Umklapp Processes Research Articles

Ground-state phase diagram of the unconventional Hubbard chain with bond–charge interaction

Using the field-theory approach combining bosonization and renormalization group, we investigate the weak-coupling phase diagram of the unconventional Hubbard chain, where in addition to a bond–charge interaction (X), the Hubbard interaction differs between odd (Uo) and even (Ue) sites. The result shows that at quarter filling, the unequal Hubbard repulsion has a great influence on low-energy charge excitations. For the attractive X, the umklapp scattering always exists, and the ground state only exhibits a spin-gapless insulating phase with dominant spin-density-wave correlations. For the repulsive X, the umklapp processes only take place if both Uo and Ue are greater than 8Xcos(π/4), and the phase diagram consists of three phases, a metallic phase with the spin-gapped singlet superconducting correlation, and two insulating phases characterized by the spin-density-wave and charge-density-wave correlations. We also present the phase diagram of the standard t-U-X chain in the case of non-half filling.

Evolution of superconductivity in twisted graphene multilayers

The group of moiré graphene superconductors keeps growing, and by now it contains twisted graphene multilayers as well as untwisted stacks. We analyze here the contribution of long-range charge fluctuations in the superconductivity of twisted double bilayers and helical trilayers, and compare the results to twisted bilayer graphene. A diagrammatic approach which depends on a few, well-known parameters is used. We find that the critical temperature and the order parameter differ significantly between twisted double bilayers and helical trilayers on one hand, and twisted bilayer graphene on the other. This trend, consistent with experiments, can be associated with the role played by moiré Umklapp processes in the different systems.

High-Mobility Compensated Semimetals, Orbital Magnetization, and Umklapp Scattering in Bilayer Graphene Moiré Superlattices.

Twist-controlled moiré superlattices (MSs) have emerged as a versatile platform for realizing artificial systems with complex electronic spectra. The combination of Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) can give rise to an interesting MS, which has recently featured a set of unexpected behaviors, such as unconventional ferroelectricity and the electronic ratchet effect. Yet, the understanding of the electronic properties of BLG/hBN MS has, at present, remained fairly limited. Here, we combine magneto-transport and low-energy sub-THz excitation to gain insights into the properties of this MS. We demonstrate that the alignment between BLG and hBN crystal lattices results in the emergence of compensated semimetals at some integer fillings of the moiré bands, separated by van Hove singularities where the Lifshitz transition occurs. A particularly pronounced semimetal develops when eight holes reside in the moiré unit cell, where coexisting high-mobility electron and hole systems feature strong magnetoresistance reaching 2350% already at B = 0.25 T. Next, by measuring the THz-driven Nernst effect in remote bands, we observe valley splitting, indicating an orbital magnetization characterized by a strongly enhanced effective gv-factor of 340. Finally, using THz photoresistance measurements, we show that the high-temperature conductivity of the BLG/hBN MS is limited by electron-electron umklapp processes. Our multifaceted analysis introduces THz-driven magnetotransport as a convenient tool to probe the band structure and interaction effects in van der Waals materials and provides a comprehensive understanding of the BLG/hBN MS.

Controlling Umklapp Scattering in a Bilayer Graphene Moiré Superlattice.

We present experimental findings on electron-electron scattering in two-dimensional moiré heterostructures with a tunable Fermi wave vector, reciprocal lattice vector, and band gap. We achieve this in high-mobility aligned heterostructures of bilayer graphene (BLG) and hBN. Around the half-full point, the primary contribution to the resistance of these devices arises from Umklapp electron-electron (Uee) scattering, making the resistance of graphene/hBN moiré devices significantly larger than that of non-aligned devices (where Uee is forbidden). We find that the strength of Uee scattering follows a universal scaling with Fermi energy and is nonmonotonically dependent on the superlattice period. The Uee scattering can be tuned with the electric field and is affected by layer polarization of BLG. It has a strong particle-hole asymmetry; the resistance when the chemical potential is in the conduction band is significantly lower than when it is in the valence band, making the electron-doped regime more practical for potential applications.

Pressure Impact on Lattice Thermal Conductivity and Its Related Parameters in CdSe Bulk, Nanowires, and Thin Film

The hydrostatic pressure effect on lattice thermal conductivity (LTC) in CdSe bulk, nanowires, and thin films was calculated. Values of LTC obtained through Callaway with that of Clapeyron equations are used to correlate those of the experimentally measured curves. Frequency‐dependent relaxation time approximation of phonon‐scattering processes was considered, which scattering due to boundary, imperfections, dislocations, electron, and other phonons via Umklapp and normal processes were used and clarified their effects. The impact of pressure on LTC is clarified according to both sample boundary dislocation concentrations. The influence of pressure is described, along with how the size dependence of the melting temperature, Debye temperature, and phonon group velocity of CdSe nanocrystals is affected. These parameters decrease as pressure increases.

Investigating the thermal properties of CrN: Theoretical insights and real ceramics

Ab-initio calculations of lattice heat capacity and phonon thermal conductivity have been performed for CrN of rock-salt structural type (paramagnetic) and for the orthorhombic antiferromagnetic (AFM) phase below TN ≈ 285 K. The comparison with experimental data below ≈30 K unveils that in contrary to the theoretically calculated heat capacity following the βT3 term with Debye temperature ΘD = 830 K, a large excess of experimental heat capacity in a form of δT2 is observed. This additional heat capacity is ascribed to magnons in the specific layered-type AFM arrangement of CrN. The experimental thermal conductivity shows a similar low-temperature excess of T2-dependence. Considering that mean free path of heat carriers is essentially limited by the grain size and calculated phonon conductivity itself is very low below ≈30 K, such additional contribution can be ascribed again to AFM magnons though theoretical model of such conductivity is still to be assessed. In spite of this complexity, our model of phonon thermal conductivity reproduces well the maximum of 7.2 Wm−1K−1 at 150 K followed by a decline tied to phonon-phonon scattering (Umklapp process). Nonetheless at higher temperatures, the experimental data evidence sudden drop at TN to ≈2 Wm−1K−1 with only slightly varying thermal conductivity at elevated temperatures, which reminds rather a behavior of amorphous or defective solids. It is proposed that atomic displacements that stabilize spin-parallel and spin-antiparallel Cr pairs in the orthorhombic AFM phase persist partially as fluctuating ones in the cubic paramagnetic state above TN, which gives rise to dynamic strain fields that cause strong phonon scattering.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces.

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Superlattices, Bonding-Antibonding, Fermi Surface Nesting, and Superconductivity

Raman and synchrotron THz absorption spectral measurements on MgB2 provide experimental evidence for electron orbital superlattices. In earlier work, we have detected THz spectra that show superlattice absorption peaks with low wavenumbers, for which spectral density evolves and intensifies after cooling below the superconducting transition temperature for MgB2. In this work, we show how these observations indicate a direct connection to superconducting properties and mechanisms. Bonding–antibonding orbital character is identified in calculated electronic band structures and Fermi surfaces consistent with superlattice structures along the c-axis. DFT calculations show that superlattice folding of reciprocal space generates Brillouin zone boundary reflections, Umklapp processes, and substantially enhances nesting relationships. Tight binding equations are compared with expected charge density waves from nesting relationships and adjusted to explicitly accommodate these linked processes. Systematic analysis of electronic band structures and Fermi surfaces allows for direct identification of Cooper pairing and the superconducting gap, particularly when the k-grid resolution of a calculation is suitably calibrated to structural parameters. Thus, we detail a robust and accurate DFT re-interpretation of BCS superconductivity for MgB2.

Defect-dominated phonon scattering processes and thermal transports of ferroelastic (Sm1-XYbX)TaO4 solid solutions

Thermal transport management is of great significance for materials such as thermal barrier coatings, photovoltaics, and thermoelectrics. In particular, tremendous efforts have been made to tailor phonon thermal transport to improve performance. This work elucidated origins of low thermal conductivity of ferroelastic (Sm1-XYbX)TaO4 solid solutions from defect-dominated phonon scattering processes. We estimated the influences of point defects (misfits in atomic weight and ionic radius, and oxygen vacancies), ferroelastic domains, and the Umklapp process on thermal transports via the corresponding phonon relaxation time (τ). The phonon relaxation time caused by point defects (τD), ferroelastic domains (τF), and the Umklapp process (τU) exhibited a trend of τD−1>τU−1>τF−1, indicating that point defects dominated the thermal transports. The effects of ferroelastic domains on thermal conductivity were estimated according to domain widths and space mismatches between neighboring domains. The approximately parallel relationship between the neighboring domains was beneficial for reducing ferroelastic t-m transition energy. Furthermore, a model was developed to derive the high-temperature thermal conductivity, and the contributions of radiative thermal conductivity were separated from the total thermal conductivity. The analysis of thermal transports can be used in other materials to estimate the contributions of different thermal transport mechanisms to the total thermal conductivity at both low and high temperatures, and to effectively tailor thermal transports according to specific applications.

Phonon hardening and the effect of phonon transport in cubic antiperovskites [formula omitted] (A = Li, Na; B = Se, Te) induced by quartic anharmonicity

In this article, we aim at the structural stability, lattice thermal conductivity, transport properties of four cubic antiperovskites A3FB using first-principle calculations combined with self-consistent phonon theory(SCP), compressive sensing techniques (CS), and Boltzmann transport equation (BTE), with a particular focus on the 4 phonon scattering rates and phonon frequency shifts caused by quartic anharmonicity. The results indicate that the strong quartic anharmonicity of alkali metal atoms Na atom harden low-frequency phonon branch to ensure dynamic stability of the material. We also find that the quartic anharmonicity play a crucial role in obtaining the reasonable temperature dependence of κL. In addition, we analyze the impact of phonon group velocities, the scattering rate and scattering phase space of 3ph and 4ph on phonon transport, revealing that the low κL of Na3FSe is due to the strong 4ph SRs which is dominated by redistribution and Umklapp processes. Our studies provide the microscopic understanding of the quartic anharmonicity together with their impact on lattice dynamics and phonon transport is not only of indispensable interest but also essential to design and explore materials with ultralow thermal conductivity.

A symmetry of potential function and non-relaxation theorems of heat flux in nonlinear lattice

We introduce a class of nonlinear lattices that have a particular symmetry called NP symmetry in their potential functions. This class includes nonlinear lattices such that the umklapp process vanishes and only the normal processes appear in their potential functions. For the NP-symmetric lattices with periodic boundary conditions, we prove non-relaxation theorems of the heat flux within both classical and quantum mechanics: the heat flux never vanishes but remains strictly positive, i.e., non-existence of the thermal resistance, when the initial state gives a positive heat flux and satisfies a certain condition. The present theorems give a rigorous theoretical basis for the widely-accepted Peierls’s hypothesis that there is no thermal resistance in a nonlinear lattice if it has no umklapp process but only the normal processes.

Effect of out-of-plane acoustic phonons on the thermal transport properties of graphene

The lattice thermal conductivity of graphene is evaluated using a microscopic model that takes into account the lattice's discrete nature and the phonon dispersion relation within the Brillouin zone. The Boltzmann transport equation is solved iteratively within the framework of three-phonon interactions without taking into account the four-phonon scattering process. The Umklapp and normal collisions are treated rigorously, thereby avoiding relaxation-time and long-wavelength approximations. The mechanisms of the failures of these approximations in predicting the thermal transport properties are discussed. Evaluation of the thermal conductivity is performed at different temperatures and frequencies and in different crystallite sizes. Reasonably good agreement with the experimental data is obtained. The calculation reveals a critical role of out-of-plane acoustic phonons in determining the thermal conductivity. The out-of-plane acoustic phonons contribute greatly and the longitudinal and transverse acoustic phonons make small contributions over a wide range of temperatures and frequencies. The out-of-plane acoustic phonons dominate the thermal conductivity due to their high density of states and restrictions governing the anharmonic phonon scattering. The selection rule severely restricts the phase space for out-of-plane phonon scattering due to reflection symmetry. The optical phonon contribution cannot be neglected at higher temperatures. Both Umklapp and normal processes must be taken into account in order to predict the phonon transport properties accurately.

Structure, electrical and thermal properties of single-crystal BaCuGdTe3.

Single crystals of the quaternary chalcogenide BaCuGdTe3 were obtained by direct reaction of elements allowing for a complete investigation of the intrinsic electrical and thermal properties of this previously uninvestigated material. The structure was investigated by high-resolution single-crystal synchrotron X-ray diffraction, revealing an orthorhombic crystal structure with the space group Cmcm. Although recently identified as a semiconductor suitable for thermoelectric applications from theoretical analyses, our electrical resistivity and Seebeck coefficient measurements show metallic conduction, the latter revealing strong phonon-drag. Temperature dependent hole mobility reveals dominant acoustic phonon scattering. Heat capacity data reveal a Debye temperature of 183 K and a very high density of states at the Fermi level, the latter confirming the metallic nature of this composition. Thermal conductivity is relatively high with Umklapp processes dominating thermal transport above the Debye temperature. The findings in this work lay the foundation for a more detailed understanding of the physical properties of this and similar multinary chalcogenide materials, and is part of the continuing effort in investigating quaternary chalcogenide materials and their suitability for use in technological applications.

Influence of secondary phases of Sb2O3 and CoSb2 on CoSb3 skutterudite to reduce thermal conductivity

The unique crystal structure, high mobility and high electrical conductivity of CoSb3 based skutterudites holds to be a promising thermoelectric (TE) material for mid-temperature thermoelectric applications. Modifying the thermal transport to lower the thermal conductivity via secondary phases/nanocomposites is an efficient method to improve its TE performance. Herein, we followed the solvothermal method to synthesize CoSb3 along with the secondary phases of Sb2O3 and CoSb2 and the thermal conductivity measurement of CoSb3 prepared by arc melting technique is compared as reference. The laser induced micro-Raman spectra reveals that phonon lifetime (τ) is decreased due to local heat from high laser power. The presence of CoSb2 and semi metallic Sb2O3 results in grain boundary/interface phonon scattering. In addition, the influence of Umklapp process in phonon transport leads to an ultralow reduction in thermal conductivity to 0.64 Wm−1K−1 at 753 K.

Unified theoretical description of out-of-equilibrium electron intraband dynamics in gold induced by femtosecond laser pulses

The modeling of the intraband dynamics of conduction electrons in gold induced by a femtosecond laser pulse is addressed. Current approaches, based on the numerical resolution of the Boltzmann equation, are only able to describe electron excitation $\mathit{or}$ relaxation processes. In the present paper, within a single formalism, our kinetic model accounts quantitatively for both excitation and relaxation processes, i.e., photon absorption, thermal conductivity, and electron-phonon coupling coefficient. We suggest that such an approach can only be built by including Umklapp processes in the description of electron collisions. In addition to normal electronic collisional processes with phonons and other electrons, the present theoretical description further includes these mechanisms: (i) a conduction electron can be scattered in the second Brillouin zone after a collision, contributing significantly to the photon absorption, and (ii) the influence of the discrete and periodic nature of the lattice on electron collisions is considered, enabling in particular the coupling between electron and transverse phonons. A good agreement of predictions of this unified modeling with experimental observations for absorption and energy transfer from electrons to the lattice is obtained. We have investigated the influence of the Umklapp processes on the imaginary part of the dielectric function, the electron-phonon coupling parameter, and the transient shape of the electron energy distribution.

Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures

In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and hBN layers, and show that it introduces avoided crossings between the TMD and hBN bands, with umklapp processes opening ‘ghost’ avoided crossings in individual bands. Comparison between density functional theory (DFT) and ARPES spectra for the MoSe2/hBN heterostructure shows that the valence bands of MoSe2 and hBN are significantly further separated in energy in experiment as compared to DFT. We then show that a novel scissor operator can be applied to the hBN valence states in the DFT calculations, to correct the band alignment and enable quantitative comparison to ARPES, explaining avoided crossings and other features of band visibility in the ARPES spectra.

Ground-state properties of the one-dimensional modified Penson–Kolb–Hubbard model

The ground-state properties of a 1D modified Penson–Kolb–Hubbard model characterized by alternating Hubbard interaction at odd and even sites are discussed by the bosonization scheme and the renormalization group analysis. At quarter filling, the quantum phase diagram consists of the charge-density-wave, spin-density-wave and singlet-superconductivity instabilities. The insulating phases come from the relevant umklapp process induced by the unequal on-site repulsion. The results provide a further understanding of occurrence of the umklapp process in 1D correlated electron systems.

Supersolid devil's staircases of spin-orbit-coupled bosons in optical lattices

We study the emergence of supersolid devil's staircases of spin-orbit-coupled bosons loaded in optical lattices. We consider two- and three-dimensional systems of pseudo-spin-1/2 bosons interacting via local spin-dependent interactions. These interactions together with spin-orbit coupling produce length scales that are commensurate to the lattice spacing. This commensurability leads to devil's staircases of supersolids, with fractal Hausdorff dimensions, which arise from uniform superfluid phases. We show that umklapp processes are essential for the existence of commensurate supersolids, and that without them the devil's staircase does not exist. Lastly, we emphasize the generality of our results, suggest experiments that can unveil these unusual predictions, and discuss potential applications to the case of Rb87.Received 14 January 2022Revised 10 May 2022Accepted 12 July 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L032023Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCold gases in optical latticesPhysical SystemsBose gasesSupersolidsAtomic, Molecular & Optical

Comparative study of the Zn1-xCdxSb and (Zn1-zCdz)13Sb10 solid solution series

Comparison of the (Zn1-zCdz)13Sb10 and Zn1-xCdxSb series allows better understanding of changes in the transport properties brought by the Cd substitution. Incorporation of a single dopant affects charge transport properties non-monotonously, and these changes are likely dependent on the intrinsic properties of the main phase. The dominant phonon scattering pathway in both solution series remains the Umklapp process even when the Zn:Cd ratio is 1:1, contrary to the expected point defect scattering. Lastly, the electron localization function (ELF) calculations suggested that the bonding character in ZnSb and Zn13Sb10 is well-described using the Zintl-Klemm concept, as a mix of covalent and ionic interactions.

High power efficiency nitrides thermoelectric device

III-Nitrides, especially InGaN, are promising for high-efficiency thermoelectric (TE) components operating at high temperatures (HTs), playing a critical role in the recovery of waste heat for sustainable energy development. However, the performance of InGaN TE materials is limited by the high thermal conductivity (k) and the conflict coupling between the power factor (PF) and the k. Here, the previously unstudied two-dimensional InGaN/GaN SL structured TE device with a high In composition of 31% was developed and demonstrated to improve the TE figure of merit (ZT, Z = PF/k) by reducing the k value without deteriorating the PF. The Seebeck coefficient (S) exhibited a value of − 365 μV/K due to the increased density of electron states near the Fermi level by the low dimensional construction. Simultaneously, a relatively low k was obtained as 7.7 W/m·K, benefitting from the alloying and SL interface scattering effect of high energy phonons. Moreover, enhancement of the Umklapp process by the space confinement effect further lowers the k. Accordingly, a record ZT value of 0.089 at 300 K was achieved, which was better than previously reported values for GaN-based TE film materials. This work provides a new material system for improving the performance of nitride TE materials at HTs and extends the fields of application in electricity harvesting from waste heat.