Hole Doping Research Articles

Piezoelectric ferromagnetism in two dimensional FeCl2

We predict that monolayer FeCl$_2$ is a two-dimensional piezoelectric ferromagnet (PFM) with easy-axis magnetism and a Curie temperature of 260 K. Our ab-initio calculations combined with data mining reveal 2H-FeCl$_2$ as the only easy-axis 2D monolayer PFM, and that its magnetic anisotropy increases many-fold with moderate hole doping. We develop an analysis based on magnetic anisotropy energy densities that explain the magnetic and doping-dependent behavior of FeCl$_2$, as well as VSe$_2$ and CrI$_3$, and can enable the design of future 2D magnetically ordered materials.

First-principles calculations to investigate transport properties of non-trivial fermions of CoSi

Recently, the electronic topology of CoSi has been reported to be present at three nodal points (two at Γ (G1, G2) and one at R (R1)) in the band structure. Considering this, we present a study where various transport coefficients are investigated by using first-principle based DFT method for the temperature (T) range 40-300 K. For the chemical potential (μ) corresponding to energies of these nodal points and at the Fermi level (EF), 3D constant energy surfaces are constructed. They have shown that the number of states available at energies of these nodal points and the EF is largest for R1 point and least for G1 point at T = 0 K. Electrical conductivity per unit relaxation time(σ/τ) and electronic thermal conductivity per unit relaxation time(κ0) have also shown similar results at different μ with rise in T. For instance, at T = 100 K, σ/τ∼0.13×1020Ω−1m−1s−1 at G1 whereas its value reaches ∼0.18×1020Ω−1m−1s−1 at R1 nodal point. However, Seebeck coefficient (S) is largest for G2 and least for G1 nodal point at any given T. The values of S are obtained to be positive at the μ corresponding to the G2, R1 and EF (except G1) which is increasing with the rise in T. Also, the dominant charge carriers at G1 point are found to be electrons for T < 225 K whereas for T > 225 K, the charge carriers are obtained to be dominated by holes. Furthermore, the doping concentrations have also been calculated for G1 (electron doping ∼2.25×1022 cm−3), G2 and R1 points (hole doping ∼4.10×1020 cm−3 and 7.22 × 1022 cm−3, respectively).

P -wave superconductivity in Luttinger semimetals

We consider the three-dimensional spin-orbit-coupled Luttinger semimetal of ``spin'' $3/2$ particles in presence of weak attractive interaction in the $l=1$ ($p$-wave) channel, and determine the low-temperature phase diagram for both particle- and hole dopings. The phase diagram depends crucially on the sign of the chemical potential, with two different states (with total angular momentum $j=0$ and $j=3$) competing on the hole-doped side, and three (one $j=1$ and two different $j=2$) states on the particle-doped side. The ground-state condensates of Cooper pairs with the total angular momentum $j=1,2,3$ are selected by the quartic, and even sextic terms in the Ginzburg-Landau free energy. Interestingly, we find that all the $p$-wave ground states appearing in the phase diagram, while displaying different patterns of reduction of the rotational symmetry, preserve time reversal. The resulting quasiparticle spectrum is either fully gapped or with point nodes, with nodal lines being absent.

Triplet Superconductivity from Nonlocal Coulomb Repulsion in an Atomic Sn Layer Deposited onto a Si(111) Substrate.

Atomic layers deposited on semiconductor substrates introduce a platform for the realization of the extended electronic Hubbard model, where the consideration of electronic repulsion beyond the on-site term is paramount. Recently, the onset of superconductivity at 4.7K has been reported in the hole-doped triangular lattice of tin atoms on a silicon substrate. Through renormalization group methods designed for weak and intermediate coupling, we investigate the nature of the superconducting instability in hole-doped Sn/Si(111). We find that the extended Hubbard nature of interactions is crucial to yield triplet pairing, which is f-wave (p-wave) for moderate (higher) hole doping. In light of persisting challenges to tailor triplet pairing in an electronic material, our finding promises to pave unprecedented ways for engineering unconventional triplet superconductivity.

Atomic Intercalation Induced Spin-Flip Transition in Bilayer CrI3.

The recent discovery of 2D magnets has induced various intriguing phenomena due to the modulated spin polarization by other degrees of freedoms such as phonons, interlayer stacking, and doping. The mechanism of the modulated spin-polarization, however, is not clear. In this work, we demonstrate theoretically and computationally that interlayer magnetic coupling of the CrI3 bilayer can be well controlled by intercalation and carrier doping. Interlayer atomic intercalation and carrier doping have been proven to induce an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in the spin-polarization of the CrI3 bilayer. Our results revealed that the AFM to FM transition induced by atom intercalation was a result of enhanced superexchange interaction between Cr atoms of neighboring layers. FM coupling induced by O intercalation mainly originates from the improved superexchange interaction mediated by Cr 3d-O 2p coupling. FM coupling induced by Li intercalation was found to be much stronger than that by O intercalation, which was attributed to the much stronger superexchange by electron doping than by hole doping. This comprehensive spin exchange mechanism was further confirmed by our results of the carrier doping effect on the interlayer magnetic coupling. Our work provides a deep understanding of the underlying spin exchange mechanism in 2D magnetic materials.

Electronic Properties of Fully Strained La1-x Sr x MnO3 Thin Films Grown by Molecular Beam Epitaxy (0.15 ≤ x ≤ 0.45).

The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La1–xSrxMnO3 (0.15 ≤ x ≤ 0.45) thin films deposited using the molecular beam epitaxy technique on 4° vicinal STO (001) substrates are probed by the combination of X-ray diffraction and various synchrotron-based spectroscopy techniques. The structural characterizations evidence a significant shift in the LSMO (002) peak to the higher diffraction angles owing to the increase in Sr doping concentrations in thin films. The nature of the LSMO Mn mixed-valence state was estimated from X-ray photoemission spectroscopy together with the relative changes in the Mn L2,3 edges observed in X-ray absorption spectroscopy (XAS), both strongly affected by doping. CTM4XAS simulations at the XAS Mn L2,3 edges reveal the combination of epitaxial strain, and different MnO6 crystal field splitting give rise to a peak at ∼641 eV. The observed changes in the occupancy of the eg and the t2g orbitals as well as their binding energy positions toward the Fermi level with hole doping are discussed. The room-temperature magnetic properties were probed at the end by circular dichroism.

Superconducting proximity effect in (7×7)R19.1∘ Ni nanoislands on Pb(111)

We have studied the proximity-induced superconductivity in $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ Ni nanoislands by combing scanning tunneling microscopy/spectroscopy (STM/STS) with density-functional theory calculation. Through depositing Ni onto Pb(111) substrate at 80 K, the monolayer Ni nanoislands with the $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ surface structure have been fabricated, where the termination of Ni atoms at the hexagonal close-packed site is energetically preferred and the filling of $3d$ orbitals from the charge transfer leads to the vanishing magnetic moment of Ni atoms. The topographic $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ lattice as well as the asymmetric height contrast in atomic unit cell have been further corroborated by the STM simulations. With high spatial and energy resolution, tunneling conductance ($dI/dU$) spectra have resolved an isotropic superconducting gap with ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}\ensuremath{\approx}1.29$ meV, which is slightly larger than ${\mathrm{\ensuremath{\Delta}}}_{Pb}\ensuremath{\approx}1.25$ meV. The temperature dependence of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$ supports the substrate-induced superconducting proximity effect according to the same transition temperature ${T}_{c}\ensuremath{\approx}7.14$ K with the Pb(111). The line spectroscopy has spatially mapped out the small increase of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$, which could be explained by an enhanced electron-phonon interaction (${V}_{ep}$) under the framework of Bardeen-Cooper-Schrieffer theory as a manifestation of the hole doping of Pb(111) from the surface Ni atoms.

Size Effects in Single- and Few-Layer MoS2 Nanoflakes: Impact on Raman Phonons and Photoluminescence.

The high optical absorption and emission of bidimensional MoS2 are fundamental properties for optoelectronic and biodetection applications and the opportunity to retain these properties in high quality nano-sized flakes would bring further possibilities. Here, a large set of single-layer and few-layer (2–3 layers) MoS2 flakes with size in the range from 10 nm to 20 μm are obtained on sapphire by vapor deposition techniques and evaluated combining the information from the Raman phonons with photoluminescence (PL) and absorption bands. The flakes have triangular shape and are found to be progressively relaxed from the tensile strain imposed by the sapphire substrate as their size is reduced. An increasing hole doping as size decreases is deduced from the blue shift of the A1g phonon, related to charge transfer from adsorbed oxygen. No clear correlation is observed between defects density and size, therefore, doping would be favored by the preferential adsorption of oxygen at the edges of the flakes, being progressively more important as the edge/surface ratio is incremented. This hole doping also produces a shift of the PL band to higher energies, up to 60 meV. The PL intensity is not found to be correlated to the size but to the presence of defects. The trends with size for single-layer and for 2–3 layer samples are found to be similar and the synthesis method does not influence PL efficiency which remains high down to 40 nm being thus promising for nanoscale photonics.

First-principles study of two-dimensional MoN2X2Y2 (X=B[formula omitted]In, Y=N[formula omitted]Te) nanosheets: The III–VI analogues of MoSi2N4 with peculiar electronic and magnetic properties

The recent discovery of MoSi2N4 nanosheet boosts researches on the layered MA2Z4 materials. Utilizing first-principles calculations, we investigate the isoelectronic and isostructural analogues of MoSi2N4, i.e. group III–VI MoN2X2Y2 (X=B∼In, Y=N∼Te) nanosheets. Nine III–VI XY combinations are revealed to form stable MoN2X2Y2 systems with robust dynamical, thermal and energetic stability. They can be indirect-gap semiconductors or metals depending on the XY compositions. There is a linear relationship between the gap size and lattice constant of semiconducting MoN2X2Y2 systems. More interestingly, a Mexican-hat-like band dispersion appears in the top valence band of MoN2X2Y2 (XY=AlO, GaO, InO) nanosheets, for which the hole doping can induce Stoner ferromagnetism and convert the systems into half-metals. For the van der Waals heterostructures formed by MoN2X2Y2 (XY=BS, AlO) and MoSi2N4 nanosheets, they exhibit versatile and strain-tunable band alignments, including the straddling-gap type-I, staggered-gap type-II, and broken-gap type-III ones. Our study demonstrates that the composition of surface layers is a new degree to manipulate the electronic structure of MA2Z4-based materials and it will bring tunable electronic and magnetic behaviours for the electrics, spintronics, and nano-device applications.

Air Annealing Process for Threshold Voltage Tuning of MoTe2 FET

A stable doping technique for modifying the conduction behaviour of two-dimensional (2D) nanomaterial-based transistors is imperative for applications based on low-power complementary oxide thin-film transistors. Achieving an ambipolar feature with a controlled threshold voltage in both the p- and n-regimes is crucial for applying MoTe2-based devices as electronic devices because their native doping states are unipolar. In this study, a simple method to tune the threshold voltage of MoTe2 field-effect transistors (FETs) was investigated in order to realise an enhancement-mode MoTe2 thin-film transistor by implementing a facile method to modulate the carrier polarity based on the oxidative properties of MoTe2 FETs. Annealing in air induced a continuous p-doping effect in the devices without significant electrical degradation. Through a precise control of the duration and temperature of the post-annealing process, the tailoring technique induces hole doping, which results in a remarkable shift in transfer characteristics, thus leading to a charge neutrality point of the devices at zero gate bias. This study demonstrates the considerable potential of air heating as a reliable and economical post-processing method for precisely modifying the threshold voltage and further controlling the doping states of MoTe2-based FETs for use in logic inverters with 2D semiconductors.

Effect of hole doping on the 120 degree order in the triangular lattice Hubbard model: a Hartree–Fock revisit

We revisit the unrestricted Hartree Fock study on the evolution of the ground state of the Hubbard model on the triangular lattice with hole doping. At half-filling, it is known that the ground state of the Hubbard model on triangular lattice develops a 120 degree coplanar order at half-filling in the strong interaction limit, i.e., in the spin 1/2 anti-ferromagnetic Heisenberg model on the triangular lattice. The ground state property in the doped case is still in controversy even though extensive studies were performed in the past. Within Hartree Fock theory, we find that the 120 degree order persists from zero doping to about 0.3 hole doping. At 1/3 hole doping, a three-sublattice collinear order emerges in which the doped hole is concentrated on one of the three sublattices with antiferromagnetic Neel order on the remaining two sublattices, which forms a honeycomb lattice. Between the 120 degree order and 1/3 doping region, a phase separation occurs in which the 120 degree order coexists with the collinear anti-ferromagnetic order in different regions of the system. The collinear phase extends from 1/3 doping to about 0.41 doping, beyond which the ground state is paramagnetic with uniform electron density. The phase diagram from Hartree Fock could provide guidance for the future study of the doped Hubbard model on triangular lattice with more sophisticated many-body approaches.

Synchronized effect of in-situ Ti doping and microwave-assisted SiOx hole transport channel on ZnFe2O4 nanocoral arrays for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting efficiency is limited by the high overpotential, severe recombination of photogenerated charges in bulk, and the surface of the photoanodes. Herein, we propose the SiOx-modified Ti:ZnFe2O4 (Ti-ZFO/SiOx) nanocorals array photoelectrode that integrates the in-situ Ti-doping and the second-order SiOx hole transport channel via successive hydrothermal and microwave methods. In addition, introduction of a cocatalyst of cobalt phosphate (Co-Pi) on Ti-ZFO/SiOx further accelerate hole transfer kinetics (surface charge seperation efficiency ∼ 90 %) in Ti-ZFO/SiOx/CoPi compared to Ti-ZFO/CoPi. The optimized Ti-ZFO/SiOx/CoPi nanocorals electrode achieves 1.6 times enhancement in photocurrent density (0.570 mA/cm2) at 1.23 VRHE than Ti-ZFO. Furthermore, Ti-ZFO/SiOx/CoPi photoelectrodes exhibited 70 and 34 µmol of H2 and O2, respectively, during 10 h real life PEC water splitting as well as and 50 h photocurrent stability. Therefore, our work is the foundational pilot for constructing the hole transport channel between the photoanode and electrolyte via the microwave method.

Hall Effect in a Doped Mott Insulator: DMFT Approximation

In the framework of dynamical mean-field theory, we analyze the Hall effect in a doped Mott insulator as a parent cuprate superconductor. We consider the partial filling (hole doping) of the lower Hubbard band and calculate the dependence of the Hall coefficient and Hall number on hole doping, determining the critical concentration for sign change of the Hall coefficient. Significant temperature dependence of the Hall effect is noted. Good agreement is demonstrated with the concentration dependence of the Hall number obtained in experiments in the normal state of YBCO.

Acoustic-phonon-mediated superconductivity in Bernal bilayer graphene

We present a systematic theory of acoustic-phonon-mediated superconductivity, which incorporates Coulomb repulsion, explaining the recent experiment in Bernal bilayer graphene under a large displacement field. The acoustic-phonon mechanism predicts that $s$-wave spin-singlet and $f$-wave spin-triplet pairings are degenerate and dominant. Assuming a spin-polarized valley-unpolarized normal state, we obtain $f$-wave spin-triplet superconductivity with a $T_c\sim 20$ mK near $n_e=-0.6\times 10^{12}$ cm$^{-2}$ for hole doping, in approximate agreement with the experiment. We further predict the existence of superconductivity for larger doping in both electron-doped and hole-doped regimes. Our results indicate that the observed spin-triplet superconductivity in Bernal bilayer graphene arises from acoustic phonons.

DFT+DMFT study of dopant effects in the heavy-fermion compound CeCoIn5

We study the dopant-induced inhomogeneity effect on the electronic properties of heavy fermionCeCoIn5using a combined approach of density functional theory (DFT) and dynamical mean-field theory (DMFT). The inhomogeneity of the hybridization between Ce-4fand conduction electrons is introduced to impose the inequivalent Ce atoms with respect to the dopant. From the DFT to the DFT+DMFT results, we demonstrate a variation of the hybridization strength depending on the hole or electron doping. A drastic asymmetric mass renormalization could be reproduced in the DFT+DMFT calculation. Finally, the calculated coherence temperature reflects the different development of the heavy quasiparticle states, depending on the dopant.

Proposal to improve Ni-based superconductors via enhanced charge transfer

Recently discovered superconductivity in hole-doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has attracted intensive attention in the field. An immediate question is how to improve its superconducting properties. Guided by the key characteristics of electronic structures of the cuprates and the nickelates, we propose that nickel chalcogenides with a similar lattice structure should be a promising family of materials. Using NdNiS$_2$ as an example, through first-principle structural optimization and phonon calculation, we find this particular crystal structure a stable one. We justify our proposal by comparing with CaCuO$_2$ and NdNiO$_2$ with regard to strength of the charge-transfer characteristics and the trend in their low-energy many-body effective Hamiltonians of doped hole carriers. This analysis indicates that nickel chalcogenides host low-energy physics closer to that of the cuprates, with stronger magnetic interaction than the nickelates, and thus they deserve further experimental exploration. Our proposal also opens up the possibility of a wide range of parameter tuning through ligand substitution among chalcogenides, to further improve superconducting properties.

Interpreting angle-dependent magnetoresistance in layered materials: Application to cuprates

The evolution of the low temperature electronic structure of the cuprate metals from the overdoped to the underdoped side has recently been addressed through Angle-Dependant Magneto-Resistance (ADMR) experiments in La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$. The results show a striking difference between hole dopings $p = 0.24$ and $p = 0.21$ which lie on either side of a putative quantum critical point at intermediate $p$. Motivated by this, we here study the theory of ADMR in correlated layered materials, paying special attention to the role of angle dependent quasiparticle weights $Z_{\mathbf{k}}$. Such a $Z_{\mathbf{k}}$ is expected to characterize a number of popular models of the cuprate materials, particularly when underdoped. Further, in the limit of weak interlayer hopping the quasiparticle weight will affect the $c$-axis transport measured in ADMR experiments. We show that proper inclusion of the quasiparticle weight does not support an interpretation of the data in terms of a $(\pi, \pi)$ spin density wave ordered state, in agreement with the lack of direct evidence for such order. We show that a simple model of Fermi surface reconfiguring across a van Hove point captures many of the striking differences seen between $p = 0.21$ and $p = 0.24$. We comment on why such a model may be appropriate for interpreting the ADMR data, despite having a large Fermi surface at $p = 0.21$, seemingly in contradiction with other evidence for a small Fermi surafce at that doping level.

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

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.

Two-orbital model for possible superconductivity pairing mechanism in nickelates

The newly synthesized strontium doped $R{\mathrm{NiO}}_{2}$ ($R$=Nd, Pr, and La) superconductors have stimulated extensive interests in understanding their pairing mechanism and pairing nature. Here we study the pairing mechanism in this family from a two-orbital model comprising the Ni- $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and $3{d}_{xy}$ orbitals, equipped with extended Hubbard interactions and induced low-energy effective superexchange interactions. We then study the pairing symmetry in this system by using large scale variational Monte Carlo approach. Our results yield the intraorbital ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave singlet pairing as the leading pairing symmetry in the nickelates, which is analogous to the cuprates. However, there exist two important differences between the physical properties of the two families due to the fact that at the low Sr-doping regime, while the Ni-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals remain half-filled and singlyins occupied to form a Mott-insulating background, the Ni-$3{d}_{xy}$ orbitals accommodate nearly all the extra doped holes, which move freely on this background. The first difference lies in the single-particle aspect: while the $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ degree of freedom remains Mott insulating with spectra weight pinned down at zero at low dopings, the $3{d}_{xy}$ one behaves as Fermi liquid with spectra weight near 1. The second difference lies in the pairing aspect: while the huge intra-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-orbital pairing gap is actually a pseudogap which has nothing to do with the SC, the small intra-$3{d}_{xy}$-orbital pairing gap serves as the true superconducting pairing gap, which is related to the ${T}_{c}$ via the BCS relation. Both differences can be verified by the angle-resolved photoemission spectrum.