Hole-doped Regime Research Articles

1D ferromagnetic edge contacts to 2D graphene/h-BN heterostructures

We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a change in sign of the signal for the electron- and hole-doped regimes, which is in contrast to the expected behavior of the spin signal. Calculations show that the contact-induced fringe fields are typically on the order of hundreds of mT, but can be reduced below 100 mT with careful optimization of the contact geometry. There may be an additional contribution from magnetoresistance effects due to tunneling anisotropy in the contacts, which needs further investigation. These studies are useful for optimization of spin injection and detection in 2D material heterostructures through 1D edge contacts.

Temperature- and doping-dependent roles of valleys in the thermoelectric performance of SnSe: A first-principles study

We theoretically investigate how each orbital and valley play a role for high thermoelectric performance of SnSe. In the hole-doped regime, two kinds of valence band valleys contribute to its transport properties: one is the valley near the U-Z line, mainly consisting of the Se-$p_z$ orbitals, and the other is the one along the $\Gamma$-Y line, mainly consisting of the Se-$p_y$ orbitals. Whereas the former valley plays a major role in determining the transport properties at room temperature, the latter one also offers comparable contribution and so the band structure exhibits multi-valley character by increasing the temperature. In the electron-doped regime, the conduction band valley around the $\Gamma$ point solely contributes to the thermoelectric performance, where the quasi-one-dimensional electronic structure along the $a$-axis is crucial. This study provides an important knowledge for the thermoelectric properties of SnSe, and will be useful for future search of high-performance thermoelectric materials.

Competing superconducting channels in iron pnictides from the strong coupling theory with biquadratic spin interactions

We study the symmetry and strength of the superconducting pairing in a two-orbital model for iron pnictides using the slave boson strong coupling approach. We show that the nearest-neighbor biquadratic interaction strongly affects the superconducting pairing phase diagram by promoting the B1g and the A1g channels. The resulting phase diagram consists of several competing pairing channels, including the isotropic A1g channel, an anisotropic B1g channel, and two pairing channels. We have investigated the evolution of superconducting states with electron doping, and find that the biquadratic interaction plays a crucial role in stabilizing the and even pure d-wave pairing in the heavily electron- and hole-doped regimes. In addition, we identify a novel orbital-B1g pairing channel, which has a s-wave form factor but a B1g symmetry. This channel has a comparable pairing amplitude to the d-wave pairing, and may strongly influence the superconducting gap anisotropy of the system in the overdoped regime. These findings are crucial in understanding the doping evolution of the superconducting gap anisotropy observed by angle resolved photoemission spectroscopy in the iron pnictides and iron chalcogenides, including the heavily K-doped BaFe2As2 and K-doped FeSe films.

Incommensurate spin correlations in highly oxidized cobaltates La2-xSrxCoO4.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.

Multicomponent Quantum Hall Ferromagnetism and Landau Level Crossing in Rhombohedral Trilayer Graphene

Using transport measurements, we investigate multicomponent quantum Hall (QH) ferromagnetism in dual-gated rhombohedral trilayer graphene (r-TLG) in which the real spin, orbital pseudospin, and layer pseudospins of the lowest Landau level form spontaneous ordering. We observe intermediate QH plateaus, indicating a complete lifting of the degeneracy of the zeroth Landau level (LL) in the hole-doped regime. In charge neutral r-TLG, the orbital degeneracy is broken first, and the layer degeneracy is broken last and only in the presence of an interlayer potential U⊥. In the phase space of U⊥ and filling factor ν, we observe an intriguing "hexagon" pattern, which is accounted for by a model based on crossings between symmetry-broken LLs.

Charge and spin quantum fluctuations in the doped strongly coupled Hubbard model on the honeycomb lattice

Field-theoretic methods are used to investigate the large-$U$ Hubbard model on the honeycomb lattice at half-filling and in the hole-doped regime. Within the framework of a functional-integral approach, we obtain the Lagrangian density associated with the charge and spin degrees of freedom. The Hamiltonian related to the charge degrees of freedom is exactly diagonalized. In the strong-coupling regime, we derive a perturbative low-energy theory suitable to describe the quantum antiferromagnetic phase (AF) as a function of hole doping. At half-filling, we deal with the underlying spin degrees of freedom of the quantum AF Heisenberg model by employing a second-order spin-wave analysis, in which case we have calculated the ground-state energy and the staggered magnetization; the results are in very good agreement with previous studies. Further, in the continuum, we derive a nonlinear $\ensuremath{\sigma}$ model with a topological Hopf term that describes the AF-VBS (valence bond solid) competition. Lastly, in the challenging doped regime, our approach allows the derivation of a $t\text{\ensuremath{-}}J$ Hamiltonian, and the analysis of the role played by charge and spin quantum fluctuations on the ground-state energy and, particularly, on the breakdown of the AF order at a critical hole doping; the results are benchmarked against recent Grassmann tensor product state simulations.

Orbital ordering transition in the single-layer manganites near half doping: a weak-coupling approach

The roles of crystal-field splitting and Jahn-Teller distortions on the orbital ordering transition are investigated in the single-layer manganites near half doping. Crystal-field splitting of energy levels favoring the $d_{3z^2-r^2}$ occupancy provides not only the correct Fermi surface topology for La$_{0.5}$Sr$_{1.5}$MnO$_4$ having a circular electron pocket around the $\Gamma$ point, but also enhances the flatness of the hole pocket around the M point thereby improving the nesting. In the presence of the circular electron pocket, Jahn-Teller distortions are found to be crucial for the transition to the transverse orbital ordering with ordering wavevector ($0.5\pi, 0.5\pi$). In the hole-doping regime $0.5 \leq x \leq 0.7$, the orbital ordering wavevector shows a linear dependence on the hole concentration in accordance with the experiments.

Electron and hole mobilities in single-layer WSe2.

Single-layer transition metal dichalcogenide WSe2 has recently attracted a lot of attention because it is a 2D semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe2 and metal contacts, which impede the injection of charges at low temperatures. Here, we report on the study of single-layer WSe2 transistors with a polymer electrolyte gate (PEO:LiClO4). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Moreover, our ohmic contacts formed at low temperatures allow us to study the temperature dependence of electron and hole mobilities. At high electron densities, a re-entrant insulating regime is also observed, a feature which is absent at high hole densities.

Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors

We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photocarriers are either electrons or holes. Both in the electron- and hole-doping regimes the photocurrent decays exponentially as a function of the distance between the illumination spot and the nearest contact, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on a same sample. Interestingly, the diffusion length of the minority carriers is several times larger in the hole accumulation regime than in the electron accumulation regime, pointing out an electron-hole asymmetry in WS2.

Doping evolution of the quasiparticle excitations in heavily hole-doped Ba1−xKxFe2As2: A possible superconducting gap with sign-reversal between hole pockets

To gain insight into the unconventional superconductivity of Fe-pnictides with no electron pockets, we measure the thermal conductivity $\kappa$ and penetration depth $\lambda$ in the heavily hole-doped regime of Ba$_{1-x}$K$_x$Fe$_2$As$_2$. The residual thermal conductivity $(\kappa/T)_{T \rightarrow 0\,{\rm K}}$ and $T$-dependence of $\lambda$ consistently indicate the fully gapped superconductivity at $x=0.76$ and the (line) nodal superconductivity at higher hole concentrations. The magnitudes of $\frac{\kappa}{T}\cdot T_c|_{T \rightarrow 0\,{\rm K}}$ and $\frac{d\lambda}{d(T/T_c)}$ at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual non-monotonic x-dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range, suggesting a doping crossover of the gap function between the s-wave states with and without sign reversal between $\Gamma$-centered hole pockets.

Spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides

We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. Multiband effect in other doping regime is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime.

Broken Symmetry Quantum Hall States in Dual-Gated ABA Trilayer Graphene

ABA-stacked trilayer graphene is a unique 2D electron system with mirror reflection symmetry and unconventional quantum Hall effect. We present low-temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν = -8, -2, 2, 6, and 10, which is in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau level (LL). Under an out-of-plane electric field E(perpendicular), we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E(perpendicular) selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene.

Scanning micro-x-ray diffraction unveils the distribution of oxygen chain nanoscale puddles in YBa2Cu3O6.33

Oxygen chain fragments are known to appear at the insulator-to-superconductor transition in YBa${}_{2}$Cu${}_{3}$O${}_{6+y}$. However, the self-organization and the size distribution of oxygen chain fragments are not known. Here, we seek to fill this gap, using scanning micro-x-ray diffraction, which is an imaging method based on advances in focusing a synchrotron radiation beam. This approach allows us to probe both real-space and $k$-space of high-quality YBa${}_{2}$Cu${}_{3}$O${}_{6.33}$ single crystals with ${T}_{c}$ $=$ 7 K. We report compelling evidence for nanoscale striped puddles, with Ortho-II structure, made of chain fragments in the basal Cu(1) plane with local oxygen concentration $y$ \ensuremath{\ge} 0.5. The size of the Ortho-II puddles spans a range between 2 and 9 nm. The real-space imaging of Ortho-II puddles granular network shows that superconductivity, at a low hole-doping regime, occurs in a network of nanoscale oxygen ordered patches, interspersed with oxygen depleted regions. The manipulation by thermal treatments of the striped Ortho-II puddles has been investigated focusing on the spontaneous symmetry breaking near the order-to-disorder phase transition at ${T}_{0}$ $=$ 350 K.

Spin Hall effect in iron-based superconductors: A Dirac-point effect

We have theoretically explored the intrinsic spin Hall effect (SHE) in the iron-based superconductor family with a variety of materials. The study is motivated by an observation that, in addition to an appreciable spin-orbit coupling in the Fe 3d states, a character of the band structure in which Dirac cones appear below the Fermi energy may play a crucial role in producing a large SHE. Our investigation does indeed predict a substantially large spin Hall conductivity in the heavily hole-doped regime such as KFe$_2$As$_2$. The magnitude of the SHE has turned out to be comparable with that for Pt despite a relatively small spin-orbit coupling, which we identify to come from a huge contribution from the gap opening induced by the spin-orbit coupling at the Dirac point, which can become close to the Fermi energy for the heavy hole doping.

Path to poor coherence in the periodic Anderson model from Mott physics and hybridization

We investigate the anomalous metal arising by hole doping the Mott insulating state of the periodic Anderson model. Using Dynamical Mean-Field Theory we show that, as opposed to the electron-doped case, in the hole-doped regime the hybridization between localized and delocalized orbitals leads to the formation of composite quasi-particles reminiscent of the Zhang-Rice singlets. We compute the coherence temperature of this state, showing its exponentially small value at low dopings. As a consequence the weakly-doped Mott state deviates from the predictions of Fermi-liquid theory already at small temperatures. The onset of the Zhang-Rice state and of the consequent poor coherence is due to the electronic structure in which both localized and itinerant carriers have to be involved in the formation of the conduction states and to the proximity to the Mott state. By investigating the magnetic properties of this state, we discuss the relation between the anomalous metallic properties and the behavior of the magnetic degrees of freedom.

Impact of the Dzyaloshinskii-Moriya interaction in strongly correlated itinerant systems

Spin-only approaches to anisotropic effects in strongly interacting materials are often insufficient for systems close to the Mott regime. Within a model context, here the consequences of the low-symmetry relevant Dzyaloshinskii-Moriya (DM) interaction are studied for strongly correlated, but overall itinerant, systems. Namely, we investigate the Hubbard bilayer model supplemented by a DM term at half filling and in the hole-doped regime. As an add-on, further results for the two- impurity Anderson model with DM interaction are also provided. The model Hamiltonians are treated by means of the rotational invariant slave boson technique at saddle point within a (cellular) cluster approach. Already small values of the anisotropic interaction prove to have a strong influence on the phases and correlation functions with increasing U. An intriguing metallic spin-flop phase is found in the doped bilayer model and a reduction of the RKKY exchange in the two-impurity model.

Absence of static stripes in the two-dimensionalt−Jmodel determined using an accurate and systematic quantum Monte Carlo approach

We examine the two-dimensional $t\ensuremath{-}J$ model by using variational approach combined with well established quantum Monte Carlo techniques [S. Sorella et al., Phys. Rev. Lett. 88, 117002 (2002).] that are used to improve systematically the accuracy of the variational ansatz. Contrary to recent density-matrix renormalization group and projected entangled-pair state calculations [P. Corboz et al., Phys. Rev. B 84, 041108(R) (2011).], a uniform phase is found for $J/t=0.4$, even when the calculation is biased with an ansatz that explicitly contains stripe order. Moreover, in the small hole doping regime, that is, $\ensuremath{\delta}\ensuremath{\lesssim}0.1$, our results support the coexistence of antiferromagnetism and superconductivity.

Exoticd-Wave Superconducting State of Strongly Hole-DopedKxBa1−xFe2As2

We investigate the superconducting phase in the K(x)Ba(1-x)Fe2As2 122 compounds from moderate to strong hole-doping regimes. Using the functional renormalization group, we show that, while the system develops a nodeless anisotropic s(±) order parameter in the moderately doped regime, gapping out the electron pockets at strong hole doping drives the system into a nodal (cos k(x) + cos k(y))(cos k(x) - cos k(y)) d-wave superconducting state. This is in accordance with recent experimental evidence from measurements on KFe2As2 which observe a nodal order parameter in the extreme doping regime. The magnetic instability is strongly suppressed.

SPECTRAL FUNCTION OF A d-p HUBBARD MODEL

This work investigates a d-p Hubbard model by the n-pole approximation in the hole-doped regime. In particular, the spectral function A(ω, k) is analyzed varying the filling, the local Coulomb interaction and the d-p hybridization. It should be remarked that the original n-pole approximation (Phys. Rev.184, 451 1969) has been improved in order to include adequately the k-dependence of the important correlation function 〈Sj·Si〉 present in the poles of the Green's functions. It has been verified that the topology of the Fermi surface (defined by A(ω = 0, k)) is deeply affected by the doping, the strength of the Coulomb interaction and also by the hybridization. Particularly, in the underdoped regime, the spectral function A(ω = 0, k) presents very low intensity close to the antinodal points (0, ±π) and (±π, 0). Such a behavior produces an anomalous Fermi surface (pockets) with pseudogaps in the region of the antinodal points. On the other hand, if the d-p hybridization is enhanced sufficiently, such pseudogaps vanish. It is precisely the correlation function 〈Sj·Si〉, present in the poles of the Green's functions, plays an important role in the underdoped situation. In fact, antiferromagnetic correlations coming from 〈Sj·Si〉 strongly modify the quasiparticle band structure. This is the ultimate source of anomalies in the Fermi surface in the present approach.

Infrared Imaging of Heat Dissipation in Graphene Transistors

We use infrared microscopy to image the temperature profile of graphene field-effect transistors functioning at high bias. We find a peak in the temperature profile, i.e. a "hot spot" appears near the drain (anode) electrode of the graphene sheet at high current while operating in the hole-doped regime. The hot spot position shifts on the graphene sheet by tuning the gate voltage into an ambipolar transport regime. This shows a direct demonstration and manipula-tion of Joule heating in graphene transistors. Comparison with a comprehensive electrical-thermal model leads to additional insight into graphene transport and energy dissipation physics.