Hole Doping Level Research Articles

Manipulable ferromagnetic properties and half-metallic behavior in hole-doped PbClI monolayer: A DFT study

The remarkable performance exhibited by two-dimensional (2D) materials holds tremendous promise for transformative applications in the domains of electronics and spintronics. This study through first-principles calculations presents a systematic investigation of the novel 2D material PbClI monolayer. It is found that the PbClI monolayer exhibits desirable semiconductor properties, characterized by a large gap of 3.75 eV. Furthermore, it has been observed that mechanical strain can effectively tune this bandgap. Importantly, due to its unique flat valence band edge, hole doping in PbClI gives rise to pronounced features of ferromagnetism and half-metallicity, and hole doping levels can effectively tune its magnetic properties, including magnetic coupling strength, easy magnetization axis, and Curie temperature. In addition, PbClI monolayer shows outstanding multidirectional piezoelectricity. The calculations serve as a pivotal theoretical framework for the realization of stable magnetism in 2D materials.The study uncovers the vast possibilities in PbClI monolayer to position it as a potential candidate in the domains of spintronics and optoelectronics.

Room temperature high thermoelectric performance of Bi-based full-Heusler compounds CsxRb[formula omitted]Bi with strong anharmonicity

Bismuth-based compounds have been identified as a class of promising candidates for the realization of high-performance thermoelectrics, derived from their exceptional electrical conductivity and intrinsic low lattice thermal conductivity κL. Here, we employed first-principles calculations, self-consistent phonon theory, and compressive sensing technique in conjunction with the Boltzmann transport equation to investigate the anharmonic thermoelectric properties of full-heusler bismuth compounds CsxRb3−xBi. As the Cs content increases, the κL of CsxRb3−xBi decreases due to impurity scattering effects, and then increases. Among them, at 300 K, Rb3Bi has the largest κL of 0.69 Wm−1K−1, while Cs2RbBi has the smallest κL of 0.45 Wm−1K−1, both of which are lower than the κL of traditional bismuth-based thermoelectric materials. The ultralow κL originates from the strong lattice anharmonicity. In addition, the coexistence of high dispersion and flat band edges leads to high thermoelectric power factor at optimal doping concentration. As a result, the largest thermoelectric figure of merit ZT values of 2.59 (4.19) at 300 (500) K were obtained for CsRb2Bi at optimal hole doping level. These findings suggest that CsxRb3−xBi compounds are superior for thermal management and thermoelectric applications.

Tc Saturation and Possible Electronic Phase Separation in Strongly Overdoped Cuprates

In order to elucidate the unusual superconducting properties of cuprates in the strongly overdoped region, i.e., at hole-doping levels p≳0.4/Cu in the CuO2 plane, we study the structural and superconducting properties of a series of Cu0.75Mo0.25Sr2YCu2O7+x powder samples oxygenated under high pressure using different concentrations of KClO3 up to 35 mol %. The analysis of X-ray diffraction data indicates a high purity ∼90% of all samples and suggests that the concentration, x, of extra oxygen atoms increases with increasing KClO3 concentration. Surprisingly, the Tc values remain nearly constant within the 80–85 K range independent of KClO3 concentration, which suggests a scenario of Tc saturation. In order to account for this unexpected behaviour, we put forward the hypothesis that overdoping enhances the density of unpaired holes, which is supported by the observation of large values of the Sommerfeld coefficient in all samples. We therefore propose a scenario of electronic phase separation between normal and superconducting holes.

Ultralow lattice thermal conductivities and excellent thermoelectric properties of hypervalent triiodides XI3 (X = Rb, Cs) discovered by machine learning method.

Thermal conductivity and power factor are key factors in evaluating heat transfer performance and designing thermoelectric conversion devices. To search for materials with ultralow thermal conductivity and a high power factor, we proposed a set of universal statistical interaction descriptors (SIDs) and developed accurate machine learning models for the prediction of thermoelectric properties. For lattice thermal conductivity prediction, the SID-based model achieved the state-of-the-art results with an average absolute error of 1.76W m-1 K-1. The well-performing models predicted that hypervalent triiodides XI3 (X = Rb, Cs) have ultralow thermal conductivities and high power factors. Combining first-principles calculations, the self-consistent phonon theory, and the Boltzmann transport equation, we obtained the anharmonic lattice thermal conductivities of 0.10 and 0.13W m-1 K-1 for CsI3 and RbI3 in the c-axis direction at 300K, respectively. Further studies show that the ultralow thermal conductivity of XI3 arises from the competition of vibrations between alkali metal atoms and halogen atoms. In addition, at 700K, the thermoelectric figure of merit ZT values of CsI3 and RbI3 are 4.10 and 1.52, respectively, at the optimal hole doping level, which indicates hypervalent triiodides are potential high performance thermoelectric materials.

Hole density dependent magnetic structure and anisotropy in Fe-pnictide superconductor

The competition between different magnetic structures in hole-doped Fe-pnicitides is explored based on an extended five-orbital Hubbard model including long-range Coulomb interactions. Our results show that the stabilized magnetic structure evolves with increasing hole doping level. Namely, the stripe antiferromagnetic phase dominates at zero doping, while magnetic structures with more antiferromagnetic linking numbers such as the staggered tetramer, staggered trimer, and staggered dimer phases become energetically favorable as the hole density increases. At a certain doping level, energy degeneracy of different magnetic structures appears, indicating strong magnetic frustration and magnetic fluctuations in the system. We suggest that the magnetic competition induced by the hole doping may explain the fast decrease of the Neel temperature T N and the moderately suppressed magnetic moment in the hole doped Fe-pnicitides. Moreover, our results show a sign reversal of the kinetic energy anisotropy as the magnetic ground state evolves, which may be the mechanism behind the puzzling sign reversal of the in-plane resistivity anisotropy in hole-doped Fe-pnicitides.

Precise dd excitations and commensurate intersite Coulomb interactions in the dissimilar cuprates YBa2Cu3O7–y and La2–xSrxCuO4

Using high-resolution extreme ultraviolet resonant inelastic X-ray scattering (EUVRIXS) spectroscopy at Cu M-edge, we observed the doping dependent spectral shifts of inter-orbital (dd) excitations of YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4. With increasing hole doping level from undoped to optimally doped superconducting compositions, the leading edge of dd excitations is found to shift towards lower energy loss in a roughly linear trend that is irrespective to the cuprate species. The magnitude of energy shift can be explained by including a 0.15 eV Coulomb attraction between Cu 3d_(x^2-y^2) electrons and the doped holes on the surrounding oxygens in the atomic multiplet calculations. The consistent energy shift between distinct cuprate families suggests that this inter-site Coulomb interaction energy scale is relatively material-independent, and provides an important reference point for understanding charge density wave phenomena in the cuprate phase diagram.

Flat-band plasmons in twisted bilayer transition metal dichalcogenides

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer ${\mathrm{MoS}}_{2}$ (tb-${\mathrm{MoS}}_{2}$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-${\mathrm{MoS}}_{2}$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasiflat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt{q}$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasiflat plasmons and higher-energy interband plasmons can coexist. For rigid tb-${\mathrm{MoS}}_{2}$ with a high doping level, strongly enhanced interband transitions quench the quasiflat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a proper hole-doping level, are essential for observing the low-damped and quasiflat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future.

Dynamic parallel spin stripes from the 1/8 anomaly to the end of superconductivity in La1.6−xNd0.4SrxCuO4

We have carried out neutron spectroscopic measurements on single crystals of ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ from $0.12\ensuremath{\le}x\ensuremath{\le}0.26$ using time-of-flight techniques. These measurements allow us to follow the evolution of parallel spin stripe fluctuations with energies less than $\ensuremath{\sim}33$ meV, from $x=0.12$ to 0.26. Samples at these hole-doping levels are known to display static (on the neutron-scattering time scale) parallel spin stripes at low temperature, with onset temperatures and intensities which decrease rapidly with increasing $x$. Nonetheless, we report remarkably similar dynamic spectral weight for the corresponding dynamic parallel spin stripes, between 5 and 33 meV, from the 1/8 anomaly near $x=0.12$, to optimal doping near $x=0.19$ to the quantum critical point for the pseudogap phase near $x=0.24$, and finally to the approximate end of superconductivity near $x=0.26$. This observed dynamic magnetic spectral weight is structured in energy with a peak near 17 meV at all dopings studied. Earlier neutron and resonant x-ray scattering measurements on related cuprate superconductors have reported both a disappearance with increasing doping of magnetic fluctuations at ($\ensuremath{\pi}$, $\ensuremath{\pi}$) wave vectors characterizing parallel spin stripe structures and persistant paramagnon scattering away from this wave vector, respectively. Our results for ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ from 0.12 $\ensuremath{\le}x\ensuremath{\le}0.26$ clearly show persistent parallel spin stripe fluctuations at and around at ($\ensuremath{\pi}$, $\ensuremath{\pi}$), and across the full range of doping studied. These results are also compared to recent theory. Together with a rapidly declining $x$ dependence to the static parallel spin stripe order, the persistent parallel spin stripe fluctuations show a remarkable similarity to the expectations of a quantum spin glass, random t-J model, recently introduced to describe strong local correlations in cuprates.

Soft memtransistor with ion transfer interface

A paradigm for soft memtransistor is demonstrated based on the ion transfer interface, consisting of an ion-rich semiconducting polymer layer on the top and a gelatin dielectric for receiving ions on the bottom. The flexible polymer memtransistor acted as an analog-type memristor without gating, and its memristive strength could be largely modulated by applying gate voltage. It is proposed that the ion redistribution across the ion transfer interface can modify the hole doping level in the polymer layer, which is responsible for the tunable memristive characteristics. Different levels of synaptic potentiation and depression were successfully emulated using the polymer memtransistor, and it is promising to extend the emulation to multi-terminal heterosynaptic plasticity.

Mapping the Phase Diagram of a YBa2Cu3O7−δ Nanowire Through Electromigration

We use electromigration (EM) to tune the oxygen content of YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) nanowi\-res. During EM, the dopant oxygen atoms in the nanowire are moved under the combined effect of electrostatic force and Joule heating. The EM current can be tuned to either deplete or replenish nanowires with oxygen, allowing fine tuning of its hole doping level. Electrical transport measurements and Kelvin probe microscopy corroborate good homogeneity of the doping level along the electromigrated nanowires. Thus, EM provides an effective method to study transport properties of YBCO in a wide doping range at the nanoscale in one and the same device.

Charge density wave and superconducting phase in monolayer InSe

In this paper, the completed investigation of a possible superconducting phase in monolayer indium selenide is determined using first-principles calculations for both the hole and electron doping systems. The hole-doped dependence of the Fermi surface is exclusively fundamental for monolayer InSe. It leads to the extensive modification of the Fermi surface from six separated pockets to two pockets by increasing the hole densities. For low hole doping levels of the system, below the Lifshitz transition point, superconductive critical temperatures $T_c \sim 55-75$ K are obtained within anisotropic Eliashberg theory depending on varying amounts of the Coulomb potential from 0.2 to 0.1. However, for some hole doping above the Lifshitz transition point, the combination of the temperature dependence of the bare susceptibility and the strong electron-phonon interaction gives rise to a charge density wave that emerged at a temperature far above the corresponding $T_c$. Having included non-adiabatic effects, we could carefully analyze conditions for which either a superconductive or charge density wave phase occurs in the system. In addition, monolayer InSe becomes dynamically stable by including non-adiabatic effects for different carrier concentrations at room temperature.

Fluctuation-frustrated flat band instabilities in NdNiO2

The discovery that Nd$_{1-x}$Sr$_x$NiO$_2$, with the CaCuO$_2$ infinite-layer structure, superconducts up to 15 K around the hole-doping level $x$=0.2 raises the crucial question of its fundamental electronic and magnetic processes. The unexplained basic feature that we address is that, for $x$=0 and as opposed to strongly antiferromagnetic (AFM) CaCuO$_2$, NdNiO$_2$ with the same structure and formal $d^9$ configuration does not undergo AFM order. We study this issue not in the conventional manner, as energetically unfavored or as frustrated magnetic order, but as an instability of the AFM phase itself. We are able to obtain the static AFM ordered state, but find that a flat-band, one-dimensional-like van Hove singularity (vHs) is pinned to the Fermi level. This situation is unusual in a non-half-filled, effectively two-band system. The vHs makes the AFM phase unstable to spin-density disproportionation, breathing and half-breathing lattice distortions, and (innate or parasitic) charge-density disproportionation. These flat-band instabilities, distant relatives of single band cuprate models, thereby inhibit but do not eliminate incipient AFM tendencies at low temperature. The primary feature is that a pair of active bands ($d_{x^2-y^2}$, $d_{z^2})$ eliminate half-filled physics and, due to instabilities, preclude the AFM phase seen in CaCuO$_2$. This strongly AFM correlated, conducting spin-liquid phase with strong participation of the Ni $d_{z^2}$ orbital, forms the platform for superconductivity in NdNiO$_2$.

Hidden magnetism at the pseudogap critical point of a cuprate superconductor

The mysterious pseudogap phase of cuprate superconductors ends at a critical hole doping level p* but the nature of the ground state below p* is still debated. Here, we show that the genuine nature of the magnetic ground state in La2-xSrxCuO4 is hidden by competing effects from superconductivity: applying intense magnetic fields to quench superconductivity, we uncover the presence of glassy antiferromagnetic order up to the pseudogap boundary p* ~ 0.19, and not above. There is thus a quantum phase transition at p*, which is likely to underlie highfield observations of a fundamental change in electronic properties across p*. Furthermore, the continuous presence of quasi-static moments from the insulator up to p* suggests that the physics of the doped Mott insulator is relevant through the entire pseudogap regime and might be more fundamentally driving the transition at p* than just spin or charge ordering.

Phonon-mediated superconductivity in aluminum-deposited graphene AlC8

It has been theoretically predicted and experimentally confirmed that graphene deposited with atoms of a univalent alkali metal such as Li (${\mathrm{LiC}}_{6}$) or divalent alkaline-earth metal such as Ca (${\mathrm{CaC}}_{6}$) can be a superconductor. For atoms of a trivalent metal such as Al, if deposited on graphene, it was predicted that ${\mathrm{AlC}}_{8}$ can be in a metallic state. Whether this compound is stable and can be made superconducting is an issue which has not been addressed and deserves further investigation. In this work, based on first-principles calculations, it is found that the phonon spectrum of ${\mathrm{AlC}}_{8}$ shows imaginary frequencies for the two lowest branches, indicating the structure is dynamically unstable. By hole doping, the imaginary frequencies basically disappear and the lattice is stabilized. Besides, biaxial tensile strain was applied to study its effect on phonon and electron-phonon coupling. With the increase of tensile strain, the high-energy phonon spectrum associated with the C-C stretching modes softens greatly and the electron-phonon coupling becomes stronger, resulting in the increase of superconducting transition temperature ${T}_{c}$ to a value of more than 22 K for a sample at the experimentally accessible hole doping ($7.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) and tensile strain ($12%$) levels. This is above the liquid hydrogen temperature of 20.3 K. Thus, besides Li and Ca deposited graphene, ${\mathrm{AlC}}_{8}$ provides another platform for realizing superconductivity in graphene.

A far from conventional Tc-p correlation in the highly doped superconducting molybdo-cuprates: The case of Mo0.3Cu0.7Sr2TmCu2O7+δ

The highly doped region of the T-p phase diagram of the high temperature superconducting cuprates, has been, somewhat surprisingly, less explored than the rest of it. Here, we present a fresh look into the superconducting properties of Mo0.3Cu0.7Sr2YCu2O7+δ, with different oxygen contents. These have been attained by two different oxidation techniques and results in samples with a very high hole doping levels in the CuO2 superconducting planes, placing these materials in a greatly doped regime. The evolution of the critical temperature (TC) with p, reflects the displacement of the superconducting region towards much higher doping levels in comparison with conventional cuprates. Moreover, the normal state transport properties are also displaced, but not in the same extension as the superconducting region, resulting in a different superconducting to normal state correspondence in this Mo0.3Cu0.7Sr2YCu2O7+δ system.

On the structural, electronic, optical and thermoelectric properties of CdIn2Se4 ordered-vacancy compound

In this paper, we report results of the structural, electronic, optical and thermoelectric properties of CdIn2Se4 compound. These properties are investigated using first-principles calculations based on the full-potential linearized plane-wave (FP-LAPW) technique within the framework of density functional theory (DFT) and semiclassical Boltzmann transport theory. During the structural optimization, the electron exchange-correlation potentials are described through Wu-Cohen (WC) functional, while the Tran-Blaha modified Becke-Johnson exchange potential is employed when calculating the electronic and related properties to obtain more accurate band gap. Electronic band structures reveal that the CdIn2Se4 compound is a direct gap Γ−Γ semiconductor, with a band gap of 1.834 ​eV. Results of the compound optical properties suggest this material as promising absorber to work under ultraviolet radiation. Finally, the thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor and dimensionless figure of merit are calculated and analyzed in details. With a proper hole doping level, the CdIn2Se4 compound can a promising candidate for thermoelectric applications with figure of merit close to unity.

Surface structures of graphene covered Cu(103)

We studied the surface structures of chemical vapor deposited (CVD) graphene on Cu(103). The graphene covered Cu surface had (103) facets parallel to the Cu[010] direction, on which triangular patterns were formed. In contrast, the bare Cu surface showed no facets. Post-growth thermal annealing in an ultra-high vacuum induced surface changes on the Cu(103) facets. The reorganization of the Cu surface by the post-growth thermal annealing led to a change in the lattice strain and hole doping level of the CVD-grown graphene.

Charge Order and Superconductivity in Underdoped YBa_{2}Cu_{3}O_{7-δ} under Pressure.

In underdoped cuprates, an incommensurate charge density wave (CDW) order is known to coexist with superconductivity. A dip in T_{c} at the hole doping level where the CDW is strongest (n_{p}≃0.12) suggests that CDW order may suppress superconductivity. We investigate the interplay of charge order with superconductivity in underdoped YBa_{2}Cu_{3}O_{7-δ} by measuring the temperature dependence of the Hall coefficient R_{H}(T) at high magnetic field and at high hydrostatic pressure. We find that, although pressure increases T_{c} by up to 10K at 2.6GPa, it has very little effect on R_{H}(T). This suggests that pressure, at these levels, only weakly affects the CDW and that the increase in T_{c} with pressure cannot be attributed to a suppression of the CDW. We argue, therefore, that the dip in T_{c} at n_{p}≃0.12 at ambient pressure is probably not caused by the CDW formation.

STM/STS study on electronic superstructures in the superconducting state of high-Tc cuprate Bi2Sr2CaCu2O8+δ

We report STM/STS measurements at 8 K in underdoped Bi2Sr2CaCu2O8+δ crystals (Tc = 76 K and hole-doping level p ∼ 0.12) whose energy spectra around the Fermi level are characterized by a two-gap structure consisting of spatially inhomogeneous pseudogap (PG) and comparatively homogeneous superconducting gap (SCG). Two electronic superstructures, checkerboard modulation (CBM) and Cu-O-Cu bond-centered modulation (BCM), are observed with mapping spectral weights at low energies within the SCG and the ratio of spectral weights at ±ΔPG (PG energy), respectively. On the basis of the present findings, we suggest that the lower-energy scale CBM is an intrinsic property of Cu-O planes and can coexist with the BCM whose characteristic energy is ∼ΔPG in identical regions in real space.

Phonon-mediated high-Tc superconductivity in hole-doped diamond-like crystalline hydrocarbon

We here predict by ab initio calculations phonon-mediated high-Tc superconductivity in hole-doped diamond-like cubic crystalline hydrocarbon K4-CH (space group I21/3). This material possesses three key properties: (i) an all-sp3 covalent carbon framework that produces high-frequency phonon modes, (ii) a steep-rising electronic density of states near the top of the valence band, and (iii) a Fermi level that lies in the σ-band, allowing for a strong coupling with the C-C bond-stretching modes. The simultaneous presence of these properties generates remarkably high superconducting transition temperatures above 80 K at an experimentally accessible hole doping level of only a few percent. These results identify a new extraordinary electron-phonon superconductor and pave the way for further exploration of this novel superconducting covalent metal.