Effect Of Umklapp Research Articles

Reducing the thermal conductivity of chemically ordered binary alloys below the alloy limit via the alteration of phonon dispersion relations

We investigate the effect of crystalline configuration on the thermal conductivity of binary Lennard-Jones based solid solutions via classical molecular dynamics simulations and harmonic lattice dynamics calculations. We show that the pronounced effect of Umklapp scattering causes the cross-plane thermal conductivity of the chemically ordered alloy (1 × 1 monolayer period superlattice) to approach the thermal conductivity of the disordered counterpart (alloy limit) at elevated temperatures. However, we find that for superlattices with thicker periods and larger acoustic mismatch between the layers, the thermal conductivity can approach a minimum that is well below the alloy limit and can even approach the theoretical minimum limit of the corresponding amorphous phase. Our simulations over a wide range of mass ratios between the species suggest two contrasting effects of increasing mass ratio: (i) flattening of modes that leads to lower group velocities and lower overall thermal conductivity and (ii) reduction in the cross-section for Umklapp scattering due to the increase in the stop bands that tends to increase the thermal conductivity. The interplay between these two mechanisms that controls the thermal conductivity is shown to be dependent on the period thickness for these superlattices.

The effect of nanoparticles on the thermal conductivity of crystalline thin films at low temperatures

This study is concerned with the prediction of the effective thermal conductivity of nanocomposite thin films consisting of nanoparticles randomly distributed in a solid matrix. Crystalline sodium chloride with embedded monodisperse silver nanoparticles is investigated as a case study for thin films where phonons are the main heat carriers. To the best of our knowledge, the equation for phonon radiative transfer is solved for the first time, with an exact scattering transport cross section of the nanoparticles as a function of frequency which was obtained from the literature. The one-dimensional equation for phonon radiative transfer based on the isotropic scaling approximation is solved on a spectral basis using the discrete ordinates method to predict the temperature profile and the heat flux across the nanocomposite thin films. The thermal conductivity is retrieved at temperatures where the effects of Umklapp and normal processes can be neglected and scattering by the particles on phonon transport dominates. The method of solution and closure laws were validated with experimental data of thermal conductivity for bulk samples at 2.53, 5.94, and 10.56 K. The effects of the film thickness (1 μm to 2.5 cm), nanoparticle diameter (5 to 100 nm) and volume fraction (0.0001 to 0.2) on the thermal conductivity of the nanocomposite thin film are investigated. The results indicate that the thermal conductivity decreases with decreasing particle radius as well as with increasing particle concentration. Finally, a dimensional analysis revealed a power-law relationship between the dimensionless thermal conductivity and a dimensionless length of the order of the acoustic thickness of the medium. These results can be used to design nanocomposite thin films for various low-temperature thermal applications by choosing optimal nanoparticle radius and volume fraction, and film thickness.

Electron correlation effects on magnetic Compton profiles of nickel in the GW approximation

The magnetic Compton profiles (MCPs) of 3d transition metal nickel are calculated by the GW approximation with FLAPW basis sets on the LSDA. The 3d valence band width narrowing, which is ascribed to the dynamical screening effects, contributes to eliminate an anomalous peak at 0.7 a.u. in the 〈110〉 direction, and also to diminish the excessive estimation of the Umklapp effects, in the LSDA MCPs. These are in good agreement with experimental observation.

Sign reversal of the quantum Hall effect and helicoidal magnetic-field-induced spin-density waves in organic conductors

Within the framework of the quantized nesting model, we study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave (SDW) phases which are experimentally observed in the quasi-one-dimensional organic conductors (TMTSF) 2 X. We discuss the conditions under which umklapp processes may explain the sign reversals (Ribault anomaly) of the quantum Hall effect (QHE) observed in these conductors. We find that the 'Ribault phase' is characterized by the coexistence of two SDWs with comparable amplitudes. This gives rise to additional long wavelength collective modes besides the Goldstone modes due to spontaneous translation and rotation symmetry breaking. These modes strongly affect the optical conductivity. We also show that the Ribault phase may become helicoidal (i.e with circularly polarized SDWs) if the strength of umklapp processes is sufficiently strong. The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears.

Confinement by umklapp scattering in two coupled chains

The effect of umklapp scattering on two coupled chains with both half-filled and doped band has been examined by renormalization group method. It has been found that electrons are confined to a single chain when the doping rate becomes small and when the correlation gap induced by umklapp scattering becomes larger than the interchain hopping energy. A crossover from an insulating state into a metallic state is discussed for several Bechgaard salts at temperatures above spin density wave state.

Universal conductance in quantum wires in the presence of Umklapp scattering

The effects of Umklapp scattering on the zero-temperature conductance in one-dimensional quantum wires are reexamined by taking into account both the screening of external potential and the non-uniform chemical potential shift due to electron-electron interaction. It is shown that in the case away from half-filling the conductance is given by the universal value, $2e^2/h$, even in the presence of Umklapp scattering, owing to these renormalization effects of external potential. The conclusion is in accordance with the recent claim obtained for the system with non-interacting leads being attached to a quantum wire.

Effect of umklapp scattering on the magnetic-field-induced spin-density waves in quasi-one-dimensional organic conductors

We study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave (FISDW) phases which are experimentally observed in the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show that the transition temperature is determined by a modified Stoner criterion which includes the effect of umklapp scattering. We determine the SDW polarization (linear or circular) by analyzing the Ginzburg-Landau expansion of the free energy. We also study how umklapp processes modify the quantum Hall effect (QHE) and the spectrum of the FISDW phases. We find that umklapp scattering stabilizes phases which exhibit a sign reversal of the QHE, as experimentally observed in the Bechgaard salts. These ``negative'' phases are characterized by the simultaneous existence of two SDW's with comparable amplitudes. As the umklapp scattering strength increases, they may become helicoidal (circularly polarized SDW's). The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears. These two characteristic properties may be utilized to detect the magnetic-field-induced helicoidal SDW phases experimentally.

Confinement of interchain hopping by umklapp scattering in two coupled chains

The effect of umklapp scattering on interchain hopping has been investigated for two-coupled chains of interacting electrons with half-filled band. By analyzing in terms of renormalization group method, we have found that interchain hopping is renormalized to zero and is confined when a gap induced by umklapp scattering becomes larger than a critical value. From a phase diagram calculated on a plane of the interchain hopping and the gap, we discuss a role of the correlation gap which has been studied in metallic state at temperatures above spin density wave state in organic conductors.

Sign Reversals of the Quantum Hall Effect and Helicoidal Magnetic-Field-Induced Spin-Density Waves in Quasi-One-Dimensional Organic Conductors

We study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave phases, which are experimentally observed in the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show that umklapp processes may naturally explain sign reversals of the quantum Hall effect (QHE) observed in these conductors. Moreover, umklapp scattering can change the polarization of the spin-density wave (SDW) from linear (sinusoidal SDW) to circular (helicoidal SDW). The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears.

Effects of Umklapp Scattering on Electronic States in One Dimension

The effects of Umklapp scattering on electronic states are studied in one spatial dimension at absolute zero. The model is basically the Hubbard model, where parameters characterizing the normal ( U ) and Umklapp ( V ) scattering are treated independently. The density of states is calculated in the t-matrix approximation by taking only the forward and Umklapp scattering into account. It is found that the Umklapp scattering causes the global splitting of the density of states. In the presence of sufficiently strong Umklapp scattering, a pole in the t-matrix appears in the upper half plane, signalling an instability towards the ` G /2-pairing' ordered state ( G is the reciprocal lattice vector), whose consequences are studied in the mean field approximation. It turns out that this ordered state coexists with spin-density-wave state and also brings about Cooper-pairs. A phase diagram is determined in the plane of V and electron filling n .

A re-examination of concepts in magnetic metals: The ‘nearly antiferromagnetic Fermi liquid’

Abstract Attempts to draw parallels between the ferromagnetic spin fluctuations of nearly ferromagnetic Fermi liquids, and the new concept of antiferromagnetic spin fluctuations in ‘nearly antiferromagnetic’ Fermi liquids are examined. It is argued that exchange interactions in Fermi liquids are normally purely ferromagnetic and that antiferromagnetic exchange and antiferromagnetic fluctuations arise from the breakdown of the Fermi-liquid theory as in the Mott insulator. Spin-density waves are a distinct phenomenon caused by umklapp effects, unrelated to local antiferromagnetic exchange interactions, and lead only to minor fluctuation effects. Experimental evidence is cited in favour of this dichotomy between two different causes for antiferromagnetism.

Effect of Umklapp Scattering on the Conductance in Quantum Wires

We investigate the system-size dependence as well as the temperature dependence of the conductance in 1D electron systems, paying particular attention to the effect of Umklapp scattering. By taking into account the renormalization of the external potential due to electron-electron interaction, the correction to the conductance, 2e^2/h, due to Umklapp scattering is estimated perturbatively. The Tomonaga-Luttinger liquid parameter appears in the conductance via the system-size and temperature dependence.

Angle-resolved photoemission from a GaAs(1-bar1-bar1-bar)-2 x 2 surface: Normal emission study.

Angle-resolved photoemission experiments using synchrotron radiation have been performed on an As-rich GaAs(111)-2×2 surface over the photon energy range 10-70 eV. The occupied bulk and surface band structures of the surface were investigated using theoretical structure plots involving the linear-muffin-tin-orbital (LMTO) calculated initial bands and free-electron-like final states. Based on a three-step model, the calculations suggest that this surface is rich in surface umklapp effects and that many strong transitions may be explained as surface umklapp transitions involving mainly the surface reciprocal lattice vector g 2×2 =2π/a(-1/3,2/3,-1/3)

Identifications of empty surface states on Au(110): Comment on ‘the 1 × 1 to 1 × 2 phase transition of Au(110): An inverse photoemission study by R. Drube, V. Dose, H. Derks and W. Heiland’

It is argued that the inverse photoemission features on Au(110)(1 × 2) attributed recently to surface umklapp effects are in fact surface states intrinsic to the unreconstructed (1 × 1) surface.

Magnetoplasmons in tunneling semiconductor superlattices.

We present a theory of magnetoplasmons in tunneling semiconductor superlattices. A new magnetoplasmon branch is predicted as a consequence of the electron tunneling in the superlattice direction. Explicit analytical results for the dispersion relation of the tunneling magnetoplasmon branch are derived in an effective-mass-like approximation, and also in a tight-binding approximation which takes into account the umklapp effects in the superlattice direction. The tunneling magnetoplasmon branch should be experimentally observable.

Low-temperature electron transport properties of the alkali metals

The current understanding of the electron-phonon and electron-electron scattering contribution to the temperature dependent part of the resistivity ϱ ( T ) of the alkali metals is reviewed. Topics treated include the effect of Umklapp scattering ( ϱ u ( T )) and phonon drag ( ϱ g ( T )) on the low temperature resistivity and thermopower. Between 0.3 and 1.3 K there is good evidence that the electron-electron scattering contribution to the resistivity ( ϱ ee ( T ) ∝ T 2 ) predominates. Below 0.3 K, ϱ ( T ) appears to change slower than T 2 for reasons unknown. Possible sample dependence of ϱ ee ( T ) and some theories pertaining to its magnitude are discussed.

Low-temperature electron transport properties of the alkali metals

The current understanding of the electron-phonon and electron-electron scattering contribution to the temperature dependent part of the resistivity ϱ ( T ) of the alkali metals is reviewed. Topics treated include the effect of Umklapp scattering ( ϱ u ( T )) and phonon drag ( ϱ g ( T )) on the low temperature resistivity and thermopower. Between 0.3 and 1.3 K there is good evidence that the electron-electron scattering contribution to the resistivity ( ϱ ee ( T ) ∝ T 2 ) predominates. Below 0.3 K, ϱ ( T ) appears to change slower than T 2 for reasons unknown. Possible sample dependence of ϱ ee ( T ) and some theories pertaining to its magnitude are discussed.

Substitutional Donors and Core Excitons in Many-Valley Semiconductors

The shallow-deep instability of substitutional donors and core excitons is discussed, with inclusion of all intervalley interactions. Shallow levels result in Si for a screened point-charge potential, because intervalley overlap and kinetic energy balance the potential-energy terms, which are severely reduced, at substitutional sites, by umklapp effects. Contrary to recent claims based on consideration of potential energy only, many-valley interactions cannot therefore be invoked to predict deep core-exciton levels in Si.

Surface umklapp effects and surface states on Ir(100)-(1×1), Ir(100)-(5×1) and Ir(111)

Abstract The (1×1)→(5×1) reconstruction of the Ir(100) surface has been observed to introduce final state surface umklapp scattering which significantly affects intensities of bulk interband transitions as well as secondary electron features observed in angle-resolved photoemission spectra. In particular, a strongly directional bulk interband transition observed in normal emission from the Ir(100)-(1×1) surface is found to be much weaker for the (5×1) reconstructed surface due to surface umklapp scattering. Previously, this emission feature had been attributed to surface state emission from Ir(100)-(1×1). For Ir(111)-(1×1) we find a Λ1 symmetry surface state at 0.4 eV below the Fermi level at the zone center Γ .

Electron hall mobility and photoconductivity of coloured KBr crystals at moderate temperatures

The current understanding of the electron-phonon and electron-electron scattering contribution to the temperature dependent part of the resistivity ϱ(T) of the alkali metals is reviewed. Topics treated include the effect of Umklapp scattering (ϱu(T)) and phonon drag (ϱg(T)) on the low temperature resistivity and thermopower. Between 0.3 and 1.3 K there is good evidence that the electron-electron scattering contribution to the resistivity (ϱee(T) ∝ T2) predominates. Below 0.3 K, ϱ(T) appears to change slower than T2 for reasons unknown. Possible sample dependence of ϱee(T) and some theories pertaining to its magnitude are discussed.