The 5d iridium-based transition metal oxides have gained broad interest because of their strong spin-orbit coupling which favors new or exotic quantum electronic states. On the other hand, they rarely exhibit more mainstream orders like ferromagnetism due to generally weak electron-electron correlation strength. Here, we show a proximity-induced ferromagnetic (FM) state with TC = 100 K and strong magnetocrystalline anisotropy in a SrIrO3 (SIO) heterostructure via interfacial charge transfer by using a ferromagnetic insulator in contact with SIO. Electrical transport allows to selectively probe the FM state of the SIO layer and the direct observation of a strong, intrinsic and positive anomalous Hall effect (AHE). For T < 20 K, the AHE displays unusually large coercive and saturation field, a fingerprint of a strong pseudospin-lattice coupling. A Hall angle, sxyAHE/sxx, larger by an order of magnitude than in typical 3d metals and a FM net moment of about 0.1 mB/Ir, is reported. This emphasizes how efficiently the nontrivial topological band properties of SIO can be manipulated by structural modifications and the exchange interaction with 3d TMOs.
It is pointed out that in ferromagnetic metals a contribution to the Hall voltage arises when a non-zero spin current exists, which is generally the case in the presence of a charge current. This contribution is independent of any scattering effects and exists down to zero temperature. The sign of the resulting Hall coefficient may be either equal or opposite to the one of the ordinary Hall coefficient depending on the band filling. This effect seems to have been left out in previous analyses of the Hall effect in ferromagnetic metals.
The local 4f-electronic spin dynamics and magnetic order in YbNi4P2 were studied by means of muon-spin relaxation measurements. Zero-field muon-spin relaxation proves static magnetic order with a strongly reduced ordered Yb3+ moment of (2.5-4.6) \times 10-2μB, below TC = 140 mK. Above TC, the muon-spin polarization P(t,B) is dominated by quasihomogeneous spin fluctuations and exhibits a time-field scaling relation P(t,B) = P(t/Bγ), indicating cooperative critical spin dynamics in the system. At T = 190 mK, slightly above TC, γ = 0.81(5), suggesting time-scale invariant power-law behavior for t…
The spin torque ferromagnetic resonance (STFMR) is one of the popular methods for measurement of the spin Hall angle (SHA). However, in order to accurately determine SHA from STFMR measurements, the acquired data must be carefully analyzed: The resonance linewidth should be determined to an accuracy of a fraction of an Oe, while the dynamical interaction leading to the measured response consists of the conventional field-induced ferromagnetic resonance (FMR), spin-torque induced FMR, and of the inverse spin Hall effect (ISHE). Additionally, the signal often deteriorates when DC current is pass…
This work is devoted to experimental study of influence of superconductivity (S) on ferromagnetism (FM) (inverse proximity effects) with the help of Polarized Neutron Reflectivity. Combining meausurements of specular and diffuse intensities it is possible to obtain full picture of magnetization change in S/FM layered systems like magnetization rotation, domain state formation, inducing of magnetization in S layer, etc. To increase weak magnetic signal we propose to use enhanced neutron standing wave regime (e.g. waveguides). Choose of materials, optimization of thicknesses of layers, estimatio…
SrRuO$_3$ heterostructures grown in the (111) direction are a rare example of thin film ferromagnets. By means of density functional theory plus dynamical mean field theory we show that the half-metallic ferromagnetic state with an ordered magnetic moment of 2$μ_{B}$/Ru survives the ultimate dimensional confinement down to a bilayer, even at elevated temperatures of 500$\,$K. In the minority channel, the spin-orbit coupling opens a gap at the linear band crossing corresponding to $\frac34$ filling of the $t_{2g}$ shell. We demonstrate that the respective state is Haldane's quantum anomalous Ha…
Transport measurements in ramp-type junctions of $YBa_2Cu_3O_y-SrRuO_3-YBa_2Cu_3O_y$ with $T_c$ values of either 80-90 K or 60-70 K are reported. In both type of junctions but without a barrier ("shorts"), the supercurrent densities at 4.2 K reached 7.5 and 3.5 MA/cm$^2$, respectively, indicating the high quality of the fabrication process. Plots of the critical current versus thickness of the ferromagnetic barrier at 4.2 K show exponential decays with decay lengths of 1.1 nm for the 90 K phase and 1.4 nm for the 60 K phase, which are much shorter than the relevant coherence lengths $ξ_F\sim 5…
We study the quantum anomalous Hall effect described by a class of two-component Haldane models on square lattices. We show that the latter can be transformed into a pseudospin triplet p+ip-wave paired superfluid. In the long wave length limit, the ground state wave function is described by Halperin's (1,1,-1) state of neutral fermions analogous to the double layer quantum Hall effect. The vortex excitations are charge e/2 abelian anyons which carry a neutral Dirac fermion zero mode. The superconducting proximity effect induces `tunneling' between `layers' which leads to topological phase tran…
A scaling relation of the anomalous Hall effect recently found in a ferromagnetic semiconductor (Ti,Co)O_2_ is compared with those of various ferromagnetic semiconductors and metals. Many of these compounds with relatively low conductivity sigma_xx_ < 10^4 ohm^-1 cm^-1 are also found to exhibit similar relation: anomalous Hall conductivity sigma_AH_ approximately scales as sigma_AH_ proportional to sigma_xx_^1.6, that is coincident with a recent theory. This relation is valid over five decades of sigma_xx_ irrespective of metallic or hopping conduction.
We have experimentally confirmed the quantum Hall ferromagnetic state with Chern number ν=0, characterized by the helical edge state, in a layered organic Dirac fermion system α-(BEDT-TTF)_2I_3. The interlayer resistance saturates at low temperatures and high magnetic fields. It does not scale with the sample cross-sectional area in the saturating region, and resonantly depends on the magnetic field direction. These results strongly suggest that the helical edge state dominates transport. This is the first observation of the topological phase in organic molecular crystals.
The anomalous Hall effect is investigated for ferromagnetic Fe3Sn2 single crystal with geometrically frustrated kagome bilayer of Fe. The scaling behavior between anomalous Hall resistivity rho_{xy}^{A} and longitudinal resistivity rho_{xx} is quadratic and further analysis implies that the AHE in Fe3Sn2 single crystal should be dominated by the intrinsic Karplus-Luttinger mechanism rather than extrinsic skew-scattering or side-jump mechanisms. Moreover, there is a sudden jump of anomalous Hall conductivity sigma_{xy}^{A} appearing at about 100 K where the spin-reorientation transition from th…
We have investigated the thermal Hall effect of magnons for various ferromagnetic insulators. For pyrochlore ferromagnetic insulators Lu$_2$V$_2$O$_7$, Ho$_2$V$_2$O$_7$, and In$_2$Mn$_2$O$_7$, finite thermal Hall conductivities have been observed below the Curie temperature $T_C$ . From the temperature and magnetic field dependences, it is concluded that magnons are responsible for the thermal Hall effect. The Hall effect of magnons can be well explained by the theory based on the Berry curvature in momentum space induced by the Dzyaloshinskii-Moriya (DM) interaction. The analysis has been ext…
For an s-wave superconductor/semiconductor/ferromagnetic-insulator structure, the proximity effect can induce the energy gap in the semiconductor rather than the superconducting pair potential of its microscope Hamiltonian. As a result, it is questionable to realize topological superconducting states in that structure.
We give a theory of the inverse spin Hall effect (ISHE) in ferromagnetic metals based on skew scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short-range spin-spin correlations. We find a relation between the ISHE and the four-spin correlations near the Curie temperature TC. Such four-spin correlations do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and ISHE, providing an essential complement to Kondo's classic theory of the AHE in met…
Recent works aiming at understanding magnetotransport phenomena in ferromagnetic III-V and II-VI semiconductors are described. Theory of the anomalous Hall effect in p-type magnetic semiconductors is discussed, and the relative role of side-jump and skew-scattering mechanisms assessed for (Ga,Mn)As and (Zn,Mn)Te. It is emphasized that magnetotransport studies of ferromagnetic semiconductors in high magnetic fields make it possible to separate the contributions of the ordinary and anomalous Hall effects, to evaluate the role of the spins in carrier scattering and localization as well as to dete…
We present an anti-ferromagnetically ordered ground state of Na$_{2}$IrO$_{3}$ based on density-functional-theory calculations including both spin-orbit coupling and on-site Coulomb interaction $U$. We show that the splitting of $e_{g}'$ doublet states by the strong spin-orbit coupling is mainly responsible for the intriguing nature of its insulating gap and magnetic ground state. Due to its proximity to the spin-orbit insulator phase, the magnetic ordering as obtained with finite $U$ is found to exhibit a strong in-plane anisotropy. The phase diagram of Na$_{2}$IrO$_{3}$ suggests a possible i…
The competition between the Zeeman energy and the Rashba and Dresselhaus spin-orbit couplings is studied for fractional quantum Hall states by including correlation effects. A transition of the direction of the spin-polarization is predicted at specific values of the Zeeman energy. We show that these values can be expressed in terms of the pair-correlation function, and thus provide information about the microscopic ground state. We examine the particular examples of the Laughlin wavefunctions and the 5/2-Pfaffian state. We also include effects of the nuclear bath.
The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well ex…
By density functional theory calculations, we predict a stable two-dimensional (2D) ferromagnetic semiconductor Cr$_2$Ge$_2$Se$_6$, where the Curie temperature $T$$_c$ can be dramatically enhanced beyond room temperature by applying a few percent strain. In addition, the anomalous Hall conductivity in 2D Cr$_2$Ge$_2$Se$_6$ and Cr$_2$Ge$_2$Te$_6$ is predicted to be comparable to that in ferromagnetic metals of Fe and Ni, and is an order of magnitude larger than that in diluted magnetic semiconductor Ga(Mn,As). Based on superexchange interactions, the enhanced $T$$_c$ in 2D Cr$_2$Ge$_2$Se$_6$ by…
In a ferromagnet, an applied electric field $\bf E$ invariably produces an anomalous Hall current ${\bf J}_H$ that flows perpendicular to the plane defined by $\bf E$ and $\bf M$ (the magnetization). For decades, the question whether ${\bf J}_H$ is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr$_2$Se$_{4-x}$Br$_x$, the resistivity $ρ$ (at low temperature) may be increased 1000 fold by varying $x$(Br), without degrading the $\bf M$. We show that ${\bf J}_H/E$ (normalized per carrier, at 5 K) remains…
The intrinsic antiferromagnetic (AFM) interlayer coupling in two-dimensional magnetic topological insulator MnBi$_2$Te$_4$ places a restriction on realizing stable quantum anomalous Hall effect (QAHE) [Y. Deng et al., Science 367, 895 (2020)]. Through density functional theory calculations, we demonstrate the possibility of tuning the AFM coupling to the ferromagnetic coupling in MnBi$_2$Te$_4$ films by alloying about 50% V with Mn. As a result, QAHE can be achieved without alternation with the even or odd septuple layers. This provides a practical strategy to get robust QAHE in ultrathin MnBi…
The quantum anomalous Hall effect (QAHE) hosts the dissipationless chiral edge states associated with the nonzero Chern number, providing potentially significant applications in future spintronics. The QAHE usually occurs in a two-dimensional (2D) system with time-reversal symmetry breaking. In this work, we propose that the QAHE can exist in three-dimensional (3D) ferromagnetic insulators. By imposing inversion symmetry, we develop the topological constraints dictating the appearance of 3D QAHE based on the parity analysis at the time-reversal invariant points in reciprocal space. Moreover, u…
We demonstrate the existance of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from Hartree-Fock Configuration Interaction method as a function of electron numbers N, magnetic field B, Zeeman energy, and tunneling barrier height. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong inter-dot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at fi…
To elucidate the origin of anomalous Hall effect (AHE) in ferromagnetic transition metals, we study the intrinsic AHE based on a multi-orbital (xz,yz)-tight-binding model. We find that a large anomalous velocity comes from the off-diagonal (inter-orbital) hopping. By this reason, the present model shows a large intrinsic anomalous Hall conductivity (AHC) which is compatible with typical experimental values in ferromagnets [100-1000 [1/Ωcm]], without necessity to assume a special band structure at the Fermi level. In good metals where resistivity ρis small, the intrinsic AHC is constant (dissip…
We study a two-dimensional electron system where the electrons occupy two conduction band valleys with anisotropic Fermi contours and strain-tunable occupation. We observe persistent quantum Hall states at filling factors $ν= 1/3$ and 5/3 even at zero strain when the two valleys are degenerate. This is reminiscent of the quantum Hall ferromagnet formed at $ν= 1$ in the same system at zero strain. In the absence of a theory for a system with anisotropic valleys, we compare the energy gaps measured at $ν= 1/3$ and 5/3 to the available theory developed for single-valley, two-spin systems, and fin…
Quantized Transport of $ν= 2/3$ Fractional Quantum Hall Edge with Disordered Superconducting Proximity
Princeton Center for Theoretical Science, Princeton University（普林斯顿大学理论科学中心）；Department of Physics, Princeton University（普林斯顿大学物理系）；Department of Electrical and Computer Engineering, Princeton University（普林斯顿大学电气与计算机工程系）
We study proximity coupling between a superconductor and counter-propagating gapless modes arising on the edges of Abelian fractional quantum Hall liquids with filling fraction $ν=1/m$ (with $m$ an odd integer). This setup can be utilized to create non-Abelian parafermion zero-modes if the coupling to the superconductor opens an energy gap in the counter-propagating modes. However, when the coupling to the superconductor is weak an energy gap is opened only in the presence of sufficiently strong attractive interactions between the edge modes, which do not commonly occur in solid state experime…
