Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3.

Layered α-RuCl3 is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this state has been reported under a modest in-plane magnetic field, such behaviour is largely inconsistent with theoretical expectations of spin liquid phases emerging only in out-of-plane fields. These predicted field-induced states have been largely out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunnelling spectroscopy, magnetotransport, electron diffraction and ab initio calculations to study the layer-dependent magnons, magnetic anisotropy, structure and exchange coupling in atomically thin samples. Due to picoscale distortions, the sign of the average off-diagonal exchange changes in monolayer α-RuCl3, leading to a reversal of spin anisotropy to easy-axis anisotropy, while the Kitaev interaction is concomitantly enhanced. Our work opens the door to the possible exploration of Kitaev physics in the true two-dimensional limit.

High-Performance Mid-IR to Deep-UV van der Waals Photodetectors Capable of Local Spectroscopy at Room Temperature.

The ability to perform broadband optical spectroscopy with subdiffraction-limit resolution is highly sought-after for a wide range of critical applications. However, sophisticated near-field techniques are currently required to achieve this goal. We bypass this challenge by demonstrating an extremely broadband photodetector based on a two-dimensional (2D) van der Waals heterostructure that is sensitive to light across over a decade in energy from the mid-infrared (MIR) to deep-ultraviolet (DUV) at room temperature. The devices feature high detectivity (>109 cm Hz1/2 W-1) together with high bandwidth (2.1 MHz). The active area can be further miniaturized to submicron dimensions, far below the diffraction limit for the longest detectable wavelength of 4.1 µm, enabling such devices for facile measurements of local optical properties on atomic-layer-thickness samples placed in close proximity. This work can lead to the development of low-cost and high-throughput photosensors for hyperspectral imaging at the nanoscale.

Giant c-axis nonlinear anomalous Hall effect in Td-MoTe2 and WTe2.

While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. The effect may arise from either Berry curvature or through various asymmetric scattering mechanisms. Here, we report the observation of an extremely large c-axis nonlinear anomalous Hall effect in the non-centrosymmetric Td phase of MoTe2 and WTe2 without intrinsic magnetic order. We find that the effect is dominated by skew-scattering at higher temperatures combined with another scattering process active at low temperatures. Application of higher bias yields an extremely large Hall ratio of E⊥/E|| = 2.47 and corresponding anomalous Hall conductivity of order 8 × 107 S/m.

Pseudogap and Weak Multifractality in 2D Disordered Mott Charge-Density-Wave Insulator.

We investigate electronic states of Se-substituted 1T-TaS2 by scanning tunneling microscopy/spectroscopy (STM/STS), where superconductivity emerges from the unique Mott-charge-density-wave (Mott-CDW) state. Spatially resolved STS measurements reveal that a pseudogap replaces the Mott gap with the CDW gaps intact. The pseudogap has little correlation with the unit-cell-to-unit-cell variation in the local Se concentration but appears globally. The correlation length of the local density of states (LDOS) is substantially enhanced at the Fermi energy and decays rapidly at high energies. Furthermore, the statistical analysis of LDOS indicates the weak multifractal behavior of the wave functions. These findings suggest a correlated metallic state induced by disorder and provide a new insight into the emerging superconductivity in two-dimensional materials.

Heterostructures of tellurium on NbSe2 from sub-monolayer to few-layer films.

As a single-elemental system, tellurium can exist stably in the form of layers with an intriguing multivalence character, which constructs a new member of the 2D family. However, the growth and electronic structure of tellurium films are still far from known at present. Here, combined with molecular beam epitaxy, scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations, we report the geometric and electronic structures of tellurium grown on NbSe2 from sub-monolayer to few-layer films. At the sub-monolayer coverage, we obtain two types of adatom-induced ordered superstructures that are strongly coupled with NbSe2. With the increase in coverage, the few-layer tellurium films adopt the α-phase form, showing internal strain-induced ripple patterns in the few-layers and bulk-like in thick layers with distinct edge geometries. The band gap of α-tellurium films decreases with the increase in thickness, which is associated with notable in-gap states. These observations, corroborated with DFT calculations, emphasize the important role of the NbSe2 substrate in modulating the structural and electronic properties of tellurium films. Moreover, the interaction between tellurium adatoms and tellurium films leads to √2 × âˆš2 surface reconstruction prior to a new monolayer, conforming to our theoretical calculations. Our work clarifies the kinetic growth of tellurium films on NbSe2 and reveals the tunability of electronic properties via substrate modulation or surface decoration.

A variable-temperature scanning tunneling microscope operated in a continuous flow cryostat.

We present a continuous flow cryostat scanning tunneling microscope (STM) which can be operated at temperatures between 4.9 K and 300 K. A variable temperature insert with 20 mm aperture is installed in the cryostat. A base temperature of 4.9 K at the STM head was achieved with helium exchange gas cooling. By using a novel design of zirconia structure, the size of the STM head was minimized to 9.2 mm in outer diameter. The high compactness and rigidity of the STM head make it less sensitive to vibrations, contributing to the high stability of the tip-sample junction. The drifting rates of the STM at 4.9 K in the X-Y plane and Z direction are 1.96 and 3.05 pm/min, respectively. The performance of the STM was demonstrated by atomically resolved imaging of graphite and NbSe2 without using external vibrational isolation. Furthermore, the dI/dV spectra of NbSe2 were resolved near its critical temperature, illustrating the formation process of the superconducting gap as a function of temperature. This STM is ultrahigh vacuum and strong magnetic field compatible, making it promising for direct application in the extreme conditions involving water-cooled magnets and hybrid magnets.

Atomically resolved probe-type scanning tunnelling microscope for use in harsh vibrational cryogen-free superconducting magnet.

We present a probe-type scanning tunnelling microscope (STM) with atomic resolution that is designed to be directly inserted and work in a harsh vibrational cryogen-free superconducting magnet system. When a commercial variable temperature insert (VTI) is installed in the magnet and the STM is housed in the VTI, a lowest temperature of 1.6 K can be achieved, at which the STM still operates well. We tested the STM in an 8 T superconducting magnet cooled with a pulse-tube cryocooler and obtained atomically resolved graphite and NbSe2 images as well as the scanning tunnelling spectrum (i.e., dI/dV spectrum) data of the latter near its critical temperature, which show the formation process of the superconducting gap as a function of temperature. The drifting rates of the STM at 1.6 K in the X-Y plane and Z direction are 1.15 and 1.71 pm/min, respectively. Noise analysis for the tunnelling current shows that the amplitudes of the dominant peaks (6.84 and 10.25 Hz) are as low as 1.5 pA.Hz-1/2 when we set the current to 0.5 nA and open the feedback loop. This is important as a cryogen-free magnet system has long been considered too harsh for any atomic resolution measurement.

Traumatic cardiac injury following sternal fracture: a case report and literature review.

Myocardial injury used to be thought as one of the major complications associated with sternal fracture even though recent studies on injuries associated with fracture of sternum are contrary to this belief. Many authors now believe the presence of sternal fracture is no longer indicative of occult injuries to the underlying structure such as the heart. However, clinicians should still maintain a high index of suspicion for the presence cardiac tamponade in cases presented as blunt chest trauma as early diagnosis and surgical intervention is vital to the patient's survival. Presented is a case of a 38-year-old female patient transferred to our hospital after being injured in a motor vehicle accident. On arrival her blood pressure (BP) was 90/50 mmHg but it then dropped to 60/30 mmHg two hours later. Although her chest roentgenography and electrocardiography (ECG) did not reveal any significant findings, the two-dimensional echocardiography was performed and revealed a moderate amount of pericardial effusion. The chest computerized tomography (CT) scan later revealed sternal fracture and cardiac tamponade. A diagnosis of cardiac rupture resulting from sternal fracture following blunt chest trauma was made. Under midline sternotomy, her right atrial rupture was repaired. The patient was reported to be doing well during a three months, post-operative follow-up.