Design and performance of an ultrahigh vacuum spectroscopic-imaging scanning tunneling microscope with a hybrid vibration isolation system

A spectroscopic imaging-scanning tunneling microscope (SI-STM) allows for the atomic scale visualization of the surface electronic and magnetic structure of novel quantum materials with a high energy resolution. To achieve the optimal performance, a low vibration facility is required. Here, we describe the design and performance of an ultrahigh vacuum STM system supported by a hybrid vibration isolation system that consists of a pneumatic passive and a piezoelectric active vibration isolation stage. We present the detailed vibrational noise analysis of the hybrid vibration isolation system, which shows that the vibration level can be suppressed below 10-8 m/sec/√Hz for most frequencies up to 100 Hz. Combined with a rigid STM design, vibrational noise can be successfully removed from the tunneling current. We demonstrate the performance of our STM system by taking high resolution spectroscopic maps and topographic images on several quantum materials. Our results establish a new strategy to achieve an effective vibration isolation system for high-resolution STM and other scanning probe microscopies to investigate the nanoscale quantum phenomena.

Dirac Nodal Line in Hourglass Semimetal Nb3SiTe6

Glide-mirror symmetry in nonsymmorphic crystals can foster the emergence of novel hourglass nodal loop states. Here, we present spectroscopic signatures from angle-resolved photoemission of a predicted topological hourglass semimetal phase in Nb₃SiTe₆. Linear band crossings are observed at the zone boundary of Nb₃SiTe₆, which could be the origin of the nontrivial Berry phase and are consistent with a predicted glide quantum spin Hall effect; such linear band crossings connect to form a nodal loop. Furthermore, the saddle-like Fermi surface of Nb₃SiTe₆ observed in our results helps unveil linear band crossings that could be missed. In situ alkali-metal doping of Nb₃SiTe₆ also facilitated the observation of other band crossings and parabolic bands at the zone center correlated with accidental nodal loop states. Overall, our results complete the system's band structure, help explain prior Hall measurements, and suggest the existence of a nodal loop at the zone center of Nb₃SiTe₆.

Spin Wave Injection and Propagation in a Magnetic Nanochannel from a Vortex Core

Spin waves can be used as information carriers with low energy dissipation. The excitation and propagation of spin waves along reconfigurable magnonic circuits is the subject of much interest in the field of magnonic applications. Here we experimentally demonstrate an effective excitation of spin waves in reconfigurable magnetic textures at frequencies as high as 15 GHz and wavelengths as short as 80 nm from Ni₈₀Fe₂₀ (Py) nanodisk-film hybrid structures. Most importantly, we demonstrate these spin wave modes, which were previously confined within a nanodisk, can now couple to and propagate along a nanochannel formed by magnetic domain walls at zero magnetic bias field. The tunable high-frequency, short-wavelength, and propagating spin waves may play a vital role in energy efficient and programmable magnonic devices at the nanoscale.

Geometric quenching of orbital pair breaking in a single crystalline superconducting nanomesh network

In a superconductor Cooper pairs condense into a single state and in so doing support dissipation free charge flow and perfect diamagnetism. In a magnetic field the minimum kinetic energy of the Cooper pairs increases, producing an orbital pair breaking effect. We show that it is possible to significantly quench the orbital pair breaking effect for both parallel and perpendicular magnetic fields in a thin film superconductor with lateral nanostructure on a length scale smaller than the magnetic length. By growing an ultra-thin (2 nm thick) single crystalline Pb nanowire network, we establish nm scale lateral structure without introducing weak links. Our network suppresses orbital pair breaking for both perpendicular and in-plane fields with a negligible reduction in zero-field resistive critical temperatures. Our study opens a frontier in nanoscale superconductivity by providing a strategy for maintaining pairing in strong field environments in all directions with important technological implications.

The design and the performance of an ultrahigh vacuum 3He fridge-based scanning tunneling microscope with a double deck sample stage for in-situ tip treatment

Scanning tunneling microscope (STM) is a powerful tool for studying the structural and electronic properties of materials at the atomic scale. The combination of low temperature and high magnetic field for STM and related spectroscopy techniques allows us to investigate the novel physical properties of materials at these extreme conditions with high energy resolution. Here, we present the construction and the performance of an ultrahigh vacuum ³He fridge-based STM system with a 7 Tesla superconducting magnet. It features a double deck sample stage on the STM head so we can clean the tip by field emission or prepare a spin-polarized tip in situ without removing the sample from the STM. It is also capable of in situ sample and tip exchange and preparation. The energy resolution of scanning tunneling spectroscopy at T = 310 mK is determined to be 400 mK by measuring the superconducting gap with a niobium tip on a gold surface. We demonstrate the performance of this STM system by imaging the bicollinear magnetic order of Fe_1+xTe at T = 5 K.

Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2

The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exists the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. We report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point at E ≅ 1.0 eV, of which the inner TSS and the partial outer TSS cross EF, on the Pb-terminated surface of this fully gapped superconductor. This discovery reveals PbTaSe2 as a promising candidate for TSC.

Compact variable-temperature scanning force microscope

A compact design for a cryogenic variable-temperature scanning force microscope using a fiber-optic interferometer to measure cantilever deflection is presented. The tip-sample coarse approach and the lateral tip positioning are performed by piezoelectric positioners in situ. The microscope has been operated at temperatures between 6 and 300 K. It is designed to fit into an 8 T superconducting magnet with the field applied in the out-of-plane direction. The results of scanning in various modes are demonstrated, showing contrast based on magnetic field gradients or surface potentials.