The phenomenon of colossal magnetoresistance in manganites is generally agreed to be a result of competition between crystal phases with different electronic, magnetic, and structural order; a competition which can be strong enough to cause phase separation between metallic ferromagnet and insulating charge modulated states [1,2,3,4,5]. Nevertheless, closer inspection of phase diagrams in many manganites reveals complex phases where the two order parameters of magnetism and charge modulation unexpectedly coexist [6,7].Here we show that such experiments can be naturally explained within a phenomenological Ginzburg-Landau theory.In contrast to models where phase separation originates from disorder [8] or as a strain induced kinetic phenomenon [9], we argue that magnetic and charge modulation coexist in new thermodynamic phases. This leads to a rich diagram of equilibrium phases, qualitatively similar to those seen in experiment. The success of this model argues for a fundamental reinterpretation of the nature of charge modulation in these materials from a localised to a more extended "charge density wave" picture. The same symmetry considerations that favour textured coexistance of charge and magnetic order may apply to many electronic systems with competing phases. The resulting "Electronically soft" phases of matter with incommensurate, inhomogeneous and mixed order may be general phenomena in correlated systems.The manganese perovskites (RE 3+ 1−x AE 2+ x MnO 3 , RE rare earth, AE alkaline earth) provide a laboratory to study the interplay of a variety of magnetic, electronic and structural phases of matter in a strongly correlated electronic system. As in many strongly correlated electronic systems, the basic paradigm for manganite physics is the competition between the delocalising effects of the electron kinetic energy and the localising effects of the Coulomb repulsion, aided by coupling to lattice degrees of freedom. When the kinetic energy is dominant, one finds a metallic ground state with ferromagnetic alignment of the core moments.When the localising effects preponderate, instead we see charge and/or orbitally ordered ground states with substantial local lattice distortions from the near cubic symmetry of the metal, along with insulating behaviour and antiferromagnetism. One may tune between these two phases by many external parameters, especially chemical substitution, but also lattice strain, and magnetic field. The competition between metal and insulator is famously evident in the phenomenon of bulk colossal magnetoresistance, where a magnetic field tunes the conductivity of the material, and even more clearly in the strong tendency toward phase separation and inhomogeneity and regimes of percolative transport.The origin of charge and orbital ordered phases is still the subject of debate. Charge modulation has been traditionally seen as the ordering of Mn 4+ and Mn 3+ ions [10]. More recently, the charge disproportionation of the Mn ions has been argued to be much smaller than one [11,12,13] but sti...

The conduction band degeneracy in Si is detrimental to quantum computing based on spin qubits, for which a nondegenerate ground orbital state is desirable. This degeneracy is lifted at an interface with an insulator, as the spatially abrupt change in the conduction band minimum leads to intervalley scattering. We present a theoretical study of the interface-induced valley splitting in Si that provides simple criteria for optimal fabrication parameters to maximize this splitting. Our work emphasizes the relevance of different interface-related properties to the valley splitting.PACS numbers: 03.67. Lx, 85.35.Gv, 71.55.Cn Semiconductor nanostructures based on GaAs and Si [1,2] are approaching the limit where device functionality relies on degrees of freedom of individual electrons. Recent progress in material processing allows precise controlled doping, band-structure engineering, and fabrication of high quality heterojunctions, which in turn pave the way for challenging applications such as the development of a scalable solid state quantum computer. The past few years witnessed tremendous experimental progress in the study of spin qubits at GaAs/AlGaAs quantum dots [1], which raises intriguing questions on the feasibility of spin qubits in Si quantum dot [3] or donor states [4] at a Si/barrier-material interface.A clear advantage of spin qubits in Si over GaAs is the long spin coherence times in Si [5]. On the other hand bulk Si conduction band edge is six-fold degenerate, a complication not present in GaAs. Near a (001) interface with a barrier material, this degeneracy is partially lifted, with the interface electron ground state remaining doubly degenerate. For electron spin qubits, the residual orbital degeneracy is an important spin decoherence source [6]. This effect can be overcome if the ground state degeneracy is significantly lifted, which occurs close to an interface that can efficiently scatter carriers between the two degenerate valleys that are near opposite ends of the Brillouin zone [7]. Measurements of the doublet splitting, or valley splitting, present significant variations among different Si/barrier samples, ranging from 0 to ∼ 1 meV [8]. In this context, a simple physical model that can help identify the relevant fabrication-related parameters in order to maximize the valley splitting is a valuable tool in assisting current experimental efforts.Theoretical approaches to describe the electronic behavior in the presence of an interface or heterojunction range from the effective mass approximation (EMA) [9,10], tight-binding models [11] to first-principles envelope function approach [12]. The present study, based on the physically motivated EMA [13], aims to identify the relevance of sample-dependent parameters to the valley splitting. Our approach, while simple, is original and permits the study of the intervalley coupling due to a single interface, not employing periodic boundary conditions. The full plane wave expansions of the Bloch functions at the two conduction band minima obtained fro...

We examine the magnetic phase diagram of iron pnictides using a five-band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Γ in photoemission experiments.

We propose a five-band tight-binding model for the Fe-As layers of iron pnictides with the hopping amplitudes calculated within the Slater-Koster framework. The band structure found in DFT, including the orbital content of the bands, is well reproduced using only four fitting parameters to determine all the hopping amplitudes. The model allows to study the changes in the electronic structure caused by a modification of the angle α formed by the Fe-As bonds and the Fe-plane and recovers the phenomenology previously discussed in the literature. We also find that changes in α modify the shape and orbital content of the Fermi surface sheets.

Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: The system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, D(x)/D(y), for the mean-field Q=(π,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that D(x)/D(y) ranges between 0.2

Prospects for the quantum control of electrons in the silicon quantum computer architecture are considered theoretically. In particular, we investigate the feasibility of shuttling donor-bound electrons between the impurity in the bulk and the Si-SiO2 interface by tuning an external electric field. We calculate the shuttling time to range from subpicoseconds to nanoseconds depending on the distance (approximately 10-50 nm) of the donor from the interface. Our results establish that quantum control in such nanostructure architectures could, in principle, be achieved.

Orbital degeneracy of the electronic conduction band edge in silicon is a potential roadblock to the storage and manipulation of quantum information involving the electronic spin degree of freedom in this host material. This difficulty may be mitigated near an interface between Si and a barrier material, where intervalley scattering may couple states in the conduction ground state, leading to non-degenerate orbital ground and first excited states. The level splitting is experimentally found to have a strong sample dependence, varying by orders of magnitude for different interfaces and samples. The basic physical mechanisms leading to such coupling in different systems are addressed here. We expand our recent study based on an effective mass approach, incorporating the full plane wave (PW) expansions of the Bloch functions at the conduction band minima. Physical insights emerge naturally from a simple Si/barrier model. In particular, we present a clear comparison between ours and different approximations and formalisms adopted in the literature, and establish the applicability of these approximations in different physical scenarios.

The electronic structure and magnetic properties of La1−xAxMnO3 are investigated. It is assumed that, at the outermost layer, the environment of the Mn ions does not have cubic symmetry. The eg orbitals are split and the double exchange mechanism is weakened. The charge state of the Mn ions is modified, and the magnetic ordering of the spins tends to be antiferromagnetic. The surface magnetization and the dependence of the transport properties through the resulting surface barrier on applied magnetic field and temperature is analyzed. 75.20.Ss,71.20.Lp,75.30.Et,75.30.Pd Doped manganites show many unusual features, the most striking being the colossal magnetoresistance in the fully ferromagnetic phase [1,2]. Extensive research shows that the transport properties, and the magnetoresistance in particular, are significantly modified at artificially created barriers [3][4][5][6][7][8][9][10] or in ceramic materials [11][12][13][14][15]. The magnetoresistance, as function of temperature, drops more rapidly than in the bulk. It has larger values at low fields, and persists at large fields, unlike in the bulk case. The relevance of the interfaces in perovskite manganites can also be inferred by comparing with transport in related materials which exhibit colossal magnetoresistance [16]. These properties have been ascribed to changes in the interface structure, although the origin of these modifications, and the resulting structure are not known.In the present paper, we analyze the simplest, and most common, modification with respect to the bulk that a surface may show: the loss of cubic symmetry around the Mn ions. It is well known that La 1−x A x MnO 3 shows a transition from a tetragonal (or orthorhombic) structure to a cubic one as the value of the doping x is increased. The systems with the highest Curie temperature have x ∼ 1 3 and are in the cubic phase. This implies that the two e g orbitals of the Mn ion are degenerate and contribute to the conduction band. In this situation, the double exchange mechanism is enhanced.The cubic symmetry is lost at the surface. If the last layer is oxygen deficient, the resulting splitting between the e g orbitals can be larger than typical Jahn-Teller splittings in La 1−x A x MnO 3 with small values of x. When one of the e g orbitals moves away from the Fermi level, the double exchange mechanism is weakened, and direct antiferromagnetic couplings between the core S = 3 2 spins can prevail. Moreover, the reduction in electronic kinetic energy can also lead to charge transfer between the surface layers and the bulk, contributing to the formation of a surface dipole. All these effects can be modified by surface spin waves, which, in turn, depend on temperature and external magnetic fields. A significant dependence of the metal-insulator transition temperature as function of oxygen pressure in thin films is found in [17].In order to investigate these features, we start from a tight binding Hamiltonian, using the two e g orbitals, d x 2 −y 2 and d 3z 2 −r 2 , which we designate x and...