Original Ground State Research Articles

Spin and surface orientation effects of work function forCr/Ni(111),Cr/Ni(100) andCr/Ni(110) metal gates

Work functions of Cr/Ni (111), Cr/Ni (100) and Cr/Ni (110) surfaces with different magnetic configurations for Cr atoms in the topmost Cr monolayer are investigated using first-principles methods based on density functional theory. The calculated results reveal that work functions vary with crystal orientations and magnetic configurations. The magnitude of the Cr magnetic moments for the three (111), (100) and (110) surfaces follows a change trend with MCr, Cr/Ni(111)< MCr, Cr/Ni(100)< MCr, Cr/Ni(110). Altering the magnetic configurations of the systems from an original ground state to an excited one will have the total energy and the Fermi level increase. Consequently, it will give rise to the reduction of the work function for the system. Moreover, the quite favorable variation range (4.92–4.41 eV) of the calculated work functions for Cr/Ni (100) system modulated by spin effect implies that the Cr/Ni (100) system may be a more promising candidate. Our work suggests that changing magnetic configurations can modulate the work functions of magnetic metal gates well.

Symmetry breaking and a dynamical property of a dipolar Bose–Einstein condensate in a double-well potential

Abstract We investigate the properties of a three-dimensional (3D) dipolar Bose–Einstein condensate (BEC) in a double-well potential (DWP). The symmetry breaking and self-trapping (SBST) phenomena that the original symmetric ground state is replaced by a new asymmetric one and localized in one of the wells are demonstrated for Dy 164 atoms in the 3D DWP by means of numerical solutions of the Gross–Pitaevskii (GP) equation. The results show that the SBST properties are affected dramatically by the magnetization direction for purely dipolar BEC. The SBST under various scattering lengths are also studied. In addition, the dynamical picture of the SBST induced by a gradual transformation of the single-well potential into a double-well is also illustrated.

Bulk-edge correspondence of entanglement spectrum in two-dimensional spin ground states

General local spin $S$ ground states, described by a Valence Bond Solid (VBS) on a two dimensional lattice are studied. The norm of these ground states is mapped to a classical O(3) model on the same lattice. Using this quantum-to-classical mapping we obtain the partial density matrix $\rho_{A}$ associated with a subsystem ${A}$ of the original ground state. We show that the entanglement spectrum of $\rho_{\rm A}$ in a translation invariant lattice is given by the spectrum of a quantum spin chain at the boundary of region $A$, with local Heisenberg type interactions between spin 1/2 particles.

Amino acid coevolution induces an evolutionary Stokes shift

The process of amino acid replacement in proteins is context-dependent, with substitution rates influenced by local structure, functional role, and amino acids at other locations. Predicting how these differences affect replacement processes is difficult. To make such inference easier, it is often assumed that the acceptabilities of different amino acids at a position are constant. However, evolutionary interactions among residue positions will tend to invalidate this assumption. Here, we use simulations of purple acid phosphatase evolution to show that amino acid propensities at a position undergo predictable change after an amino acid replacement at that position. After a replacement, the new amino acid and similar amino acids tend to become gradually more acceptable over time at that position. In other words, proteins tend to equilibrate to the presence of an amino acid at a position through replacements at other positions. Such a shift is reminiscent of the spectroscopy effect known as the Stokes shift, where molecules receiving a quantum of energy and moving to a higher electronic state will adjust to the new state and emit a smaller quantum of energy whenever they shift back down to the original ground state. Predictions of changes in stability in real proteins show that mutation reversals become less favorable over time, and thus, broadly support our results. The observation of an evolutionary Stokes shift has profound implications for the study of protein evolution and the modeling of evolutionary processes.

Nonlinear interference in a mean-field quantum model

Using similar nonlinear stationary mean-field models for both a 2D axisymmetrical Bose-Einstein Condensate and an electron pair in a parabolic trap, we propose to describe the original many-particle ground state as a one-particle mixed state (in contrast to a pure state), i.e. as a statistical ensemble of several one-particle quantum states. These quantum states are the eigenfunctions of the corresponding stationary nonlinear Schrödinger equation (hence called “nonlinear eigenstates”). Due to their nonlinearity, they are not orthogonal. Therefore, taking the simple example of a two-level system, we show that each of these two nonlinear eigenstates |i⟩ and |j⟩ occurs with a probability (or statistical weight) that is defined by their non-orthogonality ⟨i|j⟩ 0. We give the corresponding density matrix. We search for physical grounds in the interpretation of our two main results, namely, a quantum-classical nonlinear transition and the interference between two “nonlinear eigenstates”.

Electron beam fluorescence temperature measurements of N2 in a semiconductor plasma reactor

The rotational temperature of nitrogen molecules in inductively coupled plasma (ICP) discharge has been measured using the electron beam fluorescence (EBF) technique. The neutral gas temperature is an important parameter in understanding the energy balance and neutral radical uniformity in plasma processing. The EBF technique has been used to study rarefied flows, but has not been applied previously to characterize a semiconductor plasma reactor. In this work an electron beam was integrated into an inductively coupled semiconductor plasma reactor, and was used to excite neutral nitrogen molecules from the N2X 1Σg+ state into the excited molecular ion N2+B 2Σu+ state. The electron beam excitation process maps the rotational population distribution of the original ground state into the excited molecular ion state following a dipole model with ΔK=±1. The rotational temperature, which is thought to be equivalent to the neutral gas translational temperature under plasma reactor conditions, can be determined from the rotational intensity distribution of the fluorescence spectrum from the N2+B 2Σu+ state. For the range of neutral gas temperatures found in the plasma reactor a dipole model of the excitation-emission process provides good agreement with the measured spectrum. A least-square fitting procedure comparing the model and measured emission spectra is used to determine the rotation temperature. In this article the first EBF measurements of gas heating in an ICP reactor at different gas pressures (20, 50 mTorr) and plasma source input powers (250, 500, and 1000 W) are reported.

Charge transfer and photogeneration process in device consisting of safranine O dye and TiO 2 nano-particles

We have studied the effect of titanium dioxide (TiO 2) nano-particles on charge transfer processes in safranine O (SAF) dye photovoltaic device. The photoconductivity is increased by two orders and a sharp saturation is also observed. The device shows a sharp saturation in photocurrent in forward bias which indicates that exciton diffusion and dissociation at the TiO 2 interface, due the existence of internal electric field is dominant process for the collection of charge. The value of diode quality factor has been calculated also by fitting the experimental results with modified Shockley equation. The internal electric field is mainly due to the asymmetry between the work functions of the materials. Electron injection from the excited state of dye on illumination, into the conduction band of TiO 2 is followed by subsequent hole transfer from the photooxidized dye to the collecting Al electrode, regenerating the dye's original ground state. The charge transfer and photogeneration process in device have been discussed on the basis of current–voltage characteristics and photoaction action spectra of the device.

Stationary antiferromagnetic domains during magnetization reversal in an exchange-biased FeMn/Fe76Mn6C18 bilayer

Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 Å)/FeMn (100 Å) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during the prior field application, so that at H=0 the domain structure was the same as that in the original ground state. If H was off-axis, during the original field application domain growth occurred in that same direction; upon field reduction, the preferred domain growth direction was at an angle with reversed sign. These results prove the AF domain walls do not move during the motion of FM domain walls, and that an exchange spring is created parallel to the AF/FM interface as the ferromagnetic layer reverses.

Photocycle of Dried Acid Purple Form of Bacteriorhodopsin

The photocycle of dried bacteriorhodopsin, pretreated in a 0.3 M HCl solution, was studied. Some properties of this dried sample resemble that of the acid purple suspension: the retinal conformation is mostly all- trans, 15- anti form, the spectrum of the sample is blue-shifted by 5 nm to 560 nm, and it has a truncated photocycle. After photoexcitation, a K-like red-shifted intermediate appears, which decays to the ground state through several intermediates with spectra between the K and the ground state. There are no other bacteriorhodopsin-like intermediates (L, M, N, O) present in the photocycle. The K to K′ transition proceeds with an enthalpy decrease, whereas during all the following steps, the entropic energy of the system decreases. The electric response signal of the oriented sample has only negative components, which relaxes to zero. These suggest that the steps after intermediate K represent a relaxation process, during which the absorbed energy is dissipated and the protein returns to its original ground state. The initial charge separation on the retinal is followed by limited charge rearrangements in the protein, and later, all these relax. The decay times of the intermediates are strongly influenced by the humidity of the sample. Double-flash experiments proved that all the intermediates are directly driven back to the ground state. The study of the dried acid purple samples could help in understanding the fast primary processes of the protein function. It may also have importance in technical applications.

Direct Photoisomerization of the 1,6-Diphenyl-1,3,5-hexatrienes. Medium Effect on Triplet and Singlet Contributions

Quantum yields for the interconversion of the all-trans-, cis,trans,trans- and trans,cis,trans-1,6-diphenyl-1,3,5-hexatrienes (DPH) in methylcyclohexane (MCH) or acetonitrile (AN) following 366 nm excitation show these processes to be relatively inefficient. Their dependence on the concentration of the DPH reveals significant participation of triplet states in the overall process. Despite very low intersystem crossing quantum yields (0.029 and 0.010 in MCH and AN, respectively) singlet and triplet contributions in the photoisomerization of all-trans-1,6-diphenyl-1,3,5-hexatriene are roughly equal in MCH, and, for the trans,cis,trans isomer, in AN. However, in AN the cis,trans,trans isomer forms nearly exclusively by a singlet pathway from the other two isomers. The cis,cis,trans isomer, a very minor component in photostationary states, appears to form primarily from the cis,trans,trans isomer whose excited singlet state also gives another isomer, tentatively identified as ctc-DPH. The major radiationless channel of the excited singlet state of each DPH isomer is direct decay to the original ground state. Barriers to torsional relaxation of the planar lowest DPH excited singlet states (21Ag and 11Bu) must be significantly higher than previously supposed. Photoisomerization quantum yields of all-trans-DPH in the presence of fumaronitrile (FN) are also separated into singlet and triplet contributions. Fumaronitrile quenches DPH fluorescence and singlet contributions to the photoisomerization equally, but enhances DPH triplet formation and the triplet contribution to the photoisomerization. Radical cations of DPH form in AN but do not participate in isomer interconversion.

Biochemical and photochemical properties of the photophobic receptors from Halobacterium halobium and Natronobacterium pharaonis.

The phototaxis of Halobacterium halobium is initiated by two photoreceptors, the sensory rhodopsins sR-I and sR-II. An sR-II-like pigment has also been described in Natronobacterium pharaonis. In this work it was shown that N. pharaonis cells are repelled by light with a wavelength of 500 nm. A further comparison of membrane preparations from H. halobium (mutant D1) containing only sR-II and from N. pharaonis [strain SP1(28)] with a chromophoric protein (psR-II) resembling sR-II revealed substantial similarities. The biochemical and photochemical properties of the pigments are quite similar, with psR-II being more stable to external conditions such as pH and ionic strength of the buffer. Both pigments are bleached by low concentrations of hydroxylamine and can be reconstituted by the addition of all-trans-retinal. The absorption spectrum of psR-II is quite similar to sR-II including the shoulder on the short-wavelength side. After light excitation sR-II and psR-II undergo photocycles with at least three intermediates. The earliest intermediate has an absorption maximum above 520 nm and decays to a species which has a characteristic absorption (approximately 380 nm) of a deprotonated Schiff base. The final step is the regeneration of the original ground state via a red-shifted intermediate absorbing around 540 nm. From this cumulative evidence it can be concluded that, not only sR-II, but also the pigment from N. pharaonis is a photophobic photoreceptor.

Mixed procedures for generating families of isospectral Hamiltonians.

Three standard procedures for generating families of isospectral Hamiltonians, either by introducing a new ground state or by deleting the original ground state, are used in combination. I investigate the unitary transformations resulting either from using one process to insert a new state and a different process to remove it again (denoted symbolically by ${Z}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$Y), or from using one process to remove the original ground state and a different process to reintroduce a state with the original ground-state energy (denoted symbolically by ${\mathrm{ZY}}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$). Many connections are found between the resulting procedures, all of which are related to just two one-parameter families of unitary transformations, ${U}_{\ensuremath{\epsilon}}$ associated with the ${Z}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$Y processes and ${T}_{\ensuremath{\xi}}$ associated with the ${\mathrm{ZY}}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$ processes. The unitary transformations ${Z}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$Y are found to be limiting cases of the nonunitary isometric operators associated with procedures for inserting a new ground state.Several limiting cases of the new unitary transformations yield nonunitary transformations associated with deletion of the ground state. For a scattering system, I present the effects of the unitary transformations ${U}_{\ensuremath{\epsilon}}$ and ${T}_{\ensuremath{\xi}}$ on the transmission and reflection coefficients and on the norming constants of the bound states. The unitary operators ${U}_{\ensuremath{\epsilon}}$, ${T}_{\ensuremath{\xi}}$, and their inverses are shown to intertwine in a natural manner with each other and with the unitary transformations ${W}_{\ensuremath{\xi}}$ defined in an earlier paper [D. L. Pursey, Phys. Rev. D 33, 2267 (1986)]. In particular, I show that ${T}_{\ensuremath{\xi}}$=${W}_{\ensuremath{\xi}}$${T}_{0}$, and that iteration of T\ensuremath{\equiv}${T}_{0}$ generates a group of transformations isomorphic with the additive group of integers. This group is a symmetry group of a generalized formalism of the theory. This extended formalism is also invariant under a group isomorphic with the group of rotations in a plane. When either the group generated by T or the associated two-dimensional rotation group is combined with the one-parameter Lie group of transformations associated with the operators ${W}_{\ensuremath{\xi}}$, the intertwining relations show that the resulting group is a direct product. I conclude with a brief survey of remaining problems and of possible further generalizations of the theory.