Electronic Topological Transition Research Articles

On the high pressure axial ratio anomaly in zinc and the roles of temperature anddifferent electronic topological transitions

We report electronic structure calculations on zinc at variouscompressions. Our calculations show that an electronic topologicaltransition (ETT) associated with the anomaly in the axial ratio(c/a) at high pressuresis sensitive to ksampling used in the calculations, exchange–correlation potential and temperature. We showthat K-pointETT does not contribute to the anomaly, and thus disagree withthe model of contributions from many ETTs. Hence only theL-pointETT is significant. We suggest that, even though temperature smearing of theFermi function reduces the signatures of ETT, it does not wipe out the anomalyat room temperature.

Electronic topological transition in AuX2 (X = In, Ga and Al) compounds at highpressures

We present accurate x-ray diffraction data at high pressures for AuIn2,AuGa2 andAuAl2, obtained using a diamond anvil cell with the ELETTRA synchrotron source. Theresulting P–V data obtainedfrom the d-valueswere used to get the universal equation of state (UEOS), which is compared withtheoretical estimates. Deviation from linearity is evident in the UEOS curves ofAuIn2 and AuGa2, thus verifying that some of the observed anomalies inthese systems below 5 GPa are due to electronic topological transitions.

Correlation between critical temperature, pressure and hopping ratio inanisotropic low-dimensional superconductors

High pressure can strongly influence the electronic properties of a metal, bymodifying either the shape or the topology of its Fermi surface (FS).In the case of low-dimensional anisotropic superconductors, typified byhigh-Tccuprates, some quasi-2D organic salts and heavy-fermion compounds,we show that the occurrence of an electronic topological transition isgeneric to a non-monotonic pressure dependence of the critical temperatureTc. Onthe other hand, a change in shape of the FS can be correlated with the steady increaseof Tc as afunction of the ratio between next-nearest-and nearest-neighbour hopping, as is observed inthe high-Tccuprates.

Effects of an electronic topological transition for anisotropic low-dimensional superconductors

We study the superconducting properties of a two-dimensional superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T=0, the critical temperature Tc, and the impurity scattering rate are characterized by a nonmonotonic behavior, with maxima occurring close to the ETT. We derive analytical expressions for the value of such maxima both in the s-wave and in the d-wave case. Such expressions are in good qualitative agreement with the phenomenological trend recently observed for Tc^max as a function of the hopping ratio t'/t across several cuprate compounds. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness eta. Instead of vanishing at the ETT, as could be expected, thus giving rise to an increase of the fluctuation effects, in the case of momentum-independent electron-electron interaction, we find eta different from 0, as a result of an integration over the whole Fermi surface.

Features of the thermopower of Mo–Re and Mo–Re–Nb alloys and the electronic–topological transition in these systems

The anomalous behavior of the thermopower of Mo1−xRex and Mo1−x−yRexNby alloys is investigated over a wide range of temperatures and concentrations (within the limits of the solid solution). An extremum on the concentration dependence of the thermopower at 10 K, observed for both of these systems at the same electron concentration ≈6.1 electrons/atom, attests to the presence of a critical energy Ec in the electronic spectrum of Mo at which an electronic–topological transition occurs in Mo under the influence of impurities. According to the theoretical ideas about the electronic spectrum of Mo, the closest Ec>EF0 corresponds to the bottom of the band, upon the crossing of which a new electron sheet of the Fermi surface appears. In the binary systems the Re impurity causes a new sheet of the Fermi surface to appear, and in the ternary systems it disappears as the Nb impurity is added. A quantitative comparison of the theory with experiment makes it possible to determine the gap Ec−Ef0 for Mo, which is found to be ≈0.02 eV. These results correspond to those obtained previously from the superconducting characteristics.

Electronic structure and anomalous physical properties of metastable Al-Si solid solutions

Al-Si solid solutions synthesized under high pressure demonstrate striking physical properties, such as enhanced superconductivity and peculiarities of low-temperature transport coefficients. In order to understand the connection of these effects to the electronic structure changes we have performed a first-principles study of the electronic spectra and Fermi surfaces of Al-Si solid solutions. We show that two electronic topological transitions (ETT's) lead to unusual concentration dependencies of the resistivity, thermoelectric power and Hall constant of the system while a variety of other interesting phenomena such as lattice instability and superconductivity enhancement may be a result of the nesting features appearing upon Si doping. We present also the results of our theoretical calculations of the thermodynamic and transport properties of Al-Si solid solutions which are in good agreement with experiment and reproduce nicely the experimentally observed peculiarities.

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of the MgB2 ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas beyond this region the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the slope as well as a "hysteresis" effect in the frequency vs. pressure behavior. These singularities in the E2g mode behavior under pressure support the suggestion that MgB2 may undergo a pressure-induced topological electronic transition.

Axial ratio of Zn at high pressure and low temperature

High-pressure powder x-ray-diffraction experiments have been carried out on Zn with a He-pressure medium up to 18 GPa at 40 K. No anomaly in the volume dependence of the c/a axial ratio has been found within experimental errors. The c/a anomaly, which has been predicted to appear associated with the electronic topological transitions, should be extremely small. Thermal expansion of Zn rapidly decreases under high pressure, yielding nearly identical axial ratios at room and low temperatures.

Transport properties of Al–Si solid solutions: theory

Al–Si solid solutions synthesized under high pressure demonstrate striking physical properties, among which are enhanced superconductivity and peculiarities of low-temperature transport properties. To find the reason behind the latter we have performed a first-principles study of the electronic spectra and Fermi surfaces of Al–Si solid solutions. Two electronic topological transitions (ETT's), taking place in the system with increasing concentration of Si and pressure, have been revealed. Based on these data and the theory of ETT's for substitutional solid solutions we have calculated concentration dependencies of the resistivity, thermoelectric power and Hall constant. We show the results to be in quantitative agreement with experiment and to reproduce nicely the experimentally observed peculiarities.

Ab initio calculations of the equation of state and elastic constants of aluminum in the region of negative pressures

The results of ab initio calculations are presented for the specific energy, pressure, and elastic constants of an aluminum fcc single crystal with subnormal densities at T=0. Kinks in the elastic constant vs. density curves are revealed which are caused by the electronic topological transitions. An analysis of the mechanical stability of aluminum fcc crystal at negative pressures suggests that the polymorphic transition to a noncubic structure is possible. A method is suggested for taking into account the thermal nuclear excitation and the influence of zero-point vibrations on the pressure dependence of crystal density.

Effect of strain-induced electronic topological transitions on the superconducting properties of La 2 - x Sr x CuO 4 thin films

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of Tc in most cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.

Spin dynamics of a two-dimensional metal in a superconducting state: Application to the high-Tccuprates

We show that a two-dimensional (2D) metal in a superconducting (SC) state with wave-vector-dependent gap behaves as a quantum-spin liquid characterized by a collective mode of resonance type. Formally the mode exists whatever is a form of Fermi surface (FS). However, when the latter is spherical the effect is rather formal: any factor leading to an even weak electron scattering, like finite T, impurity, etc., easily kills the mode. On the contrary, for the type of FS observed in the high-${T}_{c}$ cuprates (characterized by a proximity to the saddle-point wave vectors) the resonance mode exists in a true sense being well separated from the electron-hole continuum and well defined. The effect is due to the effective 1D dimension in a proximity of electronic topological transition quantum critical point. The physical nature of the collective mode varies progressively between two possibilities. In the case of a weak interaction, the resonance mode is a typical effect of a two-particle bound state that appears slightly below the bottom of the two-particle continuum. The effect exists even in the case of infinitesimally weak interaction (of relevant sign). In the case of a strong or intermediate interaction, it is a critical mode existing as a precursor of spin-density wave (SDW) instability. The gap of the collective mode is not related directly to the value of the SC gap (being, however, limited by the latter from above) but rather to a proximity to SDW instability. We predict also a second collective mode lying inside the two-particle continuum. The existence of these two modes and the particular form of their dispersion in the case corresponding to the underdoped cuprates ${(d}_{{x}^{2}\ensuremath{\sim}{y}^{2}}$ symmetry of SC gap, exchange interaction of antiferromagnetic sign, etc.) allows one to understand the main features observed experimentally in the underdoped cuprates, namely the so-called ``41 meV resonance peak'' and ``24 meV incommensurability'' and their two-dimensional wave vector dependences, the doping evolution of the spin dynamics and many other features. The theory also predicts new features to be tested experimentally.

High-pressure thermoelasticity of body-centered-cubic tantalum

We have investigated the thermoelasticity of body-centered cubic (bcc) tantalum from first principles by using the linearized augmented plane wave (LAPW) and mixed--basis pseudopotential methods for pressures up to 400 GPa and temperatures up to 10000 K. Electronic excitation contributions to the free energy were included from the band structures, and phonon contributions were included using the particle-in-a-cell (PIC) model. The computed elastic constants agree well with available ultrasonic and diamond anvil cell data at low pressures, and shock data at high pressures. The shear modulus $c_{44}$ and the anisotropy change behavior with increasing pressure around 150 GPa because of an electronic topological transition. We find that the main contribution of temperature to the elastic constants is from the thermal expansivity. The PIC model in conjunction with fast self-consistent techniques is shown to be a tractable approach to studying thermoelasticity.

Pressure Effects on Anisotropic Low-Dimensional Superconductors Close to an Electronic Topological Transition

Hydrostatic pressure or anisotropic stress can modify the electronic properties of a metal, thus inducing a change in the topology of its Fermi surface. In the case of low-dimensional anisotropic superconductors (such as high- T c cuprates, some organic salts, or heavy Fermion compounds), such changes result in a nonmonotonic dependence of several properties on the critical parameter z , measuring the distance of the chemical potential from the electronic topological transition (ETT). This has to be contrasted with the monotonic, nearly step-like z -dependence of the same quantities in the 3D case. Such a non-monotonic behavior is in agreement with the trend observed for T c as a function of pressure and other material specific quantities in several high- T c cuprate compounds. On the other hand, higher pressures than those reported in the literature (∼ 10 kbar) should be investigated, in order to find evidence for ETT effects in the BEDT-TTF-based salts.

High-Pressure X-ray Studies of Zn at Room and Low Temperatures with a He-Pressure Medium

High-pressure powder X-ray diffraction experiments have been done on Zn with a He-pressure medium at room and low temperatures. No anomaly in the c / a axial ratio has been found even at low temperatures. This implies that the c / a anomaly, which is predicted to appear associated with the electronic topological transitions, would be extremely small.

A first-principles study of MgB2:the effect of pressure and substitution

We performed a first-principles calculation of the electronic bandstructure of MgB2, a superconductor discovered recently, byemploying a full-potential linear muffin-tin orbitalmethod. In advance, the influences of the lattice parameter on theelectronic structure are investigated with different volumes andc/a ratios. The effects of substitution on the electronicstructure are discussed in relation to the variation of the latticeparameters and electron filling which is investigated via a Mgpseudo-atom.The variations of the two-dimensional covalent σ-band andthree-dimensional metal π-band are investigated in detail atseveral high-symmetry points of the Brillouin zone (BZ). Theseresults show that the variations of the electronic structure ofMgB2 are mainly determined by the effect of the c/a ratio.Considering the variation of the Fermi surface at the M and Γpoints of the BZ, the corresponding electronic topologicaltransition (ETT) induced by the lattice and doping are discussed.The ETT at the M point should be related to the structural phasetransition in AlxMg1-xB.

Multifold enhancement of the thermoelectric figure of merit in p-type BaBiTe3 by pressure tuning

As in the combinatorial approach to materials discovery, pressure tuning allows the phase space of interaction parameters that determine materials’ properties to be explored more rapidly than by traditional means. Both the thermoelectric power and electrical conductivity of BaBiTe3 are found to increase upon compression. As a result there is a dramatic increase in the dimensionless thermoelectric figure of merit ZT from ∼0.1 to ∼0.8. We suggest a mechanism for the enhancement involving an electronic topological transition. Such transitions can be induced both by means of pressure tuning and alloying, offering a potential route to reproducing the pressure induced improved behavior reported here by chemical means.

Origin of the asymmetric spinodal decomposition in the Al-Zn system

The Fermi surface and the phase equilibria in the Al-Zn system are studied theoretically in the framework of the density-functional theory, linear muffin-tin orbital method, and the Green's function technique. Analyzing the Fermi surfaces of the random alloys, we find a series of electronic topological transitions within the stability range of the fcc solid solution. A relation between these transitions and the phase stabilities of fcc Al-Zn is established. We show that the asymmetry of the miscibility gap as well as of the mixing energy are due to the electronic topological transitions in the system.

Thermal equation of state of tantalum

We have investigated the thermal equation of state of bcc tantalum from first principles using the full-potential linearized augmented plane wave (LAPW) and mixed-basis pseudopotential methods for pressures up to 300 GPa and temperatures up to 10 000 K. The equation of state at zero temperature was computed using LAPW. For finite temperatures, mixed basis pseudopotential computations were performed for 54 atom supercells. The vibrational contributions were obtained by computing the partition function using the particle in a cell model, and the finite-temperature electronic-free energy was obtained from the LAPW band structures. We discuss the behavior of thermal equation of state parameters such as the Gr\"uneisen parameter $\ensuremath{\gamma},$ the thermal expansivity $\ensuremath{\alpha},$ and the Anderson-Gr\"uneisen parameter ${\ensuremath{\delta}}_{T}$ as functions of pressure and temperature. The calculated Hugoniot shows excellent agreement with shock-wave experiments. An electronic topological transition was found at approximately 200 GPa.

Fermi surface incommensurate nestings and phase equilibria in Cu-Pd alloys

The Cu-Pd phase diagram shows two low-temperature ordered phases corresponding to $B2$ and ${L1}_{2}$ superlattices or, regardless of the site occupation, to bcc and fcc geometrical arrangements. We show how the phase equilibria between these intermetallic compounds can be understood by introducing a hypothetical bcc random-alloy phase and a three-step transformation from $B2$ to random bcc to random fcc to ${L1}_{2}.$ The two order-disorder transformations, $B2$ to random bcc and random fcc to ${L1}_{2},$ analyzed in the framework of the concentration wave-functional theory, appear to be ruled by the sizes of the relevant Fermi surface (FS) nesting vectors in the two random-alloy systems. The transformation between the bcc and fcc solid solution phase is studied by first-principles Korringa-Kohn-Rostoker and coherent-potential approximation calculations, and it is shown to be influenced by electronic topological transitions occurring in the same systems. Our analysis points out the preeminent role played by the lattice-incommensurate FS nesting vectors that in both the random-alloy systems, give rise to frustrated concentration waves. The location of the $B2\ensuremath{-}{L1}_{2}$ coexistence at $T=0$ appears to be determined by the condition that the concentration waves of the fcc and bcc random alloys are equally frustrated.