg Isotopes with $A=20$--36

8 pags.; 4 figs.; Presented at the XXII Nuclear Physics Workshop “Marie and Pierre Curie”, Kazimierz, Dolny, Poland, September 22–27, 2015; PACS numbers: 21.60.Jz, 21.65.Ef, 21.10.Gv, 27.30.+t

Deterministic switching of ferromagnetism at room temperature using an electric field.

The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii-Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

Probing Strongly Correlated 4f-Orbital Symmetry of the Ground State in Yb Compounds by Linear Dichroism in Core-Level Photoemission

We show that the strongly correlated 4f-orbital symmetry of the ground state is revealed by linear dichroism in core-level photoemission spectra as we have discovered for YbRh2Si2 and YbCu2Si2. Theoretical analysis tells us that the linear dichroism reflects the anisotropic charge distributions resulting from crystalline electric field. We have successfully determined the ground-state 4f symmetry for both compounds from the polarization-dependent angle-resolved core-level spectra at a low temperature well below the first excitation energy. The excited-state symmetry is also probed by temperature dependence of the linear dichroism where the high measuring temperatures are of the order of the crystal-field-splitting energies.

Existence and Symmetry of Ground States to the Boussinesq abcd Systems

We consider a four-parameter family of Boussinesq systems derived by Bona et al. (J Nonlinear Sci 12:283–318, 2002). We establish the existence of the ground states which are solitary waves minimizing the action functional of the systems. We further show that in the presence of large surface tension the ground states are even up to translation.