Electron-hole Liquids Research Articles

Acoustic plasmons and isotropic short-range interaction in two-component electron liquids

Dispersion of acoustic plasmons and isotropic Landau parameters are calculated in three- and two-dimensional two-component electron-electron and electron-hole liquids at various concentration and mass ratios using Landau-Silin kinetic equation and the random phase approximation for the self-energy. It is shown that the mode propagation and the strength of quasiparticle interaction are determined by the intercomponent screening and are asymmetric with respect to charge composition of two-component liquid. The well-defined acoustic plasmon-zero sound mode arises at strong difference in concentrations between components and its renormalization by the short-range exchange-correlation interaction is negligible. The acoustic plasmon mediated interparticle effective interaction in the fast component is weak in both the three- and two-dimensional electron liquids with parabolic dispersion, so the associated plasmonic superconductivity and the formation of acoustic plasmarons are unfavorable.

Valley engineering electron-hole liquids in transition metal dichalcogenide monolayers

Electron-hole liquids (EHLs), a correlated state of matter and a thermodynamic liquid, have recently been found to exist at room temperature in suspended monolayers of ${\text{MoS}}_{2}$. Appreciably higher rates of radiative recombination inside the liquid as compared to free excitons hold promise for optoelectronic applications such as broadband lasing. In this paper, we show that leveraging the valley physics in ${\text{MoS}}_{2}$ may be a route towards achieving tunability of specific characteristics of an EHL, such as emission wavelength, linewidth, and, most importantly, the liquid density. The conditions under which EHLs form, in bulk semiconductors as well as transition metal dichalcogenide (TMDC) monolayers are quite stringent, requiring high crystal purity and cryogenic temperatures in bulk semiconductors, and suspension in monolayers. Using a simple yet powerful model for describing free excitons and show that a phase transition into the EHL state may be feasible in substrate-supported monolayer samples. More repeatable experimental realizations of EHLs may be essential to answer questions regarding the nature of electron-hole correlations and how they may be used to generate nontrivial states of light.

Electron-Hole Liquid in Monolayer Transition Metal Dichalcogenide Heterostructures

Monolayer films of transition metal dichalcogenides (in particular, MoS2, MoSe2, WS2, and WSe2) can be considered as ideal systems for the studies of high-temperature electron-hole liquids. The quasi-two-dimensional nature of electrons and holes ensures their stronger interaction as compared to that in bulk semiconductors. The screening of the Coulomb interaction in monolayer heterostructures is significantly reduced, since it is determined by the permittivities of the environment (e.g., vacuum and substrate), which are much lower than those characteristic of the films of transition metal dichalcogenides. The multivalley structure of the energy spectrum of charge carriers in transition metal dichalcogenides significantly reduces the kinetic energy, resulting in the increase in the equilibrium density and binding energy of the electron-hole liquid. The binding energy of the electron-hole liquid and its equilibrium density are determined. It is shown that the two-dimensional Coulomb potential should be used in the calculations for the electron-hole liquid.

Near Band‐Edge Optical Excitation Leading to Catastrophic Ionization and Electron–Hole Liquid in Room‐Temperature Monolayer MoS2

Atomically thin materials exhibit exotic electronic and optical properties. Strong many‐body interactions from the reduced dielectric environment lead to electronic phases that drastically change conductivity and optical response. For example, these many‐body interactions can give rise to the formation of collective states such as Mott metal–insulator transitions, electron–hole liquids and plasmas, and excitonic condensates, which typically occur at cryogenic temperatures and high excitation densities. Herein, it is demonstrated that in monolayer MoS2 at room temperature, a low‐density (1010 cm−2) excitonic gas is formed with continuous wave (CW) below‐gap optical excitation. A slight increase in the excitation fluence triggers a nanosecond phase transition into a dense electron–hole liquid state with three orders of magnitude higher carrier density. This investigation suggests that while the material is in equilibrium with the CW excitation at the threshold fluence, thermomechanical expansion combined with continuous renormalization of the band gap leads to a sudden increase of optical absorption, which initiates runaway exciton ionization and the formation of a high density electron–hole plasma. Such abrupt changes in the excitation density and carrier population can be the basis of unprecedented applications based on 2D materials.

Electron–hole liquids form at room temperature

Electron–hole liquids form at room temperature

Electron–hole liquid in low-dimensional silicon–germanium heterostructures

A brief review is given of the studies in which quasi-two-dimensional spatially-direct and dipolar electron–hole liquids in Si/SiGe/Si type-II heterostructures with a low Ge content in the SiGe layer were discovered and investigated.

Coexistence of the spin and electron-hole quantum liquids in frustrated manganites La1–y Sm y MnO3+δ

The structural, electronic, and magnetic phase transformations induced in the temperature range of 4.2–300 K by the isovalent substitution of the rare-earth Sm3+ ion for the La3+ ion with a larger radius have been investigated in the system of self-doped manganites La1–ySmyMnO3+δ (δ ∼ 0.1, 0 ≤ y ≤ 1). It has been found using the X-ray diffraction analysis that the substitution La → Sm is accompanied by a significant increase of the GdFeO3-type and Jahn-Teller-type lattice distortions. At a temperature of 300 K, an increase in the Sm concentration y leads to a concentration phase transition from the pseudocubic O* phase to the orthorhombic O’ phase, as well as to the appearance of an s-shaped anomaly of the lattice parameter a at concentrations y ≥ 0.4 and an anomalous peak in the concentration dependence of the electrical resistance R(y) near y ≈ 0.85. The magnetic T-y-〈rA〉 phase diagrams of the La1–ySmyMnO3+δ system in the temperature range of 4.2–250 K have been constructed according to the results of measurements of the temperature dependences of the direct-current (dc) magnetization M(T). It has been revealed that the samples with y ≥ 0.8 exhibit signs of the melting of the A- and CE-type modulated antiferromagnetic (AFM) structures in the form of an anomalous decrease both in the critical temperature of the transition to a frustrated AFM state and in the magnetization of the samples with an increase in the concentration y. The anomalies observed in the temperature dependences of the alternating-current (ac) dielectric permittivity of the La1–ySmyMnO3+δ samples have been explained within the existing concepts of the Bose-Einstein condensation of an electron-hole liquid in the form of metallic droplets in an excitonic insulator.

Thermodynamics of electron-hole liquids in graphene

The impact of Coulomb interactions on the chemical potential, heat capacity, and oscillating magnetic moment is studied. The cases of low and high temperatures are considered. At low temperatures, doped graphene behaves as the common Fermi liquids with the power temperature laws for thermodynamic properties. However, at high temperatures and relatively low carrier concentrations, it exhibits the collective electron-hole behavior: the chemical potential tends to its value in the undoped case going with the temperature to the charge neutrality point. Simultaneously, the electron contribution into the heat capacity tends to the constant value as in the case of the Boltzmann statistics.

Similarity and contrasts between thermodynamic properties at the critical point of liquid alkali metals and of electron-hole droplets

The recent experimental study by means of time-resolved luminescence measurements of an electron-hole liquid (EHL) in diamond by Shimano et al. [Phys. Rev. Lett. 88, 057404 (2002)] prompts us to compare and contrast critical temperature ${T}_{c}$ and critical density ${n}_{c}$ relations in liquid alkali metals with those in electron-hole liquids. The conclusion drawn is that these systems have similarities with regard to critical properties. In both cases the critical temperature is related to the cube root of the critical density. The existence of this relation is traced to Coulomb interactions and to systematic trends in the dielectric constant of the electron-hole systems. Finally a brief comparison between the alkalis and EHL's of the critical values for the compressibility ratio ${Z}_{c}$ is also given.

Electron (Hole) Liquids Flowing Through Both Heavy Fermion and Cuprate Materials in Superconducting and Normal States

In earlier work, we have considered electron (hole) liquids flowing through assemblies with antiferromagnetic fluctuations in the normal state of high-Tc cuprates. The present review has as its focus to compare and contrast these high-Tc cuprates with heavy Fermion superconductors. Both classes of materials will be considered in superconducting as well as in normal phases. In the former phase, a further focal point will be the symmetry of the pairing. In underdoped cuprates with d-wave symmetry, evidence continues to mount for preformed (2e) Bosons in the normal state, especially from neutron scattering experiments. This is in marked contrast to the heavy Fermion material UBe13, where very recent differential conductivity measurements on a UBe13-Au junction appear to exclude the formation of such precursor Bosons.

Is the Asymptotic Scenario of Precursor r space Bosons Consistent with Observation for Electron (hole) Liquids in Underdoped High-T cCuprates?

The motivation for this study is the review by Timusk and Statt [Rep. Prog. Phys. 62, 61 (1999)] on the pseudogap in high-temperature superconductors. Here, we confront the experimental data they present, plus that of Norman et al. [Nature 392, 157 (1998)] on the Fermi surface in underdoped high-Tc materials with the theoretical asymptotic scenario of precursor r space Boson formation. This reveals no inconsistencies with available observations concerning electron (hole) liquids flowing through such underdoped cuprates with their antiferromagnetic spin fluctuations.

Exciton and charge density wave formation in spatially separated electron–hole liquids

We present a microscopic many-body calculation investigating the competition between the charge density wave and exciton formation in coupled electron–hole layer systems at finite temperature. We find that as the layer separation is decreased there is a divergence at a finite wave vector in the static susceptibility, indicating an instability in the liquid to a coupled charge density wave. We also see evidence in the pair correlation function g( r ) for the liquid phase of a divergence occurring at small r when the layer separation is reduced sufficiently, which would indicate the onset of a bound state instability. However, the charge density wave instability occurs in the liquid before g( r ) can actually diverge. The unified picture which starts to emerge from our results is of a charge density wave instability occurring in the liquid phase at a layer separation larger than the exciton radius. This leaves open the possibility of a second transition from the charge density wave state to the excitonic phase.

Two-particle states in coupled quantum wires in a magnetic field

Spectrum of two charged particles in a system of two parallel quantum wires coupled by tunneling in external perpendicular magnetic field is investigated. Both cases of the same and opposite charge signs are studied. We reduce the problem to single polynomial equations depending on the symmetry properties of states and locating all the poles of the scattering amplitude in a unified manner. Field dependence of physically meaningful poles corresponding to the bound states and resonances of scattering is studied. The presence of magnetic field results in decrease of the energy splitting between symmetric and antisymmetric states of two opposite charges (say excitons), which may be regarded as the effective suppression of tunneling between wires. It is shown that in sufficiently high magnetic fields interwire resonant states appear in the spectra. Existence of such resonances should lead to important consequences for the properties of electron and electron-hole liquids in the system.

Collective modes in a quasi-one-dimensional, two-component electron liquid

Under favorable conditions, a new collective mode besides the usual plasmons may exist in degenerate electron-hole liquids. We calculate the dispersion and damping of this new mode (called the acoustic plasmon mode) in a quasi-one-dimensional, two-component electron liquid. We carry out our calculations first within the random-phase approximation, then include the effects of local-field corrections using a Hubbard-like approximation. The latter decreases the acoustic plasmon dispersion.

Many-body effects in type-II quantum-well and quantum-well-wire superlattices

Many-body phenomena related to electron-hole plasmas or liquids in type-II GaAs/AlAs superlattices grown in (100) and (311) direction are investigated by time-resolved luminescence. The band-gap-renormalization in a (100) superlattice is determined using a luminescence lineshape fit, revealing the influence of the space-charge effect on the type-II transition. The dynamics of the liquid phase which is only present in the (311) quantum-well-wire is studied. PACS: 78.66.Fd, 73.20.Dx

Electron-hole liquids and band-gap renormalization in short-period semiconductor superlattices.

In short-period superlattices, excited electrons and holes are subject to a one-dimensional perturbation with spatial dimension comparable to the bulk excitonic radius and strength comparable to the bulk excitonic binding energy. We study the stability of the electron-hole liquid and band-gap renormalization as a function of the perturbing potentials (band offsets) using the density-functional approach. We find that there is no universal band-gap renormalization in superlattices, that type-II staggered superlattices are the best candidates for observing the electron-hole liquid, and that a luminescence line in these systems should show a blue shift at intermediate electron-hole densities due to a positive differential band-gap renormalization.

Effective interactions for self-energy. II. Application to electron and electron-hole liquids.

Effective masses in an electron gas and an electron-hole liquid are calculated for different values of r/sub s/ and of the ratio M/sub h//M/sub e/ (in the case of the electron-hole liquid) using the self-energy expressions derived in the preceding paper (Ng and Singwi, Phys. Rev. B 34, 7738 (1986)). In the case of an electron gas, it is found that spin-fluctuation effects lead to an appreciable amount of mass enhancement for metallic r/sub s/ values, whereas in the case of an electron-hole liquid, both spin and density fluctuations are important in determining the quasiparticle properties.

Ultrasonic attenuation in electron-hole liquids.

A theory of the attenuation of longitudinal sound in electron-hole liquids in semiconductors, based on coupled electron and hole kinetic equations, is developed. The theory takes into account the existence of several bands of Fermi liquids, with differing charge and interaction with the lattice, the fact that the Fermi wave numbers are so small that the wave number of the sound can exceed them, and the effects of intraband collisions, which can be rapid enough to produce a significant reduction in the attenuation, even in the high-frequency regime. The theory is applied to sound propagating in the direction in a Ge electron-hole liquid, and the direction in a Si liquid. The former case, compared with the recent experiments by Dietsche, Kirch, and Wolfe (Phys. Rev. B 26, 780 (1982)), suggests that intraband collisions play an important role in the sound-attenuation process.

On the possibility of superconductivity in electron-hole liquids

It is shown that an electron-hole liquid under suitable conditions can become superconducting. This conclusion is reached by using an effective electron-electron interaction which includes vertex corrections and multiple electron-hole scattering. The mechanism for superconductivity is novel. The intermediate bosons responsible for the superconducting pairing are not acoustic plasmons but correlated pair excitations from the Fermi sea of the holes. Some materials are proposed for an experimental verification of the ideas presented here.

Theory of electron-hole liquids in two dimensions

Theory of electron-hole liquids in two dimensions