Hole-doped Region Research Articles

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure.

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Quantum-metric-induced nonlinear transport in a topological antiferromagnet.

The Berry curvature and quantum metric are the imaginary part and real part, respectively, of the quantum geometric tensor, which characterizes the topology of quantum states1. The Berry curvature is known to generate a number of important transport phenomena, such as the quantum Hall effect and the anomalous Hall effect2,3; however, the consequences of the quantum metric have rarely been probed by transport measurements. Here we report the observation of quantum-metric-induced nonlinear transport, including both a nonlinear anomalous Hall effect and a diode-like non-reciprocal longitudinal response, in thin films of a topological antiferromagnet, MnBi2Te4. Our observations reveal that the transverse and longitudinal nonlinear conductivities reverse signs when reversing the antiferromagnetic order, diminish above the Néel temperature and are insensitive to disorder scattering, thus verifying their origin in the band-structure topology. They also flip signs between electron- and hole-doped regions, in agreement with theoretical calculations. Our work provides a means to probe the quantum metric through nonlinear transport and to design magnetic nonlinear devices.

Andreev Reflection and Klein Tunneling in High-Temperature Superconductor-Graphene Junctions.

Scattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here, we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p-n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+δ}. The resonances persist with gating from p^{+}-p and p-n configurations. The suppression of the oscillation amplitude above the bias energy which is comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene-high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions.

Theoretical study of Cr2X3S3 (X = Br, I) monolayers for thermoelectric and spin caloritronics properties

High curie temperature 2D materials are important for the progress of the field of spin caloritronics. The spin Seebeck effect and conventional thermoelectric figure of merit (ZT) can give a great insight into how these 2D magnetic materials will perform in spin caloritronics applications. Here in this paper, we have systematically studied 2D Janus monolayers based on CrX3 monolayers. We obtain a ZT of 0.31 and 0.21 for the Cr2Br3S3 and Cr2I3S3 Janus monolayers. The spin Seebeck coefficient obtained at room temperature is also very high (∼1570 μVK−1 in the hole-doped region and ∼1590 μ VK−1 in the electron-doped region). The thermal conductivity of these monolayers (∼22 Wm−1 K−1 for Cr2Br3S3 and ∼16 Wm−1 K−1 for Cr2I3S3) are also very similar to other 2D semiconductor transition metals chalcogenides. These findings suggest a high potential for these monolayers in the spin caloritronics field.

Enhancement of superconductivity upon reduction of carrier density in proximitized graphene

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

Ground-state phase diagram of the t-t′-J model

We report results of large-scale ground-state density matrix renormalization group (DMRG) calculations on t-[Formula: see text]-J cylinders with circumferences 6 and 8. We determine a rough phase diagram that appears to approximate the two-dimensional (2D) system. While for many properties, positive and negative [Formula: see text] values ([Formula: see text]) appear to correspond to electron- and hole-doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The [Formula: see text] (hole-doped) region shows antiferromagnetism limited to very low doping, stripes more generally, and the familiar Fermi surface of the hole-doped cuprates. However, we find [Formula: see text] strongly suppresses superconductivity. The [Formula: see text] (electron-doped) region shows the expected circular Fermi pocket of holes around the [Formula: see text] point and a broad low-doped region of coexisting antiferromagnetism and d-wave pairing with a triplet p component at wavevector [Formula: see text] induced by the antiferromagnetism and d-wave pairing. The pairing for the electron low-doped system with [Formula: see text] is strong and unambiguous in the DMRG simulations. At larger doping another broad region with stripes in addition to weaker d-wave pairing and striped p-wave pairing appears. In a small doping region near [Formula: see text] for [Formula: see text], we find an unconventional type of stripe involving unpaired holes located predominantly on chains spaced three lattice spacings apart. The undoped two-leg ladder regions in between mimic the short-ranged spin correlations seen in two-leg Heisenberg ladders.

Chemical State Mapping of p/n-Controlled SrB6 Bulk Specimens by Soft X-ray Emission Electron Microscope

Elemental and chemical state maps of p/n-controlled SrB6 bulk specimens are presented by using a soft X-ray emission spectroscopy electron microscope. Those bulk specimens were obtained by sintering powder specimens, prepared by the molten-salt method with different compositions of initial materials. A Sr-map, a chemical shift map of B K-emission, and the spectra of characteristic regions of those materials were compared. It was observed that a local Sr deficiency caused a local hole-doped region, confirmed by a chemical shift in the B K-emission spectrum. n-type SrB6 was rather homogeneous. On the other hand, the p-type SrB6 bulk specimen was a mixture of two different p-type regions. This mixed nature originated, presumably, from an uneven Sr content of SrB6 particles prepared by the molten-salt method using a Sr-deficient starting material. A separation process of the two types of materials will realize a high-quality homogeneous p-type SrB6 bulk specimens.

Superfluid ground state phase diagram of the two-dimensional Hubbard model in the emergent Bardeen-Cooper-Schrieffer regime

In nonperturbative regimes, the superfluid instability in the two-dimensional Hubbard model can be described by an emergent BCS theory with small effective pairing constants. We compute the effective couplings using a controlled bold-line diagrammatic Monte Carlo approach, which stochastically sums all skeleton Feynman diagrams dressed in the one- and two-particle channels to high expansion orders, and map out the resulting superfluid ground-state phase diagram in a range of next-nearest-neighbor hopping $0 \leq t^{\prime} \leq 0.3t$, interaction strength $0 \leq U \leq 3t$, and lattice filling $0 \leq n \leq 2$. The phase diagram is dramatically transformed in the hole-doped region and becomes particularly rich at larger doping and $t'$. At $t'=0.3$, the weak-coupling picture with the dominant triplet pairing sharply peaked at $n \approx 0.73$ due to the Van Hove singularity is replaced by a plateau of the singlet $d_{x^2-y^2}$ paring, while for $U \gtrsim 3t$ the effective couplings are consistent with the $d_{x^2-y^2}$ high-temperature superconductivity in the hole-doped region near cuprates' optimal doping.

Enhancement of thermoelectric performance of CH3NH3PbI3 through strain driven topological phase transition and doping

The effects of compressive strain, doping and temperature on the thermoelectric properties of CH3NH3PbI3 have been investigated by employing first principles based calculations and semi-classical Boltzmann theory. CH3NH3PbI3 displays a topological phase transition under 9.9% external compressive strain. Thermoelectric studies reveal that application of strain enhances the Seebeck coefficient for electron and hole-doped regions. This increment is attributed to the strain induced band inversion and modifications in the electronic density of states at the topological transition point. The strain driven topological phase transition produces a ZT value of 0.89 for hole-doped CH3NH3PbI3 at 800 K.

Doping-dependent character and possible magnetic ordering of NdNiO2

The novel nickelate superconductors of infinite-layer type feature challenging electronic pecularities in the normal-state phase diagram with doping. Distinct many-body behavior and different dispersion regimes of the entangled $\{$Ni-$d_{z^2}$, Ni-$d_{x^2-y^2}$$\}$ orbital sector give rise to highly rich physics, which is here studied for the case of the NdNiO$_2$ system. An analysis based on advanced realistic dynamical mean-field theory unveils that the superconducting hole-doped region is the meeting place of a (self-)doped Mott insulator from the underdoped side, and a bad Hund metal from the overdoped side. Fermi-level crossing of the Ni-$d_{z^2}$ flat-band ties both regimes together to form a singular arena for unconventional superconductivity. We furthermore shed light on the intriguing problem of elusive magnetism in infinite-layer nickelates. Antiferromagnetic (AFM) order with small Ni moments is shown to be a vital competitor at low temperature. At stoichiometry, C-AFM order with ferromagnetic spin-alignment along the $c$-axis benefits from a conceivable coexistence with Kondo(-lattice) screening.

Electronic Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi2Sr2CaCu2O8 + δ**Supported by the National Natural Science Foundation of China (Grant Nos. 11888101, 11922414, and 11534007), the National Key Research and Development Program of China (Grant Nos. 2016YFA0300300 and 2017YFA0302900), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB25000000), the

High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers. A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced. Here, by in situ vacuum annealing and Rb deposition on the Bi2Sr2Ca0.6Dy0.4Cu2O8 + δ (Bi2212) sample surface to push its doping level continuously from deeply underdoped (Tc = 25 K, doping level p ∼ 0.066) to the near-zero doping parent Mott insulator, angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time. Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping, which is quite close to the upper Hubbard band. With increasing hole doping, the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior. When the chemical potential approaches the charge transfer band at a doping level of ∼0.05, the nodal spectral weight near the Fermi level increases, followed by the emergence of the coherent quasiparticle peak and the insulator–superconductor transition. Our observations provide key insights in understanding the insulator–superconductor transition in doping the parent cuprate compound and for establishing related theories.

Doping-induced disorder and conductivity anisotropy in the spin density wave state of iron pnictides

We examine the role of doped impurity atoms on the conductivity anisotropy of the spin-density wave state in iron pnictides. The conductivity is calculated in a self-consistent spin-density wave state with random impurities in a two-orbital model. We find that the increase in impurity concentration leads to an increased suppression of conductivity in both the antiferromagnetic and ferromagnetic directions. However, the conductivity anisotropy is larger in comparison to the Drude-weight anisotropy in the hole-doped regions. The difference between the two is explained in terms of the anisotropic scattering by the impurities. We demonstrate the effect of the anisotropic impurity scattering by calculating the modulation in the density of states introduced by a single impurity. It is also shown that an increase in the Drude-weight anisotropy with changing carrier concentration, which results mainly from the reconstructed band characteristics, may be directly linked to a similar anisotropy in the density-of-states modulation caused by the impurity atom.

Possible Superconductivity Induced by a Large Spin–Orbit Coupling in Carrier Doped Iridium Oxide Insulators: A Weak Coupling Approach

We study possible superconductivity in a carrier-doped iridium oxide insulator Sr$_{2}$IrO$_{4}$ based on an effective $t_{2g}$ three-orbital Hubbard model on the square lattice with a large spin-orbit coupling (SOC). Numerically solving the linearized Eliashberg equation for the superconducting (SC) gap function with the random phase approximation, we systematically examine both singlet and triplet SC gap functions with possible pairing symmetries and the parameter dependence of the superconductivity. For the realistic SOC $\lambda$ and Hund's coupling $J/U$ relevant to Sr$_{2}$IrO$_{4}$, namely, for a large $\lambda$ and small $J/U$ region, we find that the intra-band antiferromagnetic (AF) pseudospin $\bm{j}_{\text{eff}} = -\bm{l}+\bm{s}$ fluctuations favor a $d_{x^{2}-y^{2}}$-wave pseudospin $j_{\text{eff}}=1/2$ singlet pairing in the electron-doping. We also find that the $d_{x^{2}-y^{2}}$-wave pairing is more stabilized with increasing the SOC and decreasing the Hund's coupling. Furthermore, we show for a small $\lambda$ and large $J/U$ region that an $s_{\pm}$-wave singlet pairing is favored in the hole-doped region. The origin of the $s_{\pm}$-wave pairing is due to the inter-band pair scattering arising from the intra-orbital AF spin $\bm s$ fluctuations. Although the possibility of a pseudospin triplet pairing is considered, we find it always unfavorable for all parameters studied here. The experimental consequences for other strongly correlated materials with a large SOC are also discussed.

Magnetic moment evolution and spin freezing in doped BaFe2As2

Fe-Kβ X-ray emission spectroscopy measurements reveal an asymmetric doping dependence of the magnetic moments μbare in electron- and hole-doped BaFe2As2. At low temperature, μbare is nearly constant in hole-doped samples, whereas it decreases upon electron doping. Increasing temperature substantially enhances μbare in the hole-doped region, which is naturally explained by the theoretically predicted crossover into a spin-frozen state. Our measurements demonstrate the importance of Hund’s-coupling and electronic correlations, especially for hole-doped BaFe2As2, and the inadequacy of a fully localized or fully itinerant description of the 122 family of Fe pnictides.

Thermoelectric Detection of Multi-Subband Density of States in Semiconducting and Metallic Single-Walled Carbon Nanotubes.

Thermoelectric detection of a multi-subband density of states in semiconducting and metallic single-walled carbon nanotubes is demonstrated by scanning the Fermi energy from electron-doped to hole-doped regions. The Fermi energy is systematically controlled by utilizing the strong electric field induced in electric-double-layer transistor configurations, resulting in the optimization of the thermoelectric power factor.

Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor.

Superconducting circuits are exceptionally flexible, enabling many different devices from sensors to quantum computers. Separately, epitaxial semiconductor devices such as spin qubits in silicon offer more limited device variation but extraordinary quantum properties for a solid-state system. It might be possible to merge the two approaches, making single-crystal superconducting devices out of a semiconductor by utilizing the latest atomistic fabrication techniques. Here we propose superconducting devices made from precision hole-doped regions within a silicon (or germanium) single crystal. We analyse the properties of this superconducting semiconductor and show that practical superconducting wires, Josephson tunnel junctions or weak links, superconducting quantum interference devices (SQUIDs) and qubits are feasible. This work motivates the pursuit of 'bottom-up' superconductivity for improved or fundamentally different technology and physics.

Exotic spin, charge and pairing correlations of the two-dimensional doped Hubbard model: A symmetry-entangled mean-field approach

Intertwining of spin, charge and pairing correlations in the repulsive two-dimensional Hubbard model is shown through unrestricted variational calculations, with projected wavefunctions free of symmetry breaking. A crossover from incommensurate antiferromagnetism to stripe order naturally emerges in the hole-doped region when increasing the on-site coupling. Although effective pairing interactions are identified, they are strongly fragmented in several modes including d-wave pairing and more exotic channels related to an underlying stripe. We demonstrate that the entanglement of a mean-field wavefunction by symmetry restoration can largely account for interaction effects.

High-magnetic-field study of magnetic and transport properties of hole-doped NdBaCo2O5+δ

Magnetic and transport properties of the NdBaCo${}_{2}$O${}_{5+\ensuremath{\delta}}$ system are studied in the less investigated hole-doped region (0.52 \ensuremath{\delta} 0.72) and in pulsed magnetic fields up to 47 T. The conductance in zero-magnetic field and $T$ 130 K is described by a two-gap expression completed by the term of variable range hopping (VRH). The thermoelectric power $S$($T$ ) measurements confirm the magnetic phase diagram found by magnetic and electric transport investigations. It has been shown that ferromagnetic behavior of NdBaCo${}_{2}$O${}_{5+\ensuremath{\delta}}$ (0.52 \ensuremath{\delta} 0.72) is stabilized and spans in lower temperatures by increasing the external magnetic field in agreement with structural and magnetic phase diagrams found on the NdBaCo${}_{2}$O${}_{5.75}$ and GdBaCo${}_{2}$O${}_{5+\ensuremath{\delta}}$ systems. Variable range hopping mode gives good approximation in zero- and high-magnetic field 47 T. The isothermal magnetoconductance ($G$ ) of hole-doped polycrystalline NdBaCo${}_{2}$O${}_{5.72}$ is well fitted by $G$($H$, $T$ )/$G$(0) $=$ ${G}_{0}$exp($H$/${H}_{0}$) dependence at 55 K $T$ 138 K (the region where FM state appears) instead of simple linear dependence found for manganites La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$. The parameter ${H}_{0}$ is considered as the magnetic field of full spin sublattices' reorientation and $MR$ saturation. The magnetoconductance is possibly realized through the $a$-oriented grains due to the strong decrease of resistivity \ensuremath{\Delta}${\ensuremath{\rho}}_{c}$/${\ensuremath{\rho}}_{c}$ in magnetic field along the $a$ axis. The deduced reason for observed behavior is the specific origin of the antiferromagnetic (AFM)-ferromagnetic (FM) transition in layered cobaltites.

Doping dependence of spin fluctuations and electron correlations in iron pnictides

Doping dependence of the spin fluctuations and the electron correlations in the effective five-band Hubbard model for iron pnictides is investigated using the fluctuation-exchange approximation. For a moderate hole doping, we find a dominant low-energy spin excitation at Q=(\pi,0), which becomes critical at low temperature. The low-energy spin excitations in the heavily hole-doped region are characterized by weak Q dependence. The electron doping leads to an appearance of a pseudogap in spin excitation spectrum. Correspondingly, the NMR-1/T1 relaxation rate is strongly enhanced on the hole-doped side and suppressed on the electron-doped side of the phase diagram. This behavior can be to large extent understood by systematic changes of the Fermi-surface topology.

Pseudogap and superconductivity in iron-pnictides

We investigate interplay between magnetic fluctuations and superconductivity in the effective five-band Hubbard model for iron-pnictide superconductors on the basis of the fluctuation-exchange approximation. As for the normal-state properties, we find the pseudogap behavior in the NMR relaxation rate 1/T1T in the electron-doped region, while we find strong enhancement of 1/T1T in the hole-doped region. Such pseudogap behavior originates from the band structure effect, that is, existence of high density of states just below the Fermi level and its broadening by the correlation effect. Solving the superconducting Eliashberg equation, we find that the most probable candidate for the pairing symmetry is the sign-changing s-wave spin-singlet state. For small exchange coupling J, the eigenvalue is not so sensitive to carrier doping, and seems to be irrelevant with antiferromagnetic (AF) spin fluctuation. We suggest that the orbital degrees of freedom is important as the pairing mechanism as well as the AF spin fluctuation.