Broken Symmetry Ground States Research Articles

Splitting of the electronic states near EF in A4C60 compounds (A = alkali metal)

The electronic structure of A4C60 compounds (A=Na, K, Cs) has been studied using electron energy-loss spectroscopy. Both the low energy loss functions and Cls core excitation measurements reveal a strong splitting of the electronic states near the Fermi level in comparison with K3C60 or K6C60. This splitting indicates a lowering of the high degeneracy of the C60 molecular orbitals due to a broken symmetry ground state and/or the opening of a correlation gap in these systems.

Optical properties of the A 1C 60 (A = K, Rb and Cs) linear polymer fullerides

We have investigated the complete electrodynamic response of the newly discovered alkali fulleride linear polymers A 1C 60. We find clear evidence for a metal-insulator phase transition at low temperatures in Rb 1C 60 and Cs 1C 60, while the K compound remains metallic at all temperatures. The possibility of a broken symmetry ground state due to the formation of a spin density wave for the Rb and Cs compounds, as suggested by the magnetic properties, is discussed.

Skyrmion excitations in quantum Hall systems

Using finite size calculations on the surface of a sphere we study the topological (skyrmion) excitation in quantum Hall system with spin degree of freedom at filling factors around $\nu=1$. In the absence of Zeeman energy, we find, in systems with one quasi-particle or one quasi-hole, the lowest energy band consists of states with $L=S$, where $L$ and $S$ are the total orbital and spin angular momentum. These different spin states are almost degenerate in the thermodynamic limit and their symmetry-breaking ground state is the state with one skyrmion of infinite size. In the presence of Zeeman energy, the skyrmion size is determined by the interplay of the Zeeman energy and electron-electron interaction and the skyrmion shrinks to a spin texture of finite size. We have calculated the energy gap of the system at infinite wave vector limit as a function of the Zeeman energy and find there are kinks in the energy gap associated with the shrinking of the size of the skyrmion. breaking ground state is the state with one skyrmion of infinite size. In the presence of Zeeman energy, the skyrmion size is determined by the interplay of the Zeeman energy and electron-electron

The alkali-metal doped C60: what can we learn from the optical properties

We report our optical investigations on the alkali-doped C60 over a broad spectral range, extending from the far-infrared up to the ultraviolet. The occurrence of superconductivity in A3C60 compounds has raised quite a bit of controversy with respect to arguments, favouring pairing mechanism based on electron-phonon interactions with low frequency intermolar vibrations or with high frequency intramolecular modes. We discuss our experimental results within the standard Eliashberg electron-phonon theory of superconductivity, in order to single out the relevant phonon excitation for the pairing mechanism. The Eliashberg calculation strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes, in accord with the implications of the weak coupling BCS limit. Moreover, we present the electrodynamic response of the quasi-one-dimensional A1C60 compounds. Our experimental findings demonstrate that Rb1C60 and Cs1C60 undergo a metal-insulator phase transition below about 50 K, while K1C60 remains, however, metallic at all temperatures. The possibility of a broken symmetry ground state due to the formation of a spin density wave (SDW) condensate for the Rb and Cs compound, as suggested by the magnetic properties, is discussed.

Pressure tuning of the charge-density wave in the halogen-bridged transition-metal solid Pt2Br6(NH3)4.

We report the pressure dependence up to 95 kbar of Raman active stretching modes in the quasi-one-dimensional MX chain solid $Pt_2Br_6(NH_3)_4$. The data indicate that a predicted pressure-induced insulator-to-metal transition does not occur, but are consistent with the solid undergoing either a three-dimensional structural distortion, or a transition from a charge-density wave to another broken-symmetry ground state. We show that such a transition cacan be well-modeled within a Peierls-Hubbard Hamiltonian. 1993 PACS: 71.30.+h, 71.45.Lr, 75.30.Fv, 78.30.-j, 81.40.Vw

Pressure and Temperature Induced Frequency Shifts of Raman-Active Phonons in the MX Solid Pt2Br6(NH3)4

Abstract We report the dependencies on hydrostatic pressure up to 100 kbar, and on temperature from 10 K to 300 K of the frequencies of Raman-active stretching modes in the single crystal MX chain solid Pt2Br6(NH3)4 (PtBrn). The data show that a predicted pressure-induced metallization does not occur, and suggests instead that either a 3-D structural distortion or a transition to a broken-symmetry ground state different from the charge-density wave (CDW) takes place.

Study of the Electronic Structure, Metal-Metal Bonding, and Ground-State Exchange Coupling in Face-Shared Mo2X93- (X = Cl, Br, I) Dimers Using the Broken-Symmetry X.alpha.-SW Method

The results of spin-restricted SCF-Xα-SW and spin-unrestricted broken-symmetry SCF-Xα-SW calculations on the face-shared dimer complexes A3Mo2Cl9 (A = K, Rb, Cs, Me4N), A3Mo2Br9 (A = Cs, Me4N), and Cs3Mo2I9 are reported and used to discuss the electronic structure, metal-metal bonding, and magneto-structural correlations in these systems. The spin-restricted SCF-Xα-SW ground-state calculations on Cs3Mo2X9 (X = Cl, Br, I) show that the metal-metal σ and π bonding interactions are significantly reduced for the bromide and especially for the iodide complex relative to Mo2Cl9 3-, consistent with the increased metal-metal bond distances observed for both these complexes. The ground-state exchange interaction is shown to be almost entirely the result of direct overlap of magnetic orbitals with negligible contribution from superexchange effects. The Xα-SW calculations for the broken-symmetry ground state reveal that the magnetic orbitals involved in the metal-metal π interaction are almost completely localized on the metal ions. The magnetic orbitals involved in the metal-metal σ interaction, on the other hand, are partially delocalized between the two metals but still contribute significantly to the ground-state exchange interaction in agreement with earlier spectroscopic and theoretical studies. The calculated exchange coupling constants Jab for the complexes Cs3Mo2X9 (X = Cl, Br, I) support this conclusion and indicate that the effective maximum spin in the ground-state exchange levels lies between 2 and 3. A significant antiferromagnetic contribution arises from ligand → metal spin-polarization effects which accounts for the unusually large -Jab value found for the iodide complex. The magneto-structural correlations observed for the chloride and bromide complexes have been successfully modeled by initially using Mo atomic sphere radii which reproduced the experimental Jab values for Cs3Mo2Cl9 and Cs3Mo2Br9 and then adjusting the Mo sphere radii for the remaining complexes in proportion to their metal-metal bond distances relative to the Cs salts.

Do the A4C60 fullerides have a broken-symmetry ground state?

Band theory predicts both K3C60 and K4C60 to be metals; various experimental probes show that while K3C60 is indeed metallic, K4C60 appears to be insulating. The standard view of this apparent failure of the single-particle picture is that electron correlation is predominant. We describe an alternative scenario, motivated on theoretical grounds, which invokes a spin- or charge-density-wave state to explain the observed insulating behavior.

The dynamics of spin-density waves

Spin-density waves (SDWs) are broken-symmetry ground states of metals, the name referring to the periodic modulation of the spin density with period, ${\ensuremath{\lambda}}_{0}=\frac{\ensuremath{\pi}}{{k}_{F}}$, determined by the Fermi wave vector ${k}_{F}$. The state, originally postulated by Overhauser, has been found in several organic linear-chain compounds. The development of the SDW state opens up a gap in the single-particle excitation spectrum, and the ground state is close to that of an antiferromagnet, as shown by a wide range of magnetic studies. Because of the magnetic ground state and of the incommensurate periodic spin modulation (which can be thought of as two periodic charge modulations in the two spin subbands), both collective charge and spin excitations may occur. These couple to ac magnetic and electric fields, which leads to antiferromagnetic resonances and frequency-dependent collective-mode conductivity. Both have been observed in the spin-density-wave ground state. The interaction of the collective mode with impurities pins the mode to the underlying lattice, and therefore the collective-mode charge excitations occur at finite frequencies in the long-wavelength limit. The mode can also be induced to execute a translational motion upon the application of a dc field which exceeds the threshold field ${E}_{T}$. Many of the observations on the ac, and on the nonlinear dc, response are similar to those which occur in materials with a charge-density-wave ground state. At low temperatures a novel type of collective transport suggestive of a tunneling process is observed. These low-temperature phenomena remain unexplained.

Localized excitations in competing bond-order-wave, charge-density-wave, and spin-density-wave systems

The characteristics of localized excitations in quasi-one-dimensional systems are rather sensitive to the interplay between the electron-phonon (e-ph) and electron-electron (e-e) interactions giving rise to competition and possible coexistence of various symmetry-broken ground states such as the bond-order wave (BOW), the charge-density wave (CDW), and the spin-density wave (SDW). Such effects are observable in halogen-bridged mixed-valence transition-metal complexes, and can be elucidated within the Bogoliubov--de Gennes formalism using an extended Peierls-Hubbard model. The coexistence of local BOW, CDW, and SDW distortions is demonstrated in this paper for polarons and self-trapped excitons (STE) in different symmetry-broken ground states. An extensive study of localized excitations over a wide range of the on-site e-ph coupling ${\ensuremath{\lambda}}_{2}$ and the Hubbard interaction U leads to the following observations. (a) As ${\ensuremath{\lambda}}_{2}$ increases at fixed U, the number of bound states inside the gap changes from two to four for the STE case and from two to three for the polaron case. (b) Upon its further increase, one type of STE with a certain pattern of SDW distortion and charge transfer is transforming into another type of STE with a different pattern. (c) A nonmonotonic dependence of the lattice relaxation energy on ${\ensuremath{\lambda}}_{2}$ is predicted within the lattice relaxation approach developed by Su and Yu earlier, and is attributed to a crossover from the weak-coupling to strong-coupling behavior showing up as the emergence of new bound states inside the gap. Moreover, the nonradiative transition rate of STE is also calculated and is used to tentatively interpret the very short lifetime of STE in PtCl complexes. Such nonmonotonic dependence of the relaxation rate on the coupling constant may also be observed in other quasi-one-dimensional systems as well.

Origin of the broken-symmetry ground state of mixed-valence [Fe3S4]0 and (Fe4S4]3+ cores in proteins and models

Origin of the broken-symmetry ground state of mixed-valence [Fe3S4]0 and (Fe4S4]3+ cores in proteins and models

Double exchange and vibronic coupling in mixed valence systems. Origin of the broken-symmetry ground state of [Fe3S4]0 cores in proteins and models

The origin of the pair-delocalized ground state of spin S=2, observed in chemically symmetric mixed-valence [Fe 3 S 4 ] 0 cores present in proteins and synthetic models, is analyzed in the framework of an effective-Hamiltonian model, comprising terms for excess-electron transfer (leading to double-exchange coupling of the paramagnetic Fe(III) cores), vibronic coupling (trapping the excess electron), and antiferromagnetic exchange. The basic mechanisms underlying the inhomogeneous electron distributions in trinuclear mixed-valence clusters with paramagnetic ion cores are illustrated in a simple model with electronic structure d 1 -d 1 -d 2

Quantum rolling down out of equilibrium.

In a scalar field theory, when the tree level potential admits broken symmetry ground states, the quantum corrections to the static effective potential are complex. (The imaginary part is a consequence of an instability towards phase separation and the static effective potential is not a relevant quantity for understanding the dynamics). Instead, we study here the equations of motion obtained from the one loop effective action for slow rollover out of equilibrium. We considering the case in which a scalar field theory undergoes a rapid phase transition from $T_i>T_c$ to $T_f<T_c$. We find that, for slow rollover initial conditions (the field near the maximum of the tree level potential), the process of phase separation controlled by unstable long-wavelength fluctuations introduces dramatic corrections to the dynamical evolution of the field. We find that these effects slow the rollover even further

Spontaneously symmetry-broken-current-carrying states in a strongly inhomogeneous interacting two-dimensional Fermi liquid.

We examine the nature of the ground state of a two-dimensional interacting electron gas in a self-induced gauge field. We show that for a realistic form of such a many-body gauge field in two dimensions and increasing inhomogeneity of the electron density, a spontaneous current-carrying state develops. Its relation to other time-reversal broken-symmetry ground states is discussed.

Broken-symmetry ground states for the two-dimensional electron gas in a double-quantum-well system.

Working in the Hartree-Fock approximation, we calculate the ground-state energy of different states of the two-dimensional electron gas in a double-quantum-well system as a function of the separation of the wells. Our calculation takes tunneling between the wells as well as the finite thicknesses of the wells into account. In the absence of interlayer hopping the ground state at small layer separations has a spontaneously broken symmetry in which hopping matrix elements have nonzero expectation values. We find that, as the separation of the wells is increased to some critical distance ${\mathit{d}}_{\mathit{c}}$, the Hartree-Fock ground state breaks translational symmetry and the hopping order parameters rapidly diminish.

Antiferromagnetic Heisenberg-Ising ring in the presence of a magnetic flux: Relevance of domain-wall dynamics.

We consider a system of charged spinless fermions in a ring governed by the antiferromagnetic Heisenberg-Ising Hamiltonian and in the presence of a magnetic flux. We find that the two broken-symmetry ground states evolve adiabatically with increasing flux with a period corresponding to two flux quanta, while the period of the total spectrum is one flux quantum. This behavior, already observed for this system in the gapless regime [B. Sutherland and B. S. Shastry, Phys. Rev. Lett. 65, 1833 (1990)], is shown to be a natural consequence of the relevant degrees of freedom involved in the low-energy physics of this system: antiferromagnetic domain walls or solitons, with half the charge of the original particles. A space-time approach is introduced to describe the dynamics of these objects, affording a complete topological classification of space-time histories of the system. This allows a physically complete understanding of the ground-state-subspace evolution with increasing flux in the antiferromagnetic broken-symmetry regime. In this case, the flux period doubling can be explained in terms of the Berry's phase gained by the two degenerate broken-symmetry ground states upon adiabatic switching of the flux.

First-principles calculations of the three-dimensional structure and intrinsic defects in trans-polyacetylene.

We report a first-principles, local-density-functional (LDA) calculation of the electronic structure of crystalline, three-dimensional (3D) trans-(CH${)}_{\mathit{x}}$. For the perfect crystal, we find a broken-symmetry ground state having P${2}_{1}$/a space-group symmetry, corresponding to in-phase dimerization on neighboring chains within the unit cell. We show that in this structure the interchain couplings, although weak, lead to an asymmetry between the valence and conduction bands and, more importantly, give 3D character to the electronic band-edge states. We investigate several additional aspects of the electronic structure of the perfect crystal, including self-consistent optimization of the ions in the unit cell, spin polarization and electronic charge densities, interchain electron-phonon interactions, and the density of states. To study intrinsic defects in trans-(CH${)}_{\mathit{x}}$, we map our LDA results onto a multi-orbital, tight-binding model; this mapping preserves very accurately all the electronic-structure properties of the full calculation. Using a Koster-Slater Green-function technique, we are able to examine both (shallow) polaronlike and (deep) bipolaronlike lattice distortions corresponding to localized defects. We find that the 3D character of the electronic band-edge states strongly suppresses the formation of the self-trapped, localized defects characteristic of the 1D models, destabilizing polarons and possibly bipolarons as well in perfectly ordered 3D trans-(CH${)}_{\mathit{x}}$. To establish a connection with earlier work, we demonstrate that by artificially decreasing interchain effects and/or increasing the intrachain electron-phonon coupling we can cause polarons and bipolarons to form. We examine the agreement of our results for the idealized perfectly crystalline material with experimental results on real samples of trans-(CH${)}_{\mathit{x}}$ and conclude with suggestions for future work, both theoretical and experimental.

Three-dimensional structure and intrinsic defects in trans-polyacetylene.

We report first-principles local-density-functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3D trans-(CH${)}_{\mathrm{x}}$. We find a broken-symmetry ground state of P${2}_{1}$/a symmetry with in-phase dimerizations. Using a Green's-function technique, we show that the 3D character of the electronic band-edge states strongly suppresses self-trapping, destabilizing polarons, and possibly bipolarons as well in perfectly ordered 3D trans-(CH${)}_{\mathrm{x}}$. .AE

Stress and temperature dependence of the electronic properties of n-type silicon inversion layers

The ground state for n-type silicon inversion electrons at the Si (100)/SiO2 interface is studied theoretically as a function of temperature, applied uniaxial stress and gate voltage. Calculations on a model that includes charge-density-wave states under favourable conditions provide results which allow a consistent and unified description of a whole series of experimental data that otherwise lead to divergent interpretations. One novel aspect of the theory is the ability of temperature and/or stress to induce and destroy a broken-symmetry ground state with a first- and a second-order phase transition. Parameters in the theory can, in principle, be used to estimate the effectiveness of screening in the inversion layer and the strength of electron-lattice coupling at the interface.

Broken-symmetry ground states in n-type inversion layers of Ge and GaP

A theory of broken-symmetry and ground states, originally introduced to describe the electronic properties of n-type inversion layers in silicon, is applied to germanium and gallium phosphide systems where the conduction band minima possess different symmetries.