Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping.

Identifying material parameters affecting properties of ferromagnets is key to optimized materials that are better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two-dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet and explore the physical origin of this effect. Fe3-xGeTe2 nanoflakes show a significant suppression of the magnetic anisotropy with hole doping. Electronic structure measurements and calculations reveal that the chemical potential shift associated with hole doping is responsible for the reduced magnetic anisotropy by decreasing the energy gain from the spin-orbit induced band splitting. Our findings provide an understanding of the intricate connection between electronic structures and magnetic properties in two-dimensional magnets and propose a method to engineer magnetic properties through doping.

Ferromagnet-Free All-Electric Spin Hall Transistors.

The spin field-effect transistor, an essential building block for spin information processing, shows promise for energy-efficient computing. Despite steady progress, it suffers from a low-output signal because of low spin injection and detection efficiencies. We demonstrate that this low-output obstacle can be overcome by utilizing direct and inverse spin Hall effects for spin injection and detection, respectively, without a ferromagnetic component. The output voltage of our all-electric spin Hall transistor is about two orders of magnitude larger than that of previously reported spin transistors based on ferromagnets or quantum point contacts. Moreover, the symmetry of the spin Hall effect allows all-electric spin Hall transistors to effectively mimic n-type and p-type devices, opening a way of realizing the complementary functionality.

Nonlocal Spin Diffusion Driven by Giant Spin Hall Effect at Oxide Heterointerfaces.

A two-dimensional electron gas emerged at a LaAlO3/SrTiO3 interface is an ideal system for "spin-orbitronics" as the structure itself strongly couple the spin and orbital degree of freedom through the Rashba spin-orbit interaction. One of core experiments toward this direction is the nonlocal spin transport measurement, which has remained elusive due to the low spin injection efficiency to this system. Here we bypass the problem by generating a spin current not through the spin injection from outside but instead through the inherent spin Hall effect and demonstrate the nonlocal spin transport. The analysis on the nonlocal spin voltage, confirmed by the signature of a Larmor spin precession and its length dependence, displays that both D'yakonov-Perel' and Elliott-Yafet mechanisms involve in the spin relaxation at low temperature. Our results show that the oxide heterointerface is highly efficient in spin-charge conversion with exceptionally strong spin Hall coefficient γ ∼ 0.15 ± 0.05 and could be an outstanding platform for the study of coupled charge and spin transport phenomena and their electronic applications.

Formation and magnetic properties of InFeP:Ag nanorods fabricated with noble metal Ag using an ion milling method.

The formation, including the density and height of the InFeP:Ag nanorods doped with noble metal Ag using an ion milling method, was preponderantly determined from transmission electron microscopy and x-ray diffraction analyses. We investigate, in particular, the enhanced ferromagnetism of the well-aligned InFeP:Ag nanorods. Auger electron spectroscopy and x-ray photoelectron spectroscopy measurements were carried out in order to investigate the incorporation of Ag and to verify the local chemical bonding of the InFeP:Ag nanorods. The variation of FWHM for the double-crystal x-ray rocking curve and triple-axis diffraction peaks demonstrates that noble metal Ag is incorporated into the InFeP:Ag nanorods. The noticeable ferromagnetic signature (M-H curve) of the InFeP:Ag nanorods is observed and T c persists up to almost 350 K (3.9 × 10-4 emu g-1), as determined by temperature-dependence magnetization (M-T curve) measurements. This study suggests that the InFeP:Ag nanorods should be a potential candidate for the application of spintronic devices.

Interaction between Rashba and Zeeman effects in a quantum well channel.

The applied field induced Zeeman effect interferes with Rashba effect in a quantum well system. The angle dependence of Shubnikov-de Haas oscillation shows that the in-plane term of the applied field changes the intrinsic Rashba induced spin splitting. The total effective spin-orbit interaction parameter is determined by the vector sum of the Rashba field and the applied field.

Gate-controlled spin-orbit coupling in InAs/InGaAs quantum well structures.

We have investigated gate electric field controlled Rashba spin-orbit coupling (SOC) constant (alpha) in In0.53Ga0.47As and InAs-inserted quantum well (QW) structures. More than three times larger gate controllability of a in the InAs-inserted QW has been observed compared to the In0.53Ga0.47As QW. The enhanced gate controllability of alpha directly results from the larger zero-field SOC in narrow band gap InAs QW. Furthermore, the lower contact resistance and higher electron mobility imply that the InAs QW is a more promising channel for spintronic device applications.

Boltzmann Switching MoS2 Metal-Semiconductor Field-Effect Transistors Enabled by Monolithic-Oxide-Gapped Metal Gates at the Schottky-Mott Limit.

A gate stack that facilitates a high-quality interface and tight electrostatic control is crucial for realizing high-performance and low-power field-effect transistors (FETs). However, when constructing conventional metal-oxide-semiconductor structures with two-dimensional (2D) transition metal dichalcogenide channels, achieving these requirements becomes challenging due to inherent difficulties in obtaining high-quality gate dielectrics through native oxidation or film deposition. Here, a gate-dielectric-less device architecture of van der Waals Schottky gated metal-semiconductor FETs (vdW-SG MESFETs) using a molybdenum disulfide (MoS2) channel and surface-oxidized metal gates such as nickel and copper is reported. Benefiting from the strong SG coupling, these MESFETs operate at remarkably low gate voltages, <0.5 V. Notably, they also exhibit Boltzmann-limited switching behavior featured by a subthreshold swing of ≈60 mV dec-1 and negligible hysteresis. These ideal FET characteristics are attributed to the formation of a Fermi-level (EF) pinning-free gate stack at the Schottky-Mott limit. Furthermore, authors experimentally and theoretically confirm that EF depinning can be achieved by suppressing both metal-induced and disorder-induced gap states at the interface between the monolithic-oxide-gapped metal gate and the MoS2 channel. This work paves a new route for designing high-performance and energy-efficient 2D electronics.

Single crystalline ß-Ag2Te nanowire as a new topological insulator.

A recent theoretical study suggested that Ag(2)Te is a topological insulator with a highly anisotropic Dirac cone. Novel physics in the topological insulators with an anisotropic Dirac cone is anticipated due to the violation of rotational invariance. From magnetoresistance (MR) measurements of Ag(2)Te nanowires (NWs), we have observed Aharanov-Bohm (AB) oscillation, which is attributed to the quantum interference of electron phase around the perimeter of the NW. Angle and temperature dependences of the AB oscillation indicate the existence of conducting surface states in the NWs, confirming that Ag(2)Te is a topological insulator. For Ag(2)Te nanoplates (NPLs), we have observed high carrier mobility exceeding 22,000 cm(2)/(V s) and pronounced Shubnikov-de Haas (SdH) oscillation. From the SdH oscillation, we have obtained Fermi state parameters of the Ag(2)Te NPLs, which can provide valuable information on Ag(2)Te. Understanding the basic physics of the topological insulator with an anisotropic Dirac cone could lead to new applications in nanoelectronics and spintronics.

Feasibility of placing a modified fully covered self-expandable metal stent above the papilla to minimize stent-induced bile duct injury in patients with refractory benign biliary strictures (with videos).

BACKGROUND: Endoscopic placement of fully covered self-expandable metal stents (FCSEMS) has been attempted to manage benign biliary strictures, but currently available FCSEMSs may be associated with unintended complications, including de novo strictures, in patients with normal life expectancy. OBJECTIVE: To evaluate the feasibility of an intraductally placed modified FCSEMS to minimize stent-induced bile duct injury in patients with benign biliary strictures. DESIGN: Prospective observational clinical feasibility study. SETTING: Tertiary-care academic center. PATIENTS: This study involved 21 patients with symptomatic benign biliary strictures in whom conventional endoscopic management failed. INTERVENTION: Strictured segments were 15 mm above the ampulla of Vater. The modified FCSEMS has convex margins, a lasso, and an anti-migrating waist on the central portion. Stents were placed entirely above the papilla and removed after 3 to 5 months. MAIN OUTCOME MEASUREMENTS: Success, complications, removability, midterm outcome. RESULTS: FCSEMSs were successfully placed inside the bile duct in all patients. No episodes of pancreatitis, cholangitis, or sepsis were noted during the stenting period. Stent migration occurred in 4 patients (19.0%), but 3 were asymptomatic during follow-up. All stents were removed successfully with rat-tooth forceps without complications. Post-stenting cholangiograms showed improvement of strictures in 20 of 21 patients, without de novo focal stricture. The clinical success rate was 95.2%, with one recurrent stricture. LIMITATIONS: The small number and lack of comparison with other types of FCSEMSs. CONCLUSION: Temporary intraductal placement of a newly modified FCSEMS effectively improved strictures and prevented potential stent-induced complications in patients with benign biliary strictures. Controlled large-scale trials are needed to confirm the long-term efficacy.

Effects of Interfacial Oxidization on Magnetic Damping and Spin-Orbit Torques.

We investigate the effects of interfacial oxidation on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques in heavy-metal (Pt)/ferromagnet (Co or NiFe)/capping (MgO/Ta, HfOx, or TaN) structures. At room temperature, the capping materials influence the effective surface magnetic anisotropy energy density, which is associated with the formation of interfacial magnetic oxides. The magnetic damping parameter of Co is considerably influenced by the capping material (especially MgO) while that of NiFe is not. This is possibly due to extra magnetic damping via spin-pumping process across the Co/CoO interface and incoherent magnon generation (spin fluctuation) developed in the antiferromagnetic CoO. It is also observed that both antidamping and field-like spin-orbit torque efficiencies vary with the capping material in the thickness ranges we examined. Our results reveal the crucial role of interfacial oxides on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques.

Interface Engineering of Magnetic Anisotropy in van der Waals Ferromagnet-based Heterostructures.

Interface engineering is an effective approach to tune the magnetic properties of van der Waals (vdW) magnets and their heterostructures. The prerequisites for the practical utilization of vdW magnets and heterostructures are a quantitative analysis of their magnetic anisotropy and the ability to modulate their interfacial properties, which have been challenging to achieve with conventional methods. Here we characterize the magnetic anisotropy of Fe3GeTe2 layers by employing the magnetometric technique based on anomalous Hall measurements and confirm its intrinsic nature. In addition, on the basis of the thickness dependences of the anisotropy field, we identify the interfacial and bulk contributions. Furthermore, we demonstrate that the interfacial anisotropy in Fe3GeTe2-based heterostructures is locally controlled by adjacent layers, leading to the realization of multiple magnetic behaviors in a single channel. This work proposes that the magnetometric technique is a useful platform for investigating the intrinsic properties of vdW magnets and that functional devices can be realized by local interface engineering.

Orbital torque in magnetic bilayers.

The orbital Hall effect describes the generation of the orbital current flowing in a perpendicular direction to an external electric field, analogous to the spin Hall effect. As the orbital current carries the angular momentum as the spin current does, injection of the orbital current into a ferromagnet can result in torque on the magnetization, which provides a way to detect the orbital Hall effect. With this motivation, we examine the current-induced spin-orbit torques in various ferromagnet/heavy metal bilayers by theory and experiment. Analysis of the magnetic torque reveals the presence of the contribution from the orbital Hall effect in the heavy metal, which competes with the contribution from the spin Hall effect. In particular, we find that the net torque in Ni/Ta bilayers is opposite in sign to the spin Hall theory prediction but instead consistent with the orbital Hall theory, which unambiguously confirms the orbital torque generated by the orbital Hall effect. Our finding opens a possibility of utilizing the orbital current for spintronic device applications, and it will invigorate researches on spin-orbit-coupled phenomena based on orbital engineering.

Endoscopic removal of an enterolith causing afferent loop syndrome using electrohydraulic lithotripsy.

Electrohydraulic lithotripsy is a very useful method for fragmenting biliary stones and it can be used for endoscopic removal of difficult biliary stones. Acute afferent loop syndrome induced by enterolith is very rare, and surgical treatment is the usual choice for this condition. We describe a patient with acute afferent loop syndrome, which was induced by an enterolith after a Billroth II gastrectomy. We used electrohydraulic lithotripsy to endoscopically remove the enterolith.

Rashba Effect in Functional Spintronic Devices.

Exploiting spin transport increases the functionality of electronic devices and enables such devices to overcome physical limitations related to speed and power. Utilizing the Rashba effect at the interface of heterostructures provides promising opportunities toward the development of high-performance devices because it enables electrical control of the spin information. Herein, the focus is mainly on progress related to the two most compelling devices that exploit the Rashba effect: spin transistors and spin-orbit torque devices. For spin field-effect transistors, the gate-voltage manipulation of the Rashba effect and subsequent control of the spin precession are discussed, including for all-electric spin field-effect transistors. For spin-orbit torque devices, recent theories and experiments on interface-generated spin current are discussed. The future directions of manipulating the Rashba effect to realize fully integrated spin logic and memory devices are also discussed.

Direct peroral cholangioscopy using an ultra-slim upper endoscope for the treatment of retained bile duct stones.

OBJECTIVES: Intracorporeal lithotripsy must be used under direct visual control with an additional endoscope in order to ensure safety and precise targeting during the removal of difficult-to-extract bile duct stones using conventional methods. The currently available "mother-baby" scope system has several disadvantages. We evaluated the feasibility and efficacy of direct peroral cholangioscopy (POC) using an ultra-slim upper endoscope for electrohydraulic lithotripsy (EHL) or laser lithotripsy (LL) for the treatment of difficult-to-extract bile duct stones. METHODS: A total of 18 patients who were unsuccessfully treated for common bile duct (CBD) stones using conventional endoscopy, including mechanical lithotripsy (ML), underwent EHL or LL under intraductal balloon-guided direct POC using an ultra-slim upper endoscope. The balloon catheter was used to maintain access while an ultra-slim upper endoscope was advanced directly into the bile duct. EHL or LL was performed until it became possible to capture the fragmented stones in a basket. Endoscopic removal of stone fragments by duodenoscopy was attempted in the same session. RESULTS: The overall success rate of bile duct clearance by lithotripsy under direct POC by a single endoscopist was 88.9% (16 of 18). Stone fragmentation under direct POC was successfully performed in nine patients using EHL and in seven patients using LL. The average number of treatment sessions required to complete stone removal was 1.6. ML was performed to complete stone removal in 5 of 18 (27.8%) patients. Procedure-related complications were not observed. CONCLUSIONS: Lithotripsy with EHL or LL under direct POC involving the use of an ultra-slim endoscope by a single endoscopist seems to be an effective and safe treatment for select patients with difficult-to-extract CBD stones.

Effect of biliary stenting combined with ursodeoxycholic acid and terpene treatment on retained common bile duct stones in elderly patients: a multicenter study.

OBJECTIVES: For frail, elderly patients with large impacted common bile duct (CBD) stones, long-term treatment with biliary stenting provides palliation. Biliary stenting with choleretic agents such as ursodeoxycholic acid (UDCA) and terpene preparations may promote CBD stone size reduction. We studied the effectiveness of biliary stenting combined with UDCA and a terpene preparation as a medical treatment for difficult-to-remove CBD stones in patients older than 65 years in this multicenter, prospective study. METHODS: A total of 28 elderly patients with CBD stones refractory to conventional endoscopic removal, including mechanical lithotripsy, underwent endoscopic placement of a straight 10-F plastic biliary stent. Each patient was administered 600 mg of UDCA and 300 mg of a terpene preparation daily for a mean of 6 months. After 6 months of medication following the initial endoscopic retrograde cholangiopancreatography (ERCP), a second ERCP was performed and endoscopic stone removal was again attempted. Differences in stone size and CBD diameter before and after biliary stenting and medication were compared. The complete stone removal rate after treatment was obtained. RESULTS: The mean size (transverse x longitudinal diameter) of the CBD stones was initially 21.6 x 29.5 mm, and it decreased significantly to 12.2 x 20.1 mm at the second ERCP (P<0.01). The mean CBD diameter measured initially at the cystic duct insertion level was 23.2 mm, and it decreased significantly to 19.5 mm at the second ERCP (P<0.01). After biliary stenting with medication, endoscopic stone removal was successful in 26 of 28 patients (92.8%), with a mean of 1.7 subsequent ERCP sessions. CONCLUSIONS: Endoscopic biliary stenting with a period of combined UDCA and terpene preparation administration seems to be a safe and effective method for retained CBD stones in elderly patients. A prospective study with randomization and a control group is required to confirm these results.

The utility of a multibending endoscope for selective cannulation during ERCP in patients with a Billroth II gastrectomy (with video).

BACKGROUND: Selective cannulation in patients with a Billroth II gastrectomy is still challenging. OBJECTIVE: To evaluate the usefulness of a multibending, forward-viewing endoscope (M-scope) for selective cannulation during diagnostic or therapeutic ERCP in patients with a Billroth II gastrectomy. DESIGN: Case series. SETTING: Tertiary center. PATIENTS: Fourteen patients having biliary disease with a Billroth II gastrectomy in whom selective cannulation failed when using a conventional forward-viewing endoscope. INTERVENTIONS: In all cases, we attempted selective biliary cannulation for ERCP with a single-bending, forward-viewing endoscope for 10 minutes. After failure with the conventional endoscope, we retried selective cannulation with the M-scope for 10 minutes. After cannulation, the diagnostic or therapeutic endoscopic procedures were performed. MAIN OUTCOME MEASUREMENTS: We assessed the success rate of selective cannulation, the possibility of therapeutic approaches, and procedure-related complications. RESULTS: In all cases, we successfully reached the ampulla of Vater with the M-scope. The overall success rate of selective cannulation with the M-scope was 92.9% (13/14). One patient developed mild pancreatitis. Therapeutic procedures such as sphincterotomy, balloon dilatation, stone removal, and biliary drainage were all possible. LIMITATIONS: Small number of patients; uncontrolled, single-center study. CONCLUSIONS: The M-scope seems to be helpful for selective cannulation during ERCP in patients with a Billroth II gastrectomy. All diagnostic and therapeutic procedures were possible through the M-scope.

Overtube-balloon-assisted direct peroral cholangioscopy by using an ultra-slim upper endoscope (with videos).

BACKGROUND: The "mother-baby" endoscope system currently used for peroral cholangioscopy (POC) has several limitations. Endoscopic direct cholangioscopy when using an ultra-slim upper endoscope with a guidewire to maintain access has been reported, but appropriate accessories are required to increase the success rate. Herein, we describe a novel method, overtube-balloon-assisted endoscopy, for direct POC. OBJECTIVE: To evaluate the feasibility and usefulness of direct POC when using an ultra-slim upper endoscope with an overtube balloon to maintain access. SETTING: A single center. DESIGN: Case series. PATIENTS: Twelve patients with biliary disease. INTERVENTIONS: Direct POC by using an ultra-slim upper endoscope and diagnostic and/or therapeutic procedures. MAIN OUTCOME MEASUREMENTS: Success rate of this technique, diagnostic or therapeutic feasibility, and complications. RESULTS: Overtube-balloon-assisted direct POC was performed successfully in 10 of 12 patients (83.3%). The procedure revealed 4 common bile duct (CBD) stones, 4 benign biliary strictures, 1 polypoid tumor, and 1 cholangiocarcinoma. Five patients underwent forceps biopsy under direct visualization of the intraductal lesion. Laser lithotripsy was successfully performed in 1 patient. No procedure-related complication occurred. LIMITATIONS: A small number of patients and no comparison with conventional cholangioscopy. CONCLUSIONS: The overtube balloon appears to be a useful accessory in direct POC when using an ultra-slim upper endoscope. However, further development of a slim overtube or other accessories is necessary to improve the success rate of direct POC.

Combined endoscopic stent-in-stent placement for malignant biliary and duodenal obstruction by using a new duodenal metal stent (with videos).

BACKGROUND: Self-expandable metal stents (SEMSs) are an effective palliative treatment for malignant biliary or duodenal strictures, but endoscopic stenting for combined malignant biliary and duodenal obstruction remains technically difficult. OBJECTIVE: To evaluate the feasibility and clinical success rate of endoscopic double stent-in-stent placement by using a new duodenal metallic stent for the management of malignant biliary and duodenal obstructions. DESIGN: Prospective, observational clinical feasibility study. SETTING: Tertiary referral center. MAIN OUTCOME MEASUREMENT: Technical and clinical success, complications, midterm outcome. PATIENTS AND METHODS: Eight patients with inoperable malignant biliary and duodenal strictures were enrolled. Endoscopic placement of a biliary SEMS was performed through the mesh of the duodenal stent. RESULTS: The duodenal strictures were in the first part of the duodenum in 3 patients (type 1) and in the second part in 5 patients (type 2). Duodenal stents were successfully deployed in all patients. Endoscopic placement of a biliary SEMS through the mesh of the duodenal stent was successful in 7 (87.5%) of 8 patients. In 2 of 3 patients with type 2 duodenal strictures with failed bile duct cannulation, biliary stenting was successful by using the rendezvous procedure. Early complications occurred in 1 patient. The overall median survival after combined stenting was 91 days (range 36-314 days). LIMITATIONS: Small number of patients and no comparison with other enteral stents. CONCLUSIONS: Biliary stenting through the new duodenal metallic stent is technically feasible and has a high success rate. Combined endoscopic biliary and duodenal stent-in-stent placement is a promising solution for the palliation of malignant biliary and duodenal obstruction.

Gate-tunable giant nonreciprocal charge transport in noncentrosymmetric oxide interfaces.

A polar conductor, where inversion symmetry is broken, may exhibit directional propagation of itinerant electrons, i.e., the rightward and leftward currents differ from each other, when time-reversal symmetry is also broken. This potential rectification effect was shown to be very weak due to the fact that the kinetic energy is much higher than the energies associated with symmetry breaking, producing weak perturbations. Here we demonstrate the appearance of giant nonreciprocal charge transport in the conductive oxide interface, LaAlO3/SrTiO3, where the electrons are confined to two-dimensions with low Fermi energy. In addition, the Rashba spin-orbit interaction correlated with the sub-band hierarchy of this system enables a strongly tunable nonreciprocal response by applying a gate voltage. The observed behavior of directional response in LaAlO3/SrTiO3 is associated with comparable energy scales among kinetic energy, spin-orbit interaction, and magnetic field, which inspires a promising route to enhance nonreciprocal response and its functionalities in spin orbitronics.