Hongtao Yuan

Stanford University | SU · Geballe Laboratory for Advanced Materials and SLAC

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photo...

Many-body interactions between quasiparticles (electrons, excitons, and phonons) have led to the emergence of new complex correlated states and are at the core of condensed matter physics and material science. In low-dimensional materials, unique electronic properties for these correlated states could significantly affect their optical properties....

The technological appeal of van der Waals ferromagnetic materials is the ability to control magnetism under external fields with desired thickness towards novel spintronic applications. For practically useful devices, ferromagnetism above room temperature or tunable magnetic anisotropy is highly demanded but remains challenging. To date, only a few...

Strain engineering can serve as a powerful technique for modulating the exotic properties arising from the atomic structure of materials. Examples have been demonstrated that one-dimensional (1D) structure can serve as a great platform for modulating electronic band structure and phonon dispersion via strain control. Particularly, in a van der Waal...

Single-crystalline α-Sn films on the wafer scale are epitaxially grown on InSb(001) substrates by molecular-beam epitaxy. The Berry phase for α-Sn is extracted to be −0.64π from the quantum oscillations of the magnetoconductivity. Angle- and temperature-dependent Shubnikov–de Haas oscillations substantiate that the α-Sn film has a spherical Fermi s...

Identifying air-stable two-dimensional (2D) ferromagnetism with high Curie temperature (Tc) is highly desirable for its potential applications in next-generation spintronics. However, most of the work reported so far mainly focuses on promoting one specific key factor of 2D ferromagnetism (Tc or air stability), rather than comprehensive promotion o...

The recent discovery of ferromagnetism in two-dimensional van der Waals crystals has provoked a surge of interest in the exploration of fundamental spin interaction in reduced dimensions. However, existing material candidates have several limitations, notably lacking intrinsic room-temperature ferromagnetic order and air stability. Here, motivated...

Van der Waals interfaces can be formed by layer stacking without regard to lattice constants or symmetries of individual building blocks. We engineered the symmetry of a van der Waals interface of tungsten selenide and black phosphorus and realized in-plane electronic polarization that led to the emergence of a spontaneous photovoltaic effect. Spon...

Topological superconductors (TSCs), with the capability to host Majorana bound states that can lead to non-Abelian statistics and application in quantum computation, have been one of the most intensively studied topics in condensed matter physics recently. To date, only a few compounds have been proposed as candidates of intrinsic TSCs, such as dop...

Topological superconductors (TSCs), with the capability to host Majorana bound states that can lead to non-Abelian statistics and application in quantum computation, have been one of the most intensively studied topics in condensed matter physics recently. Up to date, only a few compounds have been proposed as candidates of intrinsic TSCs, such as...

Atomically thin oxychalcogenides have been attracting intensive attention for their fascinating fundamental properties and application prospects. Bi2O2Se, a representative of layered oxychalcogenides, has emerged as an air‐stable high‐mobility 2D semiconductor that holds great promise for next‐generation electronics. The preparation and device fabr...

An amendment to this paper has been published and can be accessed via a link at the top of the paper

The original HTML version of this Article omitted to list Harold Y. Hwang as a corresponding author and incorrectly listed Adrian G. Swartz as a corresponding author. This has been corrected in the HTML version of the Article. The PDF version was correct from the time of publication.

Quantum ground states that arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional metallic behaviour often observed adjacent to superconductivity. At the superconducting LaAlO3/SrTiO3 interface, a metallic ground state emerges upon the coll...

Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layere...

Infrared detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of new photoelectric response materials. The rise of two-dimensional (2D) materials, thanks to their distinct electronic structure, extreme dimensional confinement and strong light-matter intera...

Quantum ground states which arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional (2D) metallic behaviour often observed adjacent to superconductivity. At the superconducting LaAlO3/SrTiO3 interface, a metallic ground state emerges upon th...

Ionic liquids and gels have attracted attention for a variety of energy storage applications, as well as for high performance electrolytes for batteries and super-capacitors. Although the electronic structure of ionic electrolytes in these applications is of practical importance for device design and improved performance, the understanding of the e...

Doped semiconductors are the most important building elements for modern electronic devices1. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface2,3. Besides, the emergence of two-dimensional (2D) materials enables the realizatio...

Electric double layer (EDL) gating with liquid electrolyte has been a powerful tool widely used to explore emerging interfacial electronic phenomena. Due to the large EDL capacitance, a high carrier density up to 1014 cm-2 can be induced, directly leading to the realization of field-induced insulator to metal (or superconductor) transition. However...

Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) superconductivity at/near the atomically thin limit. In particular, gate-induced interfacial superconductivity realized by the use of an electric-double-layer transistor (EDLT) has greatly extended the capability to electrica...

Superconductors at the atomic two-dimensional (2D) limit are the focus of an enduring fascination in the condensed matter community. This is because, with reduced dimensions, the effects of disorders, fluctuations, and correlations in superconductors become particularly prominent at the atomic 2D limit; thus such superconductors provide opportuniti...

Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching th...

High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal...

Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the co...

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations...

Two-dimensional transition metal dichalcogenides are emerging with tremendous potential in many optoelectronic applications due to their strong light-matter interactions. To fully explore their potential in photoconductive detectors, high responsivity and weak signal detection are required. Here, we present high responsivity phototransistors based...

Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~...

Electrostatic modification of functional materials by electrolytic gating has demonstrated a remarkably wide range of density modulation, a condition crucial for developing novel electronic phases in systems ranging from complex oxides to layered chalcogenides. Yet little is known microscopically when carriers are modulated in electrolyte-gated ele...

The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodet...

Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼...

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as top candidates for post-silicon electronics. While most of 2D TMDs exhibit isotropic behavior, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here, we present atomically th...

We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600-nm-thick, non-absorbing silica spacer layer. An ionic...

Lattice structure and symmetry of two-dimensional (2D) layered materials are of key importance to their fundamental mechanical, thermal, electronic, and optical properties. Raman spectroscopy, as a convenient and nondestructive tool, however has its limitations on identifying all symmetry allowing Raman modes and determining the corresponding cryst...

Electron occupation of orbitals in two-dimensional (2D) layered materials controls the magnitude and anisotropy of the interatomic electron transfer and exerts a key influence on the chemical bonding modes of 2D layered lattices. Therefore, their orbital occupations are believed to be responsible for massive variations of the physical and chemical...

The development of two-dimensional (2D) materials has been experiencing a renaissance since the adventure of graphene. Layered transition metal dichalcogenides (TMDs) are now playing increasingly important roles in both fundamental studies and technological applications due to their wide range of material properties from semiconductors, metals to s...

Graphene nanoribbons (GNRs) are promising building blocks for high-performance electronics due to their high electron mobility and dimensionality-induced bandgap. Despite many past efforts, direct synthesis of GNRs with controlled dimensions and scalability remains challenging. Here we report the scalable synthesis of GNRs using electrospun polymer...

Doped indium tin oxide (ITO) behaves as a Drude metal with a plasma frequency that is controlled by its free carrier density. In this work, we systematically tune this frequency across the mid-infrared range by annealing treatments in a reducing environment that produce high electron concentrations (similar to 10(21) cm(-3)). The changes in ITO's o...

The ability to detect light over a broad spectral range is central for practical optoelectronic applications, and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here we demonstrate a linear-dichroic b...

The valley degree of freedom in layered transition-metal dichalcogenides provides an opportunity to extend the functionalities of spintronics and valleytronics devices. The achievement of spin-coupled valley polarization induced by the non-equilibrium charge-carrier imbalance between two degenerate and inequivalent valleys has been demonstrated the...

We demonstrate a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The light absorption in the screen can be actively tuned by electrically gating the carrier density in the graphene layer with an ionic liquid/gel. The screen was designed to achieve maximum absorption at a t...

Field-effect transistors that employ an electrolyte in place of a gate dielectric layer can accumulate ultrahigh-density carriers not only on a well-defined channel (e.g., a two-dimensional surface) but also on any irregularly shaped channel material. Here, on thin films of 95% pure metallic and semiconducting single-walled carbon nanotubes (SWNTs)...

Electric field tuning of superconductivity has been a long-standing issue in solid state physics since the invention of the field-effect transistor (FET) in 1960. Owing to limited available carrier density in conventional FET devices, electric-field-induced superconductivity was believed to be possible in principle but impossible in practice. Howev...

Taking advantages of broad tunability of carrier density in electric-double-layer transistors (EDLTs) with ionic-liquid gating, we demonstrate evidence of parallel conduction from both p-type bulk and n-type surface in Mg-doped InN EDLTs by comparing their transport properties, especially Hall effect, with those in non-doped InN. Large anomalous os...

Electrical manipulation and read-out of quantum states in zero-dimensional nanostructures by nano-gap metal electrodes is expected to bring about innovation in quantum information processing. However, electrical tunability of the quantum states in zero-dimensional nanostructures is limited by the screening of gate electric fields. Here we demonstra...

Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct bandgap tunability, giant spin splitting, and valley related physics. However, the batch production of layered WS2 is under developed (as compared with that of MoS2) for exploring these fundamental issues and developin...

Since the discovery of room temperature ferromagnetism in (Ti,Co)O2, the mechanism has been under discussion for a decade. Particularly, the central concern has been whether or not the ferromagnetic exchange interaction is mediated by charge carriers like (Ga,Mn)As. Recent two studies on the control of ferromagnetism in anatase (Ti,Co)O2 at room te...

Transition-metal dichalcogenides such as WSe2 and MoS2 have electronic band structures that are ideal for hosting many exotic spin–orbit phenomena. Here we investigate the possibility to generate and modulate a giant Zeeman-type spin polarization in WSe2 under an external electric field. By tuning the perpendicular electric field applied to the WSe...

Thin film GdBiTe3 has been proposed as a candidate material in which to observe the quantum anomalous Hall effect. As a thermal non-equilibrium deposition method, molecular beam epitaxy (MBE) has the ability to incorporate large amounts of Gd into Bi2Te3 crystal structures. High-quality rhombohedral (GdxBi1−x)2Te3 films with substitutional Gd conce...

Electrically manipulating electron spins based on Rashba spin-orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DES) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by m...

Electric field induced ferromagnetism at room temperature in cobalt-doped titanium dioxide was demonstrated by means of electric double layer transistor. This result represents that a carrier-mediated exchange coupling plays a principal role in the high temperature ferromagnetism in this compound. Accordingly, this compound is a promising material...

We here report the result of in situ magnetization measurements of electrochemical cells at low temperatures. Upon applying voltages between the electrodes of the electrochemical cells, we observed shielding diamagnetic signals from several materials, indicating superconducting transitions. The superconducting states can be induced both electrochem...

In order to clarify its electric double layer (EDL) structure under external voltage, an interface between the ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide and a Au (111) surface was studied by x-ray reflectometry. A clear change in reflectivity was observed as a function of applied voltage. The el...

Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin-orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L(φ) and spin relaxation length L(so) of carriers can be effectively modulated with gate bias....

Recent success in isolating single-walled carbon nanotubes (SWNTs) of narrow chirality distribution enabled making pure metallic (m-) and semiconducting (s-) SWNT films. Such films are expected to reflect the nature of individual SWNTs, that is their one dimensional subband structure. Therefore, it is interesting to investigate electronic transport...

Ferromagnetic semiconductor is one of the most attractive materials for semiconductor spintronics because of the controllability of both charge and spin degrees of freedom. Electric field effect of magnetism in the ferromagnetic semiconductors such as (Ga,Mn)As has been demonstrated only at low temperature due to their low Curie temperatures. In th...

Generation of spin polarized electrons is the most critical step for developing spintronics applications. As an electric and nonmagnetic way to realize spin polarization in energy bands, spin-orbit interaction (SOI) has been widely used for spin manipulation in two-dimensional systems. For example, Rashba-type energy splitting with in-plane-polariz...

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V.