A magnetic field typically suppresses superconductivity by either breaking Cooper pairs via the Zeeman effect or inducing vortex formation. However, under certain circumstances, a magnetic field can stabilize superconductivity instead. This seemingly counterintuitive phenomenon is associated with magnetic interactions and has been extensively studied in three-dimensional materials. By contrast, this phenomenon, hinting at unconventional superconductivity, remains largely unexplored in two-dimensional systems, with moiré-patterned graphene being the only known example. Here, we report the observation of reentrant superconductivity at the epitaxial (110)-oriented LaTiO 3 -KTaO 3 interface. This phenomenon occurs across a wide range of charge carrier densities, which, unlike in three-dimensional materials, can be tuned in situ via electrostatic gating. We propose that the observed reentrant superconductivity can arise from an interplay between strong spin-orbit coupling and a magnetic field–driven modification of the Fermi surface. Our findings provide insight into reentrant superconductivity and establish a robust platform for exploring unconventional superconducting phenomena in two-dimensional systems.

Based on ab initio calculations, we demonstrate that a Mott insulator LaTiO3 (LTO), not inspected previously as an altermagnetic material, shows the characteristic features of altermagnets, i.e., (i) fully compensated antiferromagnetism and (ii) k-dependent spin-split electron bands in the absence of spin-orbit coupling. The altermagnetic ground state of LTO is protected by the crystal symmetry and specifically ordered d orbitals of Ti ions with the orbital momentum l = 2. The altermagnetism occurs when sites of Ti pairs in the unit cell are occupied by single electrons with m = -1, sz = +1/2 and m = +1, sz = -1/2 per site, with m and sz- being the z-component of the orbital momentum and spin, respectively. By further simulating orbital disorder within the Green's function method, we disclose its damaging character on the spin splitting and the resulting altermagnetism. When the single-electron spin-polarized state at each Ti site is contributed almost equally by two or three t2g orbitals, LTO becomes antiferromagnetic. The effect of the spin-orbit coupling, which can cause orbital disorder and suppress altermagnetism, is discussed.

Intriguingly, conducting perovskite interfaces between ordinary band insulators are widely explored, whereas similar interfaces with Mott insulators are still not quite understood. Here, we address the (001), (110), and (111) interfaces between the LaTiO_{3} Mott, and large band gap KTaO_{3} insulators. Based on first-principles calculations, we reveal a mechanism of interfacial conductivity, which is distinct from a formerly studied one applicable to interfaces between polar wideband insulators. Here, the key factor causing conductivity is the matching of oxygen octahedra tilting in KTaO_{3} and LaTiO_{3} which, due to a small gap in the LaTiO_{3} results in its sensitivity to the crystal structure, yields metallization of its overlayer and following charge transfer from Ti to Ta. Our findings, also applicable to other Mott insulators interfaces, shed light on the emergence of conductivity observed in LaTiO_{3}/KTaO_{3} (110) where the "polar" arguments are not applicable and on the emergence of superconductivity in these structures.

Design of epitaxial interfaces is a pivotal way to engineer artificial structures where new electronic phases can emerge. Here we report a systematic emergence of interfacial superconducting state in epitaxial heterostructures of LaTiO3 and KTaO3. The superconductivity transition temperature increases with decreasing the thickness of LaTiO3. Such behavior is observed for both (110) and (111) crystal oriented structures. For thick samples, the finite resistance developing below the superconducting transition temperature increases with increasing LaTiO3 thickness. Consistent with previous reports, the (001) oriented heterointerface features high electron mobility of 250 cm2/Vs and shows no superconducting transition down to 40 mK. Our results imply a non-trivial impact of LaTiO3 on the superconducting state and indicate how superconducting KTaO3 interfaces can be integrated with other oxide materials.

2D phases of matter have become a new paradigm in condensed matter physics, bringing in an abundance of novel quantum phenomena with promising device applications. However, realizing such quantum phases has its own challenges, stimulating research into non-traditional methods to create them. One such attempt is presented here, where the intrinsic crystal anisotropy in a "fractional" perovskite, EuxTaO3 (x = 1/3 - 1/2), leads to the formation of stacked layers of quasi-2D electron gases, despite being a 3D bulk system. These carriers possess topologically non-trivial spin textures, indirectly controlled by an external magnetic field via proximity effect, making it an ideal system for spintronics, for which several possible applications are proposed. An anomalous Hall effect with a non-monotonic dependence on carrier density is shown to exist, signifying a shift in band topology with carrier doping. Furthermore, quantum oscillations in charge conductivity and oscillating thermoelectric properties are examined and proposed as routes to experimentally demonstrate the quasi-2D behavior. A quasi-2D electron gas of charge carriers is realized in a bulk 3D oxide perovskite. Furthermore, these carriers have magnetically controllable topological spin textures and are of a single orbital character, making it an ideal platform for spintronic devices.image

In a hybrid system of topological insulator (TI)/superconductor (SC), the proximity-induced topological superconductivity is expected to appear at the interface. Here we propose and demonstrate that a TI/SC hybrid Bi2Te3/PdTe2 heterostructure serves as a platform for exploring topological superconductivity with various features: all made of tellurium compounds, epitaxial growth, and a small charge transfer interface. In the Bi2Te3/PdTe2 heterostructure films, we observe large nonreciprocal charge transport near the superconducting transition temperature under a transverse in-plane magnetic field. The observation indicates the interplay between the topological surface state and superconductivity, suggesting that the Bi2Te3/PdTe2 heterostructure is a candidate for a topological superconductor. Also observed is an unexpected sign reversal of the nonreciprocal coefficient when the in-plane magnetic field is slightly tilted toward the out-of-plane direction. The analysis reveals that the sign reversal occurs with the change of dominant vortex type, that is, the change from spontaneous vortices to external-field induced ones.

We report experimental signatures of topological transitions among the Weyl semimetal states of pyrochlore Pr2Ir2O7, where the Kondo coupling between the Jr topological electrons and the spin-ice-like orders of Pr moments plays a decisive role. The magnetic-field dependence of resistivity and the Hall conductivity exhibits a plateau and a sharp jump associated with a magnetic-field hysteresis, similar to a liquid-gas-like transition in a dipolar spin-ice system. Furthermore, the Kondo coupling is controlled by hydrostatic pressure, revealing that the field-induced displacement of Weyl points in the momentum space strongly depends on the respective electronic state as well as on the Kondo coupling strength. These observations pave a route toward the engineering of band topology in hybrid quantum materials with relativistic conduction electrons and localized magnetic moments.

In semiconducting materials, electrostatic gating and light illumination are widely used stimuli to tune the electronic properties of the system. Here, we show a significant enhancement of photoresponse at the conducting interface of LaVO3-SrTiO3 under the simultaneous application of light and negative gate bias voltage, in comparison to their individual application. On the other hand, the LaVO3-SrTiO3 interface remains largely insensitive to light illumination, when a positive gate bias voltage is applied. Our X-ray diffractometer, Raman spectroscopy and photoemission measurements show that unlike the LaAlO3-SrTiO3 interface, migration of oxygen vacancies is not the prime mechanism for the enhanced photoresponse. Rather, we suggest that the photoresponse of our system is intrinsic and this intrinsic mechanism is a complex interplay between band filling, electric field at the interface, strong electron interaction due to mottness of LaVO3 and modification of conducting channel width.

Spin-orbit coupling (SOC) is pivotal for various fundamental spin-dependent phenomena in solids and their technological applications. In semiconductors, these phenomena have been so far studied in relatively weak electron-electron interaction regimes, where the single electron picture holds. However, SOC can profoundly compete against Coulomb interaction, which could lead to the emergence of unconventional electronic phases. Since SOC depends on the electric field in the crystal including contributions of itinerant electrons, electron-electron interactions can modify this coupling. Here we demonstrate the emergence of SOC effect in a high-mobility two-dimensional electron system in a simple band structure MgZnO/ZnO semiconductor. This electron system features also strong electron-electron interaction effects. By changing the carrier density with Mg-content, we tune the SOC strength and achieve its interplay with electron-electron interaction. These systems pave a way to emergent spintronic phenomena in strong electron correlation regime and to the formation of novel quasiparticles with the electron spin strongly coupled to the density.

The origin of the conductivity at the interface of two insulating perovskite oxides is a matter of intensive studies. The conductivity generated at the interface of insulating LaVO3 (LVO) and SrTiO3 (STO) is explained in terms of polar catastrophe. Here, the authors grown LVO films on (001) TiO2-terminated STO substrate employing pulsed laser deposition technique and demonstrate a transition from conducting to insulating interface by changing the La-stoichiometry by only 1%, whereby such transition takes place for La-deficient films. The effect of cation (non)stoichiometry of LVO film on both carrier density and mobility is studied and compared with previously reported LaAlO3-STO interface. This observation suggests a revisit to the explanation of possible origin of such conductivity beyond the polar catastrophe scenario and can be instrumental in search for novel conducting interfaces.

Oxygen vacancies play a crucial role in the conductivity of oxides. Here, we report the photoresponse of the electron doped surface of Ar + bombarded oxygen vacant (001) KTaO 3 (KTO) single crystal. The bombardment time defines the amount of oxygen vacancies and hence the electron doping level. The time evolution of photoresponse to daylight illumination remains independent of the carrier density and follows the biexponential function. By contrast, the amplitude of the photoresponse increases with the decreasing charge carrier density. The samples show distinct responses in terms of amplitude as well as response time to the illumination with laser light of wavelengths 633, 532, and 405nm. The defect states distribution within the bandgap is calculated with the photoconductivity relaxation, which involves deep sensitizing hole traps. The combined results of electrical conductivity, photoconductivity, atomic force microscopy, and Kelvin probe force microscopy suggest that the conductivity produced on the KTO surface is not continuous throughout the surface. Rather, Ar + bombardment creates oxygen deficiency patches that are oriented along some preferential crystal orientations and interconnected with each other, thus producing percolating conducting channels on the surface of the sample. Under light illumination, photocarriers are generated in these conducting channels.

We report the fabrication of both antidot lattices and unidirectional stripe patterns upon molecular beam epitaxy grown MgZnO/ZnO heterostructures. The magnetoresistance of these high mobility devices exhibits commensurability oscillations associated with ballistic transport of carriers executing orbital motion within the geometry of the imposed modulation.

Interactions between the constituents of a condensed matter system can drive it through a plethora of different phases due to many-body effects. A prominent platform for it is a dilute two-dimensional electron system in a magnetic field, which evolves intricately through various gaseous, liquid and solid phases governed by Coulomb interaction. Here we report on the experimental observation of a phase transition between the composite fermion liquid and adjacent magnetic field induced phase with a character of Wigner solid. The experiments are performed in the lowest Landau level of a MgZnO/ZnO two-dimensional electron system with attributes of both a liquid and a solid. An in-plane magnetic field component applied on top of the perpendicular magnetic field extends the Wigner-like phase further into the composite fermion liquid phase region. Our observations indicate the direct competition between a composite fermion liquid and a Wigner solid formed either by electrons or composite fermions.

Anomalous Hall effect, a manifestation of Hall effect occurring in systems without time-reversal symmetry, has been mostly observed in ferromagnetically ordered materials. However, its realization in high-mobility two-dimensional electron system remains elusive, as the incorporation of magnetic moments deteriorates the device performance compared to non-doped structure. Here we observe systematic emergence of anomalous Hall effect in various MgZnO/ZnO heterostructures that exhibit quantum Hall effect. At low temperatures, our nominally non-magnetic heterostructures display an anomalous Hall effect response similar to that of a clean ferromagnetic metal, while keeping a large anomalous Hall effect angle θAHE≈20°. Such a behaviour is consistent with Giovannini-Kondo model in which the anomalous Hall effect arises from the skew scattering of electrons by localized paramagnetic centres. Our study unveils a new aspect of many-body interactions in two-dimensional electron systems and shows how the anomalous Hall effect can emerge in a non-magnetic system.

We have investigated the quantum transport properties of high-mobility electrons and holes in atomically thin black phosphorus ambipolar devices. The two-dimensional hole system exhibits unambiguously the quantum Hall effect in a magnetic field up to 30 T, while the electron system shows clearly developing Hall plateaus at integer Landau level filling factors accompanied by R-xx oscillations, signaling the onset of the quantum Hall effect. By bringing the spin-resolved Landau levels of the electron system to a coincidence, we determine an electron spin susceptibility to be chi(se) = m*g* = 1.1 +/- 0.03, which, combined with the electron mass m* = 0.39m(0), yields a Lande g factor g* = 2.8 +/- 0.2. The enhancement of spin susceptibility in the black phosphorus two-dimensional electron system is around 50% compared with band susceptibility, which agrees well with various two-dimensional charge-carrier systems with weak spin-orbit coupling, suggesting the important role played by the exchange interaction.

3.Univ. of Tokyo, 4. IMR, Tohoku Univ.) E-mail: kei.takahashi@riken.jp 遷移金属酸化物は、電子相関が強く多様な物性を示すため半導体デバイスを凌駕することが期 待されている。近年、パルスレーザー堆積法(Pulsed Laser Deposition: PLD)によるエピタキシャ ル技術が飛躍的に発達したことにより、遷移金属酸化物薄膜の原子レベルで急峻な界面が合成可 能になり超格子による機能化や界面二次元電子の創出などの研究成果が数多く報告されている[1]。 しかし、PLD は熱力学的に非平衡性の強いプロセスであるため、局所平衡が成り立つプロセスで 合成されたバルク結晶に比べ、原理的に結晶性が悪くなってしまう。例えば金属や半導体の高結 晶性の指標である残留抵抗や電子移動度を比べると、ほとんどの場合バルク単結晶の方が薄膜よ り優れている。そのためこれまでの研究では、期待される物性や機能が薄膜化による結晶性の低 下で発現しないことが多かった。 本研究では、この問題を解決することで新たなブレークスルーを目指し、酸化物薄膜の結晶性 を飛躍的に向上させる成長方法を開発した。分子線エピタキシー(Molecular Beam Epitaxy: MBE) は、成長時の原子の運動エネルギーが低く平衡プロセスに近い。しかし、これまでの遷移金属酸 化物のMBE成長では、遷移金属の蒸気圧が低く原子フラックスを安定的に得ることが困難である ため、得られる薄膜の結晶性も他の成長法と大差ないことが多く、優位性が顕在化していなかっ た。我々は数年前から、MBEで遷移金属単体を蒸発させるのではなく、蒸気圧の高い揮発性の有 機金属ガスを用いたガスソース MBE が有効であると考えて開発に取り組んできた。先行研究[2] を参考に、半導体レーザーを用いた基板加熱による高温成長を組みあわせたガスソースMBEを開 発した。これまでに、デルタドープ SrTiO3の二次元電子移動度の向上による量子ホール効果の観 察[3]、希薄電子ドープ EuTiO3の異常ホール効果の観察、Laドープ BaTiO3薄膜の極性金属の実現 に成功している。これらの結果は、他の成長法で合成した結晶性の低い薄膜では得られなかった ものであり、結晶性の高い薄膜が合成可能なガスソースMBEが酸化物のポテンシャルを引き出す 強力なツールであることを示している。講演では、これまでに得られた結果と将来の展望につい て議論する。 [1] H. Y. Hwang et al. “Emergent phenomena at oxide interfaces” Nature Mater. 11, 103 (2012). [2] J. Son et al. “Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm Vs” Nature Mater. 9, 482 (2010). [3] Y. Matsubara et al. submitted. 第63回応用物理学会春季学術講演会 講演予稿集 (2016 東京工業大学 大岡山キャンパス) 20p-H111-2

We demonstrate that a field-effect transistor (FET) made of few-layer black phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum exhibits a room-temperature hole mobility of 5200 cm2/(Vs), being limited just by the phonon scattering. At cryogenic temperatures, the FET mobility increases up to 45 000 cm2/(Vs), which is five times higher compared to the mobility obtained in earlier reports. The unprecedentedly clean h-BN-BP-h-BN heterostructure exhibits Shubnikov-de Haas oscillations and a quantum Hall effect with Landau level (LL) filling factors down to v = 2 in conventional laboratory magnetic fields. Moreover, carrier density independent effective mass of m* = 0.26 m0 is measured, and a Landé g-factor of g = 2.47 is reported. Furthermore, an indication for a distinct hole transport behavior with up- and down-spin orientations is found.

The quantum Hall effect is a macroscopic quantum phenomenon in a two-dimensional electron system. The two-dimensional electron system in SrTiO 3 has sparked a great deal of interest, mainly because of the strong electron correlation effects expected from the 3 d orbitals. Here we report the observation of the quantum Hall effect in a dilute La-doped SrTiO 3 -two-dimensional electron system, fabricated by metal organic molecular-beam epitaxy. The quantized Hall plateaus are found to be solely stemming from the low Landau levels with even integer-filling factors, ν =4 and 6 without any contribution from odd ν ’s. For ν =4, the corresponding plateau disappears on decreasing the carrier density. Such peculiar behaviours are proposed to be due to the crossing between the Landau levels originating from the two subbands composed of d orbitals with different effective masses. Our findings pave a way to explore unprecedented quantum phenomena in d -electron systems.

Microwave induced resistance and photovoltage oscillations were investigated in MgxZn1-xO/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35-120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m* = (0.35 +/- 0.01)m(0), which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps.

The fractional quantum Hall (FQH) effect emerges in high-quality two-dimensional electron systems exposed to a magnetic field when the Landau-level filling factor, ν e , takes on a rational value. Although the overwhelming majority of FQH states have odd-denominator fillings, the physical properties of the rare and fragile even-denominator states are most tantalizing in view of their potential relevance for topological quantum computation. For decades, GaAs has been the preferred host for studying these even-denominator states, where they occur at ν e = 5/2 and 7/2. Here we report an anomalous series of quantized even-denominator FQH states outside the realm of III–V semiconductors in the MgZnO/ZnO 2DES electron at ν e = 3/2 and 7/2, with precursor features at 9/2; all while the 5/2 state is absent. The effect in this material occurs concomitantly with tunability of the orbital character of electrons at the chemical potential, thereby realizing a new experimental means for investigating these exotic ground states.