Two-step stabilization of orbital order and the dynamical frustration of spin in the model charge-transfer insulator KCuF3

We report a combined experimental and theoretical study of KCuF3, which offers - because of this material's relatively simple lattice structure and valence configuration (d9, i.e., one hole in the d-shell) - a particularly clear view of the essential role of the orbital degree of freedom in governing the dynamical coupling between the spin and lattice degrees of freedom. We present Raman and x-ray scattering evidence that the phase behaviour of KCuF3 is dominated above the Neel temperature (T_N = 40 K) by coupled orbital/lattice fluctuations that are likely associated with rotations of the CuF6 octahedra, and we show that these orbital fluctuations are interrupted by a static structural distortion that occurs just above T_N. A detailed model of the orbital and magnetic phases of KCuF3 reveals that these orbital fluctuations - and the related frustration of in-plane spin-order-are associated with the presence of nearly degenerate low-energy spin-orbital states that are highly susceptible to thermal fluctuations over a wide range of temperatures. A striking implication of these results is that the ground state of KCuF3 at ambient pressure lies near a quantum critical point associated with an orbital/spin liquid phase that is obscured by emergent Neel ordering of the spins; this exotic liquid phase might be accessible via pressure studies.Comment: 13 pages, 3 figure

The Hubbard model within the equations of motion approach

The Hubbard model has a special role in Condensed Matter Theory as it is considered as the simplest Hamiltonian model one can write in order to describe anomalous physical properties of some class of real materials. Unfortunately, this model is not exactly solved except for some limits and therefore one should resort to analytical methods, like the Equations of Motion Approach, or to numerical techniques in order to attain a description of its relevant features in the whole range of physical parameters (interaction, filling and temperature). In this manuscript, the Composite Operator Method, which exploits the above mentioned analytical technique, is presented and systematically applied in order to get information about the behavior of all relevant properties of the model (local, thermodynamic, single- and two- particle ones) in comparison with many other analytical techniques, the above cited known limits and numerical simulations. Within this approach, the Hubbard model is shown to be also capable to describe some anomalous behaviors of the cuprate superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference

DERIVATION OF A SINGLE-BAND MODEL FOR CUO2 PLANES BY A CELL-PERTURBATION METHOD

A cell-perturbation method is developed for CuO2 planes in the cuprate superconductors described by a d-p model. It is shown that a single-band t-t'-J-J' model accurately describes the low-energy physics and how the parameters of this model vary with those of the underlying d-p model. The method is similar in spirit to Anderson's original treatment of superexchange [Phys. Rev. 115, 2 (1959)], where the exchange interaction is obtained in second order rather than given by the usual fourth-order result of ordinary perturbation theory (a poor approximation). It is shown that O-O hopping can appreciably affect the absolute and relative magnitudes of the effective single-band parameters and that a regime with J approximately t is quite conceivable. Although triplet (intermediate) states can appreciably enhance the magnitude of the "diagonal" effective hopping t', an effective single-band description should remain valid for all reasonable estimates of the underlying d-p parameters. Correction terms involving hole pairs on neighboring cells are derived and shown to be small. For the "undoped" case, an estimate is made of the critical charge-transfer energy for an insulator-metal transition

Fabricating a set of semiconducting nanowires, and electric device comprising a set of nanowires

The method of fabricating a set of semiconducting nanowires ( 10 ) having a desired wire diameter (d) comprises the steps of providing a set of pre-fabricated semiconducting nanowires ( 10 '), at least one pre-fabricated semiconducting nanowire having a wire diameter (d') larger than the desired wire diameter (d), and reducing the wire diameter of the at least one pre-fabricated nanowire ( 10 ') by etching, the etching being induced by light which is absorbed by the at least one pre-fabricated nanowire ( 10 '), a spectrum of the light being chosen such that the absorption of the at least one pre-fabricated nanowire being significantly reduced when the at least one pre-fabricated nanowire reaches the desired wire diameter (d). The electric device ( 100 ) may comprise a set of nanowires ( 10 ) having the desired wire diameter (d). The apparatus ( 29 ) may be used to execute the method according to the invention

Nanostructure, electronic device having such nanostructure and method of preparing nanostructures

The compound nanotubes of InP or another II-VI or III-V material show a very large blueshift. Thus, devices with photoluminescent and electroluminescent effects in the visible range of the electromagnetic spectrum are provided by the inclusion of such nanotubes

Semiconductor device with tunable energy band gap

The present invention relates to a semiconductor device in which energy band gap can be reversibly varied. An idea of the present invention is to provide a device, which is based on a semiconducting material (306) in mechanical contact with a material that exhibits a reversible volume change when properly addressed, e.g. a phase change material (307). The device can, for example, be implemented in light emitting, switching and memory in applications. The semiconducting material can be reversibly strained by applying a local volume expansion to the phase change material. The resulting band gap variation of the semiconducting material can be utilized to tune the color of the light emitted from e.g. an LED or a laser. In other fields of application, contact resistance in semiconductor junctions can be controlled, and this feature is highly advantageous in memories and switches

Electron source with low energy spread

The invention provides an electron source suitable for use in a charged-particle apparatus, in which source a beam of electrons can be extracted from an electrode that is subjected to at least one of an electric potential, thermal excitation and photonic excitation, whereby at least part of the electrode comprises semiconductor material having a conduction band that is quantized into discrete energy levels. Such a source enjoys a relatively low energy spread, typically much smaller than that of a Cold Field Emission Gun (CFEG). Said semiconductor material may, for example, comprise a semiconductor nanowire. Examples of suitable semiconductor materials for such a nanowire include InAs and GaInAs