Dilute Hole Research Articles

Light-induced ferromagnetism in moiré superlattices.

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena1-3. The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle1-3, electrical field4-6, moiré carrier filling7-11 and interlayer coupling12. Here we report that optical excitation can highly tune the spin-spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS2 /WSe2 moiré superlattices. Near the filling factor of -1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes9, and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter13-19.

Efficient enhancement of thermoelectric performance of CdTe via dilute hole doping together with heavy isoelectronic doping

Thermoelectric performances of p type and isoelectronically doped CdTe are investigated using first principles method and Boltzmman transport theory. For the supercell Cd32Te32, when one N atom is doped into it, the maximal S and ZeT are only 156 μV/K and 0.76, respectively, but the ZeT values are much smaller at 300–700 K for P, As and Sb dopings. However, if eight Sr atoms are further doped into Cd32Te31M (M= pnicogen) to substitute Cd atoms, all the four maximal S values exceed 242 μV/K, and the ZeT reaches 1.5 at 1000 K for N doping, but for P, As and Sb dopings the values do not exceed 1.0. The large S arises from the steep but low DOS at EF. The enhancement in ZeT results from the S dominating over σ/κe. The number and distribution of Sr atoms have an impact on the ZeT values. Adding one or two O atoms further can hoist the ZeT at 700 K and 800 K significantly. When one Cl atom and one Na atom are doped into Cd24Sr8Te31N, the ZeT reaches 1.7 at 1000 K. The results show that hole doping combined with heavily isoelectronic doping is efficient on improving the electronic figures of merit of semiconductors if appropriate doped elements are chosen. The way of doping assumed here should be applicable for other semiconductor systems.

Reentrant insulating phases in the integer quantum Hall regime

Quantum Hall measurements are performed in a dilute high-quality GaAs two-dimensional hole system with carrier densities of (4.4--$5.3)\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$. With the ${r}_{s}$ value approximately 20, the interaction effect is already strong even in the absence of magnetic field. In the integer quantum Hall regime up to 2 T, a series of reentrant insulating phases (RIPs) are observed between fillings 1, 2, 3, and 4, with peak locations periodic in $1/B$. Moreover, the RIP peaks are also characterized by a striking inductance in response to ac excitations, which is likely due to the onset of a Wigner crystal state. The multiple RIP peaks indicate a more complex phase diagram than previously perceived.

Spin polarization in two dimensional hole gas GaAs/AlGaAs

In this paper, we have studied the magnetoresistance in a dilute two-dimensional hole gas in GaAs/AlGaAs within a parallel magnetic field. To do this, we tried to define the normalized resistivity, ρ(B)/ρ(0), as a function as B/Bχ and B/Bcross, where Bχ is a scaling parameter, and Bcross is determined by the bend of the resistivity curves versus B2. This study shows that there is no dependence between Bχ and Bcross for various hole densities. In order to examine the variation of the susceptibility, χ, we have access the product m*g* versus the hole density. The later is found to be almost constant. In this work we have reanalyzed the data obtained by Kumar et al., which were published in Ref. [4].

Monopoles, confinement and charge localization in the t–J model with dilute holes

We present a quantum field theoretic description on the t–J model on a square lattice with dilute holes (i.e. near half-filling), based on the compact mutual Chern–Simons gauge theory. We show that, due to the presence of non-perturbative monopole plasma configuration from the antiferromagnetic background, holons (carrying electric charge) are linearly confined and strongly localized even without extrinsic disorder taken into account. Accordingly, the translation symmetry is spontaneously broken at ground state. Such an exotic localization is distinct from Anderson localization and essentially rooted in intrinsic Mott physics of the t–J model. Finally, a finite-temperature phase diagram is proposed. The metal–insulator transition observed in in-plane resistivity measurement is identified to a confinement–deconfinement transition from the perspective of gauge theory. The transition is characterized by the order parameter “Polyakov-line”.

Effective mass and spin susceptibility of dilute two-dimensional holes in GaAs

We report effective hole mass (${m}^{*}$) measurements through analyzing the temperature dependence of Shubnikov--de Haas oscillations in dilute (density $p\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{10}$ cm${}^{\ensuremath{-}2}$, ${r}_{s}\ensuremath{\sim}6$) two-dimensional (2D) hole systems confined to a 20-nm-wide, (311)A GaAs quantum well. The holes in this system occupy two nearly degenerate spin subbands whose ${m}^{*}$ we measure to be $\ensuremath{\sim}$0.2 (in units of the free electron mass). Despite the relatively large ${r}_{s}$ in our 2D system, the measured ${m}^{*}$ is in reasonably good agreement with the results of our energy band calculations, which do not take interactions into account. We then apply a sufficiently strong parallel magnetic field to fully depopulate one of the spin subbands, and measure ${m}^{*}$ for the populated subband. We find that this latter ${m}^{*}$ is close to the ${m}^{*}$ we measure in the absence of the parallel field. We also deduce the spin susceptibility of the 2D hole system from the depopulation field, and we conclude that the susceptibility is enhanced by about 50$%$ relative to the value expected from the band calculations.

Lattice-driven magnetoresistivity and metal-insulator transition in single-layered iridates

Sr${}_{2}$IrO${}_{4}$ exhibits an insulating state driven by spin-orbit interactions. We report two phenomena, namely, a large magnetoresistivity in Sr${}_{2}$IrO${}_{4}$ that is extremely sensitive to the orientation of magnetic field but exhibits no apparent correlation with the magnetization, and a robust metallic state that is induced by dilute electron (La${}^{3+}$) or hole (K${}^{+}$) doping for Sr${}^{2+}$ ions in Sr${}_{2}$IrO${}_{4}$. Our structural, transport, and magnetic data reveal that a strong spin-orbit interaction alters the balance between the competing energies so greatly that (1) the spin degree of freedom alone is no longer a dominant force, (2) the underlying transport properties delicately hinge on the Ir-O-Ir bond angle via a strong magnetoelastic coupling, and (3) a highly insulating state in Sr${}_{2}$IrO${}_{4}$ is proximate to a metallic state, and the transition is governed by lattice distortions that can be controlled via either the magnetic field or chemical doping.

모형가스터빈 연소기에서 분무각 변화에 따른 연료농도 및 화염온도 분포

KIVA3V를 이용하여 모형 가스터빈 연소기에서 Jet-A의 분무와 연소에 관한 수치해석을 하였다. 연소기 형상은 6개의 희석홀을 가지며, 스월효과를 고려하였다. 연소 특성을 해석하기 위해 열적 NO 발생을 고려하였다. 다양한 분무각에 대해서 Jet-A 증기, 화염온도 분포와 NO 발생량에 대하여 비교하였다. 분무각이 커질수록 1차 영역에서 연료 증기가 비교적 고르게 나타나게 되며 증발이 더욱 빠르게 되고, 주변 기체와 연료 증기 사이의 혼합이 증가한다. 그 결과 고온부가 넓게 나타나게 되고, 이에 의해 열적 NO 발생이 증가한다. Jet-A spray and combustion were numerically analyzed in annular type combustor model using KIVA3V. The combustor geometry have 6 dilute holes. Swirl effect and thermal NO were considered in this investigation to analyze mixing and combustion characteristics. Fuel vapor, flame temperature, NO generation were investigated for various spray angle. As increase of spray angle, Jet-A vapor appeared uniformly in primary zone and evaporation rate was increased. Mixing between fuel vapor and ambient gas was enhanced as increase of spray angle. As a result, high temperature region appeared widely and thermal NO generation rate was increased.

Tunneling between dilute GaAs hole layers

We report interlayer tunneling measurements between very dilute two-dimensional GaAs hole layers. Surprisingly, the shape and temperature dependence of the tunneling spectrum can be explained with a Fermi liquid-based tunneling model, but the peak amplitude is much larger than expected from the available hole band parameters. Data as a function of parallel magnetic field reveal additional anomalous features, including a recurrence of a zero-bias tunneling peak at very large fields. In a perpendicular magnetic field, we observe a robust and narrow tunneling peak at total filling factor ${\ensuremath{\nu}}_{T}=1$, signaling the formation of a bilayer quantum Hall ferromagnet.

Anisotropic In-Plane Magnetoresistance and Spin Susceptibility of Dilute Two-Dimensional Holes in (100) GaAs Heterostructure

The in-plane magnetoresistance of dilute two-dimensional holes in a (100) GaAs heterostructure has been measured for various directions of a parallel magnetic field. The magnetoresistance exhibits a strong anisotropy, and changes its field dependence at a field corresponding to the full spin depopulation. The spin susceptibility deduced from the depopulation field also shows a strong anisotropy at higher densities while the anisotropy is absent at a lower density.

Formula omitted] noise in a dilute GaAs two-dimensional hole system in the insulating phase

We have measured the resistance and the 1 / f resistance noise of a two-dimensional low-density hole system in a high mobility GaAs quantum well at low temperature. At densities lower than the metal–insulator transition one, the temperature dependence of the resistance is either power-like or simply activated. The noise decreases when the temperature or the density increase. These results contradict the standard description of independent particles in the strong localization regime. On the contrary, they agree with the percolation picture suggested by higher density results. The physical nature of the system could be a mixture of a conducting and an insulating phase. We compare our results with those of composite thin films.

Effects of interactions and disorder on the compressibility of two-dimensional electron and hole systems

The compressibility χ of dilute two-dimensional electron and hole gases in GaAs semiconductor structures has been studied in the ranges of the interaction parameter r s = 1 –2.5 and r s = 10 –30 for the electron and hole system, respectively. Nonmonotonic dependence of χ - 1 with an upturn at low carrier densities is observed. Despite the large difference in r s the behavior of χ - 1 in both systems can be accurately described by the theory of nonlinear screening of disorder by the carriers.

Magnetically induced metallic phase in superconducting tantalum films

We have studied the electronic transport properties of homogeneously disordered superconducting tantalum thin films in magnetic fields. The films exhibit three distinct transport regimes in the zero temperature limit which we identify as superconducting, metallic, and insulating phases. The metallic phase is unexpected. The transport characteristics of this metallic phase are found to be similar to those of MoGe films and high mobility dilute two-dimensional electrons or holes confined in semiconductor interface or transistor geometry.

Direct and inverse domain structures in ferrofluids

The work deals with the theoretical study of domain structures in a ferrofluid, filling a thin flat gap with hard non-magnetic walls, placed into magnetic field, perpendicular to the gap plane. Two experimentally observed types of these structures are considered. The first one is the direct domain structure, presenting a system of dense cylindrical-like domains in a relatively dilute surrounding ferrofluid. The second one is the inverse structure, i.e., the system of dilute cylindrical holes in the dense simply connected environment. We estimate dependencies of diameter of the domains on applied magnetic field, on the gap thickness and other parameters of the system. The diagrams, corresponding to the transition from the direct to inverse structure, are calculated.

Temperature Dependent Weak Field Hall Resistance in Two-Dimensional Carrier Systems

Using the Drude-Boltzmann semiclassical transport theory, we calculate the weak-field Hall resistance of a two-dimensional system at low densities and temperatures, assuming carrier scattering by screened random charged impurity centers. The temperature-dependent 2D Hall coefficient shows striking nonmonotonicity in strongly screened systems, and, in particular, we qualitatively explain the recent puzzling experimental observation of a decreasing Hall resistance with increasing temperature in a dilute 2D hole system. We predict that the impurity scattering limited Hall coefficient will eventually increase with temperature at higher temperatures.

Coulomb drag near the metal-insulator transition in two dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the ${T}^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single-layer resistivity, suggesting they both arise from the same origin. In addition, layer-spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single-layer resistivity.

Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas

We have studied the temperature dependent phonon emission rate P(T) of a strongly interacting (r(s) > or =22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that P(T) changes from P(T) approximately T5 to P(T) approximately T7 above 100 mK, indicating a crossover from screened piezoelectric (PZ) coupling to screened deformation potential (DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is almost 20 times stronger than expected from theory. The density dependence of P(T) shows that it is easier to cool low-density 2D holes in GaAs than higher density 2D hole systems.

Spin Polarization Dependence of the Coulomb Drag at Largers

We find that the temperature dependence of the drag resistivity between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Our main observation is that near the apparent metal-insulator transition, the temperature dependence of the drag, given by Talpha, weakens significantly with the application of a parallel magnetic field (B(||)), with alpha saturating at half its zero field value for B(||)>B(*), where B(*) is the polarization field.

Insulating behavior of dilute two-dimensional holes in GaAs under an in-plane magnetic field

Insulating resistivity of high mobility dilute two-dimensional holes in GaAs under an in-plane magnetic field exhibits a weak temperature dependence when the carrier density is higher than the critical density of zero-field metal-insulator transition. When the density is below the critical density, the temperature dependence is stronger and exhibits a crossover from the Mott variable range hopping to the Efros-Shklovskii variable range hopping as the field is increased.

Temperature and Magnetic-Field-Enhanced Hall Slope of a Dilute 2D Hole System in the Ballistic Regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.