Search

Due to the coexistence of many emergent phenomena, including 2D superconductivity and a large Rashba spin-orbit coupling, 5d transition metal oxides based two-dimensional electron systems (2DESs) have been prospected as one of the potential intrants for modern electronics. However, despite the lighter electron mass, the mobility of carriers, a key requisite for high-performance devices, in 5d-oxides devices remains far behind their 3d-oxides analogs. The carriers mobility in these oxides is significantly hampered by the inevitable presence of defects generated during the growth process. Here, we report very high mobility ($\sim$ 22650 cm$^2$V$^{-1}$s$^{-1}$) of 5d-2DES confined at the LaAlO$_3$/KTaO$_3$ interface. The high mobility, which is beyond the values observed in LaAlO$_3$/SrTiO$_3$ and $\gamma$-Al$_2$O$_3$/SrTiO$_3$ systems in the same carrier-density range, is achieved using the ionic-liquid gating at room temperature. We postulate that the ionic-liquid gating affects the oxygen vacancies and efficiently reduces any disorder at the interface. Investigating density and mobility in a broad range of back-gate voltage, we reveal that the mobility follows the power-law $\mu \propto n^{1.2}$, indicating the very high quality of ionic-liquid-gated LaAlO$_3$/KTaO$_3$ devices, consistent with our postulate. Further, the analysis of the quantum oscillations measured in high magnetic fields confirms that the high-mobility electrons occupy the electronic sub-bands emerging from the Ta:5d orbitals of KTaO$_3$., Comment: 20 pages, 6 figures

Superconductivity can be destroyed by a magnetic field with an upper bound known as the Pauli-limit in spin-singlet superconductors. Almost all the discovered superconductors are spin-singlet, with the highest transition temperature $T_c$ at ambient pressure achieved in the cuprate family. The closest cuprate analogue is the recently discovered infinite-layer nickelate, which hosts substantial structural and electronic similarity to the cuprate. A previous magnetotransport study on Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ has observed an isotropic Pauli-limited upper critical field. Here, we report a large violation (>2 times) of Pauli-limit in every crystallographic directions in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ regardless of the doping $x$. Such a large violation of the Pauli-limit in all directions in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ is unexpected and unlikely accounted by a Fulde Ferrell-Larkin-Ovchinnikov (FFLO)-state, strong spin-orbit-coupling, strong-coupling or a large pseudogap. On the other hand, in agreement with the previous report, we observe a Pauli-limiting critical field in Nd$_{1-x}$Sr$_x$NiO$_2$ and the superconducting anisotropy decreases as doping increases, suggesting a spin-singlet pairing. Therefore, superconductivity in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ could be driven by a non-spin-singlet Cooper pairing mechanism with an attractive high-$T_c$ at 10 K, an order of magnitude higher than the known spin triplet superconductors, favourably extending the application of spin-triplet superconductivity in topological matter, non-dissipative spintronics, and quantum computing., Comment: 21 pages, 4 main figures, 2 extended data

We have investigated the illumination effect on the magnetotransport properties of a two-dimensional electron system at the LaAlO$_3$/SrTiO$_3$ interface. The illumination significantly reduces the zero-field sheet resistance, eliminates the Kondo effect at low-temperature, and switches the negative magnetoresistance into the positive one. A large increase in the density of high-mobility carriers after illumination leads to quantum oscillations in the magnetoresistance originating from the Landau quantization. The carrier density ($\sim 2 \times 10^{12}$ cm$^{-2}$) and effective mass ($\sim 1.7 ~m_e$) estimated from the oscillations suggest that the high-mobility electrons occupy the d$_{xz/yz}$ subbands of Ti:t$_{2g}$ orbital extending deep within the conducting sheet of SrTiO$_3$. Our results demonstrate that the illumination which induces additional carriers at the interface can pave the way to control the Kondo-like scattering and study the quantum transport in the complex oxide heterostructures.

The quasi two-dimensional electron gas (q-2DEG) at oxide interfaces provides a platform for investigating quantum phenomena in strongly correlated electronic systems. Here, we study the transport properties at the high-mobility (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_{0.3}$/SrTiO$_{0.3}$ (LSAT/STO) interface. Before oxygen annealing, the as-grown interface exhibits a high electron density and electron occupancy of two subbands: higher-mobility electrons ($\mu_1\approx{10^4}$ cm$^2$V$^{-1}$s$^{-1}$ at 2 K) occupy the lower-energy $3d_{xy}$ subband, while lower-mobility electrons ($\mu_1\approx{10^3}$ cm$^{2}$V$^{-1}$s$^{-1}$ at 2 K) propagate in the higher-energy $3d_{xz/yz}$-dominated subband. After removing oxygen vacancies by annealing in oxygen, only a single type of 3dxy electrons remain at the annealed interface, showing tunable Shubnikov-de Haas (SdH) oscillations below 9 T at 2 K and an effective mass of $0.7m_e$. By contrast, no oscillation is observed at the as-grown interface even when electron mobility is increased to $50,000$ cm$^{2}$V$^{-1}$s$^{-1}$ by gating voltage. Our results reveal the important roles of both carrier mobility and subband occupancy in tuning the quantum transport at oxide interfaces., Comment: 22 pages, 4 figures

We report on the stability of the quantum Hall plateau in wide Hall bars made from a chemically gated graphene film grown on SiC. The $\nu=2$ quantized plateau appears from fields $B \simeq 5$ T and persists up to $B \simeq 80$ T. At high current density, in the breakdown regime, the longitudinal resistance oscillates with a $1/B$ periodicity and an anomalous phase, which we relate to the presence of additional electron reservoirs. The high field experimental data suggest that these reservoirs induce a continuous increase of the carrier density up to the highest available magnetic field, thus enlarging the quantum plateaus. These in-plane inhomogeneities, in the form of high carrier density graphene pockets, modulate the quantum Hall effect breakdown and decrease the breakdown current., Comment: accepted in Physical Review Letter in November, 2016

We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in temperature range from 4.2 K up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence subbands, and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.

The Integer Quantum Hall Effect (IQHE) is a distinctive phase of two-dimensional electronic systems subjected to a perpendicular magnetic field. Thus far, the IQHE has been observed in semiconductor heterostructures and in mono- and bi-layer graphene. Here we report on the IQHE in a new system: trilayer graphene. Experimental data are compared with self-consistent Hartree calculations of the Landau levels for the gated trilayer. The plateau structure in the Hall resistivity determines the stacking order (ABA versus ABC). We find that the IQHE in ABC trilayer graphene is similar to that in the monolayer, except for the absence of a plateau at filling factor v=2. At very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder., Comment: 5 pages, 4 figures

We report on magneto-transport measurement in disordered graphene under pulsed magnetic field of up to 57T. For large electron or hole doping, the system displays the expected anomalous Integer Quantum Hall Effect (IQHE) specific to graphene up to filling factor $\nu=2$. In the close vicinity of the charge neutrality point, the system breaks up into co-existing puddles of holes and electrons, leading to a vanishing Hall and finite longitudinal resistance with no hint of divergence at very high magnetic field. Large resistance fluctuations are observed near the Dirac point. They are interpreted as the the natural consequence of the presence of electron and hole puddles. The magnetic field at which the amplitude of the fluctuations are the largest is directly linked to the mean size of the puddles.

Measurements of basal plane longitudinal rho_b(B) and Hall rho_H(B) resistivities were performed on highly oriented pyrolytic graphite (HOPG) samples in pulsed magnetic field up to B = 50 T applied perpendicular to graphene planes, and temperatures 1.5 K < T < 4.2 K. At B > 30 T and for all studied samples, we observed a sign change in rho_H(B) from electron- to hole-like. For our best quality sample, the measurements revealed the enhancement in rho_b(B) for B > 34 T (T = 1.8 K), presumably associated with the field-driven charge density wave or Wigner crystallization transition. Besides, well defined plateaus in rho_H(B) were detected in the ultra-quantum limit revealing the signatures of fractional quantum Hall effect in graphite., Comment: 15 pages, including 4 figures

We find that the polarization field, B_chi, obtained by scaling the weak-parallel-field magnetoresistance at different electron densities in a dilute two-dimensional electron system in (111) silicon, corresponds to the spin susceptibility that grows strongly at low densities. The polarization field, B_sat, determined by resistance saturation, turns out to deviate to lower values than B_chi with increasing electron density, which can be explained by filling of the upper electron subbands in the fully spin-polarized regime.

We report on the magnetic properties of thoroughly characterized Zn1-xCoxO epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials., Comment: 4 pages, 4 figures

In this paper we demonstrate the application of high magnetic fields to study the magnetic properties of low dimensional spin systems. We present a case study on the series of 2-leg spin-ladder compounds Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T = Ni, Co, Fe and Mn. In all compounds the transition metal is in the $T^{2+}$ high spin configuation. The localized spin varies from S=1 to 3/2, 2 and 5/2 within this series. The magnetic properties were examined experimentally by magnetic susceptibility, pulsed high field magnetization and specific heat measurements. The data are analysed using a spin hamiltonian description. Although the transition metal ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. This behaviour can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder. All compounds exhibit magnetic field driven ground state changes which at very low temperatures lead to a multistep behaviour in the magnetization curves. In the Co and Fe compounds a strong axial anisotropy induced by the orbital magnetism leads to a nearly degenerate ground state and a strongly reduced critical field. We find a monotonous decrease of the intradimer magnetic exchange if the spin quantum number is increased.

Magnetotransport properties of p-InMnAs layers are studied in pulsed magnetic fields up to 30 T. Samples were prepared by the laser deposition and annealed by ruby laser pulses. Well annealed samples show p-type conductivity while they were n-type before the annealing. Surprisingly the anomalous Hall effect resistance in paramagnetic state (T>40 K) and in strong magnetic fields (B > 20 T) appears to be greater than that in ferromagnetic state (T <= 40 K), while the longitudinal resistance rises with the temperature decrease. The negative magnetoresistance saturates in magnetic fields higher then 10T at T near 4 K only, whereas the saturation fields of the anomalous Hall effect resistance are much less (around 2 T at 30K). The total reduction of resistance exceeds 10 times in magnetic fields around of 10T. The obtained results are interpreted on the base of the assumptions of the non-uniform distribution of Mn atoms acting as acceptors, the local ferromagnetic transition and the percolation-like character of the film conductivity, which prevailed under conditions of the strong fluctuations of the exchange interaction. Characteristic scales of the magneto-electric nonuniformity are estimated using analysis of the mesoscopic fluctuations of the non-diagonal components of the magnetoresistivity tensor., Comment: 12 pages, 5 figures

We report the synthesis, crystal structure and magnetic properties of the S=1 2-leg spin-ladder compound Na_2Ni_2(C_2O_4)_3(H_2O)_2. The magnetic properties were examined by magnetic susceptibility and pulsed high field magnetization measurements. The magnetic excitations have been measured in high field high frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. The analysis of the temperature dependent magnetization data leads to a magnetic exchange constant of J=43 K along the rungs of the ladder and an average value of the g-factor of 2.25. From the ESR measurements, we determined the single ion anisotropy to D=11.5 K. The validity of the isolated dimer model is supported by Quantum Monte Carlo calculations, performed for several ratios of interdimer and intradimer magnetic exchange and taking into account the experimentally determined single ion anisotropy. The results can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder., Comment: 8 pages, 10 figures

Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a \mu_4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ~25 T., Comment: revised version, accepted for publication in Physical Review B

We report an electron spin resonance (ESR) study of the s=1/2-Heisenberg pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At sub-Terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few percent of ''extra'' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic ordering at T_N = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below T_N of the order of 0.3 - 0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu2O 3 at weak magnetic fields H_{sf} ~ 3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce the antiferromagnetic correlations and may even suppress the long-range magnetic order in CaCu2O3. ESR data support scenarios with a significant role of the ''extra'' spin states for the properties of low-dimensional quantum magnets., Comment: revised version, accepted for publication in the New Journal of Physics

High-field magnetic measurements performed with the use of magnetooptical Kerr effect (MOKE) in the polar configuration as well as high-field and low-field magnetotransport studies were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. The structural investigations by means of high resolution XRD were also performed. We observe significant changes in magnetoresistivity curves, magnetization and strain introduced by the annealing., Comment: International Conference on Magnetism, ICM 2003, Roma; to be published on Journal of Magnetism and Magnetic Materials

High- and low-field magneto-transport measurements, as well as SQUID measurements of magnetization, were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280 0C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga1-xMnxAs annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga1-xMnxAs., Comment: 2-nd International Conference on Physics and Application of Spin Related Phenomena In Semoconductors, Wurzburg 23-26 July 2002, to be published in a special issue of the Journal of Superconductivity/Novel Magnetism

The Anomalous Hall Effect was investigated in IV-VI ferromagnetic semimagnetic semiconductors of Sn1-xMnxTe codoped with either Eu or Er. The analysis of experimental data: Hall resisitivity and magnetization showed that AHE coefficient RS depends on temperature, its value decreases with the temperature increase. We observe that above ferromagnet-paramagnet transition temperature RS changes sign. We discuss the possible physical mechanisms responsible for observed temperature dependence of RS, particularly change of the sign., Comment: 2nd International Conference on Physics and Application of Spin Related Phenomena In Semiconductors, Wurzburg 23-26 July 2002, to be published in aspecial issue of the Journal of Superconductivity/Novel Magnetism

We present the results of low temperature (LT) annealing studies of Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy in a wide range of Mn concentrations (0.01

We report on unique electronic properties in Fe-SiO2 nanocomposite thin films in the vicinity of the percolation threshold. The electronic transport is dominated by quantum corrections to the metallic conduction of the Infinite Cluster (IC). At low temperature, mesoscopic effects revealed on the conductivity, Hall effect experiments and low frequency electrical noise (random telegraph noise and 1/f noise) strongly support the existence of a temperature-induced Quantum Size Effect (QSE) transition in the metallic conduction path. Below a critical temperature related to the geometrical constriction sizes of the IC, the electronic conductivity is mainly governed by active tunnel conductance across barriers in the metallic network. The high 1/f noise level and the random telegraph noise are consistently explained by random potential modulation of the barriers transmittance due to local Coulomb charges. Our results provide evidence that a lowering of the temperature is somehow equivalent to a decrease of the metal fraction in the vicinity of the percolation limit., Comment: 21 pages, 8 figures