Showing papers on "Ground state published in 2004"

A tetranuclear 3d-4f single molecule magnet: [CuIILTbIII(hfac)2]2.

Abstract: We report now the first single molecule magnet (SMM) consisting of d−f elements. The present study demonstrates that the synthesis of the d−f polynuclear molecule is a very promising approach to SMMs. (1) The d−f polynuclear molecule can be easily synthesized by the assembly reaction of the d-component and the f-component, (2) the high-spin ground state can be generated by a smaller number of metal ions than the d complex, and (3) the molecular magnetic anisotropy is easily derived from the f-component.

Oligoacenes: Theoretical Prediction of Open-Shell Singlet Diradical Ground States

Abstract: A series of oligoacenes from benzene to decacene were studied computationally with DFT and CASSCF methods. In contrast to the common view that acenes are closed-shell systems or may have a triplet ground state, these results offer the first theoretical predictions for the singlet ground state and diradical character for oligoacenes. The nature of the ground states of these molecules arises from the disjoint nature of the NBMOs that are singly occupied in the diradical.

Inhomogeneous superconductivity in condensed matter and QCD

Abstract: Inhomogeneous superconductivity arises when the species participating in the pairing phenomenon have different Fermi surfaces with a large enough separation. In these conditions it could be more favorable for each of the pairing fermions to stay close to its Fermi surface and, unlike the usual BCS state, for the Cooper pair to have a nonzero total momentum. For this reason, in this state the gap varies in space, the ground state is inhomogeneous, and a crystalline structure might be formed. This situation was considered for the first time by Fulde and Ferrell (1964) and Larkin and Ovchinnikov (1964), after whom the corresponding state is called the LOFF state. The spontaneous breaking of the space symmetries in the vacuum state is a characteristic feature of this phase and is associated with the presence of long-wavelength excitations of zero mass. The situation described here is of interest both in solid-state and in elementary-particle physics, in particular in quantum chromodynamics at high density and low temperature. This review presents the theoretical approach to the LOFF state and its phenomenological applications using the language of the effective field theories.

Charge Ordering, Commensurability, and Metallicity in the Phase Diagram of the Layered NaxCoO2

Abstract: The phase diagram of nonhydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2), then to a "Curie-Weiss metal" (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.

Hall-effect evolution across a heavy-fermion quantum critical point.

Abstract: A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet1,2,3. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type4,5, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism6,7,8. Here we report measurements of the low-temperature Hall coefficient (RH)—a measure of the Fermi surface volume—in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. RH undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.

Universality of entropy scaling in one dimensional gapless models.

Abstract: We consider critical models in one dimension. We study the ground state in the thermodynamic limit (infinite lattice). We are interested in an entropy of a subsystem. We calculate the entropy of a part of the ground state from a space interval $(0,x)$. At zero temperature it describes the entanglement of the part of the ground state from this interval with the rest of the ground state. We obtain an explicit formula for the entropy of the subsystem at any temperature. At zero temperature our formula reproduces a logarithmic formula, discovered by Vidal, Latorre, Rico, and Kitaev for spin chains. We prove our formula by means of conformal field theory and the second law of thermodynamics. Our formula is universal. We illustrate it for a Bose gas with a delta interaction and for the Hubbard model.

Dynamics of charged particles and their radiation field

Abstract: This book provides a self-contained and systematic introduction to classical electron theory and its quantization, non-relativistic quantum electrodynamics. The first half of the book covers the classical theory. It discusses the well-defined Abraham model of extended charges in interaction with the electromagnetic field, and gives a study of the effective dynamics of charges under the condition that, on the scale given by the size of the charge distribution, they are far apart and the applied potentials vary slowly. The second half covers the quantum theory, leading to a coherent presentation of non-relativistic quantum electrodynamics. Topics discussed include non-perturbative properties of the basic Hamiltonian, the structure of resonances, the relaxation to the ground state through emission of photons, the non-perturbative derivation of the g-factor of the electron and the stability of matter.

Validation of Exchange-Correlation Functionals for Spin States of Iron Complexes

Abstract: Spin state energies of iron complexes are important for biochemical applications such as the catalytic cycle of cytochrome P450. Due to the size of these systems and the presence of iron, accurate computational results can be obtained only with density functional theory (DFT). Validation of exchange-correlation (xc) DFT functionals for predicting the correct spin ground state of iron complexes is a rather unexplored area. In this contribution we report a systematic study on the performance of several xc functionals for seven iron complexes that are experimentally found to have either a low, intermediate, or high spin ground state. Standard xc functionals like LDA, BLYP, and PBE are found to disfavor high spin states, whereas hybrid and some meta-GGA functionals do provide the correct spin ground state for all molecules. Recently improved pure DFT functionals such as Handy's optimized exchange (OPTX) also perform well. The origin for the apparent performance of the DFT functionals has been addressed and seems to be related to the inclusion of fourth-order terms (s 4 ) of the dimensionless (or reduced) density gradient s in the exchange functional.

Relativistic quantum chemistry: the multiconfigurational approach

Abstract: A multiconfigurational approach to the quantum chemistry of heavy element compounds is described. Relativistic effects are treated in two steps, both based on the Douglas–Kroll Hamiltonian. Scalar terms are included in the basis set generation and are used to determine wave functions and energies, which include static (through the use of the CASSCF method) and dynamic correlation effects (using multiconfigurational perturbation theory, CASPT2). Spin–orbit coupling is treated in a configuration interaction model, which uses CASSCF wave functions as the basis states. The method is shown to work for all atoms of the periodic system, with the possible exception of the heavier fifth row main group atoms. Illustrative results are presented for the main group atoms (spin–orbit splittings), the electronic spectrum of the iridium atom, the ground state of Tl2 and Pb2, and for the electronic spectrum of PbO. Some applications in actinide chemistry are also discussed.

Laser Cooling of a Nanomechanical Resonator Mode to its Quantum Ground State

Abstract: We show that it is possible to cool a nanomechanical resonator mode to its ground state. The proposed technique is based on resonant laser excitation of a phonon sideband of an embedded quantum dot. The strength of the sideband coupling is determined directly by the difference between the electron-phonon couplings of the initial and final states of the quantum dot optical transition. Possible applications of this scheme include generation of nonclassical states of mechanical motion.

Efficient deactivation of a model base pair via excited-state hydrogen transfer.

Abstract: We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1 ππ* state and the electronic ground state with a 1 ππ* charge-transfer state that is strongly stabilized by the transfer of a proton.

Porphyrin-fullerene linked systems as artificial photosynthetic mimics.

Abstract: We have prepared a variety of porphyrin–fullerene linked systems to mimic photoinduced energy and electron transfer (ET) processes in photosynthesis. Photodynamical studies on porphyrin and analogs–fullerene linked systems have revealed the acceleration of photoinduced electron transfer and charge-shift and the deceleration of charge recombination, which is reasonably explained by the small reorganization energies of electron transfer in fullerenes. In this context, we have proposed two strategies, photoinduced single-step and multi-step electron transfers, for prolonging the lifetime of a charge-separated state in donor–acceptor linked systems. The single-step ET strategy allowed a zinc chlorin–fullerene linked dyad to extend the lifetime up to 120 seconds in frozen PhCN at 123 K, which is the longest value of charge separation ever reported for donor–acceptor linked systems. Unfortunately, however, the quantum yield of formation of the charge-separated state was as low as 12%, probably due to the decay of the precursor exciplex state to the ground state rather than to the favorable complete charge-separated state. In contrast, the multi-step ET strategy has been successfully applied to porphyrin–fullerene linked triads, tetrads, and a pentad. In particular, a ferrocene–porphyrin trimer–fullerene pentad revealed formation of a long-lived charge-separated state (0.53 s in frozen DMF at 163 K) with an extremely high quantum yield (83%), which is comparable to natural bacterial reaction centers. These results not only provide valuable information for a better understanding of photoinduced energy and electron transfer processes in photosynthesis, but also open the door for the development of photoinitiated molecular devices and machines.

Tunable nonlocal spin control in a coupled-quantum dot system.

Abstract: The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (known as the Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, was realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate nonlocal spin control by suppressing and splitting Kondo resonances in one quantum dot by changing the electron number and coupling of the other dot. The results suggest an approach to nonlocal spin control that may be relevant to quantum information processing.

Relativistic atomic natural orbital type basis sets for the alkaline and alkaline-earth atoms applied to the ground-state potentials for the corresponding dimers

Abstract: New basis sets of the atomic natural orbital (ANO) type have been developed for the atoms Li–Fr and Be–Ra. The ANOs have been obtained from the average density matrix of the ground states and the lowest excited states of the atom, the positive ion, and the dimer at its equilibirium geometry. Scalar realtivisitc effects are included through the use of a Douglas–Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of the ground-state potentials for the dimers. Computed bond energies are accurate to within 0.05 eV for the alkaline dimers and 0.02 eV for the alkaline-earth dimers (except for Be2).

Ab initio studies on the photophysics of the guanine?cytosine base pair

Abstract: The low-lying excited singlet states of the Watson–Crick form of the guanine–cytosine base pair have been investigated with multi-reference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). The reaction paths and energy profiles for single proton transfer from guanine to cytosine in the 1ππ* guanine-to-cytosine charge-transfer state and for twisting of the CC double bond of the cytosine ring in the locally excited 1ππ* state of cytosine have been explored by excited-state geometry optimization using the configuration-interaction-with-singles (CIS) method and single-point energy calculations at the CASPT2 level. Avoided crossings of the 1ππ* potential-energy functions with the electronic ground-state potential-energy function have been identified along both reaction paths. The results suggest the existence of low-lying conical intersections of the 1ππ* potential-energy surface with the S0 surface which become accessible by possibly barrierless single proton transfer as well as out-of-plane deformation of cytosine and may trigger an ultrafast radiationless decay to the ground state. The relevance of these results for the rationalization of the photostability of the genetic code is briefly discussed.

Radiationless Decay of Excited States of Uracil through Conical Intersections

Abstract: The electronically excited singlet states of uracil have been studied theoretically using ab initio multireference configuration interaction methods focusing on the mechanism for radiationless decay back to the ground state. The first excited state with a significant oscillator strength is calculated to be the S2 state, corresponding to an excitation π → π*, while a dark state, S1 (nO → π*), exists below S2. Conical intersections have been located between the S2 and S1 states 0.9 eV below the vertical excitation to S2 and between the S1 and the ground state, ca. 1.8 eV below the vertical excitation energy to S2. These conical intersections are connected with each other and the Franck Condon region by pathways that exhibit no barriers and provide for a nonradiative decay to the ground state.

Single-molecule magnets: a large Mn30 molecular nanomagnet exhibiting quantum tunneling of magnetization.

Abstract: The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn12O12(O2CCH2But)16(H2O)4] (1; 8MnIII, 4MnIV) from CH2Cl2/MeNO2 causes its conversion to [Mn30O24(OH)8(O2CCH2But)32(H2O)2(MeNO2)4] (2; 3MnII, 26MnIII, MnIV). The structure of 2 consists of a central, near-linear [Mn4O6] backbone, to either side of which are attached two [Mn13O9(OH)4] units. Peripheral ligation around the resulting [Mn30O24(OH)8] core is by 32 ButCH2CO2-, 2 H2O, and 4 MeNO2 groups. The molecule has crystallographically imposed C2 symmetry. Variable-temperature and -field magnetization (M) data were collected in the 1.8−4.0 K and 0.1−0.4 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 5, D = −0.51 cm-1 = −0.73 K, and g = 2.00, where D is the axial zero-field splitting parameter. AC susceptibility measurements in the 1.8−7.0 K range in a zero DC field and a 3.5 G AC field oscillating at frequencie...

Time-dependent density functional theory based on a noncollinear formulation of the exchange-correlation potential.

Abstract: In this study we have introduced a formulation of time-dependent density functional theory (TDDFT) based on a noncollinear exchange-correlation potential. This formulation is a generalization of conventional TDDFT. The form of this formulation is exactly the same as that of the conventional TDDFT for the excitation energies of transitions that do not involve spin flips. In addition, this noncollinear TDDFT formulation allows for spin-flip transitions. This feature makes it possible to resolve more fully excited state spin multiplets, while for closed-shell systems, the spin-flip transitions will result in singlet-triplet excitations and this excitation energy calculated from this formulation of TDDFT is exactly the same as that from ordinary TDDFT. This formulation is applied to the dissociation of H2 in its 1Σg+ ground state and 1Σu+ and 3Σu− excited states with 3Σu− (Ms=+1) as the reference state and the multiplets splitting of some atoms.

Preference for vibrational over translational energy in a gas-surface reaction.

Abstract: State-resolved gas-surface reactivity measurements revealed that vibrational excitation of ν3 (the antisymmetric C-H stretch) activates methane dissociation more efficiently than does translational energy. Methane molecules in the vibrational ground state require 45 kilojoules per mole (kJ/mol) of translational energy to attain the same reactivity enhancement provided by 36 kJ/mol of ν3 excitation. This result contradicts a key assumption underlying statistical theories of gas-surface reactivity and provides direct experimental evidence of the central role that vibrational energy can play in activating gas-surface reactions.

Vanishing Hall resistance at high magnetic field in a double-layer two-dimensional electron system.

Abstract: At total Landau level filling factor nu(tot)=1 a double-layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low-temperature limit This finding supports the prediction that the ground state at nu(tot)=1 is an excitonic superfluid

Entanglement in a second-order quantum phase transition

Abstract: We consider a system of mutually interacting spins 1/2 embedded in a transverse magnetic field which undergoes a second-order quantum phase transition. We analyze the entanglement properties and the spin squeezing of the ground state and show that, contrarily to the one-dimensional case, a cusplike singularity appears at the critical point ${\ensuremath{\lambda}}_{c}$ in the thermodynamical limit. We also show that there exists a value ${\ensuremath{\lambda}}_{0}g~{\ensuremath{\lambda}}_{c}$ above which the ground state is not spin squeezed despite a nonvanishing concurrence.

Entanglement in XY Spin Chain

Abstract: We consider the ground state of the XY model on an infinite chain at zero temperature. Following Bennett, Bernstein, Popescu, and Schumacher we use entropy of a sub-system as a measure of entanglement. Vidal, Latorre, Rico and Kitaev conjectured that von Neumann entropy of a large block of neighboring spins approaches a constant as the size of the block increases. We evaluated this limiting entropy as a function of anisotropy and transverse magnetic field. We used the methods based on integrable Fredholm operators and Riemann-Hilbert problem. The entropy is singular at phase transitions.

On the thermoelectricity of correlated electrons in the zero-temperature limit

Abstract: The Seebeck coefficient of a metal is expected to display a linear temperature dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1 K. We put under scrutiny the magnitude of this term in different families of strongly interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalization contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly correlated electron systems.

Excitation-Wavelength-Dependent Fluorescence Behavior of Some Dipolar Molecules in Room-Temperature Ionic Liquids

Abstract: The fluorescence behavior of several dipolar molecules has been studied in three room-temperature ionic liquids, namely, [BMIM][BF4], [EMIM][BF4], and [BMIM][PF6], as a function of the excitation wavelength. Although a large majority of these systems show normal fluorescence behavior with no excitation wavelength dependence, a few systems surprisingly exhibit fairly strong excitation-wavelength-dependent fluorescence behavior in these media. The excitation-wavelength-dependent shift of the fluorescence maximum is measured to be between 10 and 35 nm. The various fluorescence parameters of the systems have been carefully examined to determine the factors that contribute to this kind of behavior, generally not observed in conventional media. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited-state relaxation processes, viz., solvation and energy transfer, are responsible for the excitation-wavelength-dependent fluor...

Finite-size scaling exponents of the Lipkin-Meshkov-Glick model.

Abstract: We study the ground state properties of the critical Lipkin-Meshkov-Glick model. Using the Holstein-Primakoff boson representation, and the continuous unitary transformation technique, we compute explicitly the finite-size scaling exponents for the energy gap, the ground state energy, the magnetization, and the spin-spin correlation functions. Finally, we discuss the behavior of the two-spin entanglement in the vicinity of the phase transition.

Intermediate state representation approach to physical properties of electronically excited molecules.

Abstract: Propagator methods provide a direct approach to energies and transition moments for (generalized) electronic excitations from the ground state, but they do not usually allow one to determine excited state wave functions and properties. Using a specific intermediate state representation (ISR) concept, we here show how this restriction can be overcome in the case of the algebraic–diagrammatic construction (ADC) propagator approach. In the ISR reformulation of the theory the basic ADC secular matrix is written as a representation of the Hamiltonian (or the shifted Hamiltonian) in terms of explicitly constructable states, referred to as intermediate (or ADC) states. Similar intermediate state representations can be derived for operators other than the Hamiltonian. Together with the ADC eigenvectors, the intermediate states give rise to an explicit formulation of the excited wave functions and allow one to calculate physical properties of excited states as well as transition moments for transitions between dif...

Magnetic-field-induced condensation of triplons in Han Purple pigment BaCuSi2O6.

Abstract: Besides being an ancient pigment, BaCuSi2O6 is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes BaCuSi2O6 an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.

On the thermoelectricity of correlated electrons in the zero-temperature limit

Abstract: The Seebeck coefficient of a metal is expected to display a linear temperature-dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1K. We put under scrutiny the magnitude of this term in different families of strongly-interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalisation contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly-correlated electron systems.

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of pyrrole

Abstract: The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 < λphot < 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Excitation at the longer wavelengths within this range results in (vibronically induced) population of the 11A2(πσ*) excited state, but once λphot ≤ 225 nm the electric dipole allowed 11B2 ← X1A1(π* ← π) transition becomes the dominant absorption. All of the total kinetic energy release (TKER) spectra so derived show a ‘fast’ peak, centred at TKER ∼7000 cm−1. Analysis of the structure evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, reveals selective population of specific vibrational levels of the pyrrolyl co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N–H bond strength in pyrrole: D0 = 32850 ± 40 cm−1, and the enthalpy of formation of the pyrrolyl radical: ΔfH0°(C4H4N) = 301.9 ± 0.5 kJ mol−1. The recoil anisotropy of the fast H atom photofragments formed following excitation to, and dissociation on, the 11A2(πσ*) potential energy surface (PES) is seen to depend upon the vibrational level of the pyrrolyl co-fragment. This observation, and the finding that the mean TKER associated with these fast H + pyrrolyl fragments is essentially independent of λphot, can be explained by assuming that, upon N–H bond fission, the skeletal vibrational motions in pyrrole(11A2) molecules evolve adiabatically into the corresponding modes of the ground state pyrrolyl fragment. A second, ‘slow’ peak is increasingly evident in TKER spectra recorded at shorter photolysis wavelengths, and becomes the dominant feature once λphot ≤ 218 nm. This component exhibits no recoil anisotropy; its TKER profile is reminiscent of that observed in many other dissociations that yield H atoms by ‘statistical’ decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at these shorter excitation wavelengths is rationalised by assuming two possible decay routes for pyrrole molecules excited to the 1B2(ππ*) state. One involves fast 11B2 11A2 radiationless transfer and subsequent fragmentation on the 11A2 PES, yielding ‘fast’ H atoms (and pyrrolyl co-fragments)–reminiscent of behaviour seen at longer excitation wavelengths where the 11A2 PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, either by successive 11B2 11A2 X1A1 couplings mediated by conical intersections between the relevant PESs or, possibly, by an as yet unrecognised direct 11B2 X1A1 coupling, and subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding ‘slow’ H atoms (together with, most probably, cyanoallyl co-fragments).