Condensed Matter

Superconducting nanowire single-photon detector (SNSPD) with near-unity system efficiency is a key enabling, but still elusive technology for numerous quantum fundamental theory verifications and quantum information applications. The key challenge is to have both a near-unity photon-response probability and absorption efficiency simultaneously for the meandered nanowire with a finite filling ratio, which is more crucial for NbN than other superconducting materials (e.g., WSi) with lower transition temperatures. Here, we overcome the above challenge and produce NbN SNSPDs with a record system efficiency by replacing a single-layer nanowire with twin-layer nanowires on a dielectric mirror. The detector at 0.8 K shows a maximal system detection efficiency (SDE) of 98% at 1590 nm and a system efficiency of over 95% in the wavelength range of 1530-1630 nm. Moreover, the detector at 2.1K demonstrates a maximal SDE of 95% at 1550 nm using a compacted two-stage cryocooler. This type of detector also shows the robustness against various parameters, such as the geometrical size of the nanowire, and the spectral bandwidth, enabling a high yield of 73% (36%) with an SDE of >80% (90%) at 2.1K for 45 detectors fabricated in the same run. These SNSPDs made of twin-layer nanowires are of important practical significance for batch production.

A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridised in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behaviour was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behaviour. The measurement approach would be a useful diagnostic for axion dark matter searches, which plan on using superconducting materials but need to know precisely the internal magnetic field.

We report theoretical and experimental results on transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP exhibits the nonsymmorphic symmetries and is an anisotropic three-dimensional superconductor. This conclusion is supported by magnetoresistance experimental data as well as by the investigation of the superconducting fluctuations of the conductivity in the presence of a magnetic external field, both underlining a three dimensional behavior.

The idea that preformed Cooper pairs could exist in a superconductor above its zero-resistance state has been explored for unconventional, interface, and disordered superconductors, yet direct experimental evidence is lacking. Here, we use scanning tunneling noise spectroscopy to unambiguously show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature T C in the disordered superconductor titanium nitride, by observing a clear enhancement in the shot noise that is equivalent to a change of the effective charge from 1 to 2 electron charges. We further show that spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above T C that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons.

A conventional superconductor (SC) in the two-dimensional (2D) limit has a low transition temperature T c , and low superfluid density (SFD), resulting in fragile superconductivity [1-3]. Previous investigations using highly disordered granular films have also shown a rapid suppression of both T c and the SFD with thickness reduction, eventually resulting in a superconductor-insulator transition [4-8]. The emergence of single crystal films, however, reveals surprises: at a thickness of only five atoms, Pb films still show remarkably high superfluid rigidity with robust superconductivity [9], indicating the need for a close examination of phase rigidity in single crystal superconducting films. Using Indium 7 – √ × 3 – √ on Si(111) as a single layer superconductor [10], we study phase fluctuations by in situ measurement of both the macroscopic SFD and the microscopic quasi-particle excitation spectrum. We demonstrate a quantitative control of the superfluid phase rigidity by systematically increasing point defects. We further reveal how the density and morphology of defects impact the superconducting order parameter at both local and global scales. We measure both the Bardeen-Cooper-Schrieffer (BCS) [11] and Berezinskii-Kosterlitz-Thouless (BKT) [12-15] transition temperatures as the phase rigidity is varied, from which a 2D SC phase diagram is established, with generic features applicable to other ultrathin superconducting systems.

Pair density wave (PDW) states are defined by a spatially modulating superconductive order-parameter. To search for such states in transition metal dichalcogenides (TMD) we use high-speed atomic-resolution scanned Josephson-tunneling microscopy (SJTM). We detect a PDW state whose electron-pair density and energy-gap modulate spatially at the wavevectors of the preexisting charge density wave (CDW) state. The PDW couples linearly to both the s-wave superconductor and to the CDW, and exhibits commensurate domains with discommensuration phase-slips at the boundaries, conforming to those of the lattice-locked commensurate CDW. Nevertheless, we find a global δΦ∼±2π/3 phase difference between the PDW and CDW states, possibly owing to the Cooper-pair wavefunction orbital content. Our findings presage pervasive PDW physics in the many other TMDs that sustain both CDW and superconducting states.

The origin of the intermediate anomalous metallic state in two-dimensional superconductor materials remains enigmatic. In the present paper, we observe such a state in a series of ∼ 9.0 nm thick Ta x (SiO 2 ) 1−x ( x being the volume fraction of Ta) nanogranular films. At zero field, the x ≳ 0.75 films undergo a Berezinskii-Kosterlitz-Thouless transition as transform from normal to superconducing states upon cooling. For the x ≲ 0.71 films, the resistance increases with decreasing temperature from 2 K down to 40 mK. A normal state to anomalous metallic state transition is observed in the x ≃ 0.73 film, i.e., near the transition temperature, the resistance of the film decreases sharply upon cooling as if the system would cross over to superconducting state, but then saturates to a value far less than that in normal state. When a small magnetic field perpendicular to the film plane is applied, the anomalous metallic state occurs in the x ≳ 0.75 films. It is found that both disorder and magnetic field can induce the transition from superconductor to anomalous metal and their influences on the transition are similar. For the the magnetic field induced case, we find the sheet resistance R □ (T,H) ( T and H being the temperature and the magnitude of magnetic field) data near the crossover from the anomalous metal to superconductor and in the vicinity of the anomalous metal to insulator transition, respectively, obey unique scaling laws deduced from the Bose-metal model. Our results strongly suggest that the anomalous metallic state in the Ta x (SiO 2 ) 1−x granular films is bosonic and dynamical gauge field fluctuation resulting from superconducting quantum fluctuations plays a key role in its formation.

We report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the Néel temperature TN = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was achieved by a [100]-oriented biasing field favoring a single-domain state during cooling across TN. Our specific heat and magnetization measurements confirm the bulk character of this AFM phase transition. On longer time scales, however, we observe that the field-biased AFM state is highly unstable even at the lowest temperature of T = 3 K. After switching off the biasing field, the XMLD signal decays exponentially with a time constant {\tau} = 1506 s. The initial XMLD signal is restored only upon repeating a cycle consisting of heating and field-cooling through TN. We explain the time effect by a gradual formation of a multi-domain state with 90 deg rotated AFM domains, promoted by structural disorder, facilitating the motion of twin-domains. Significant disorder in our Fe1+yTe sample is evident from our X-ray diffraction and specific heat data. The stability of magnetic phases in Fe-chalcogenides is an important material property, since the Fe(Te1-xSex) phase diagram shows magnetism intimately connected with superconductivity.

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that attracts considerable attention. A proposed explanation of the origin of the high-field phase is the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field phase has been observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesized FeSe single crystals with different levels of disorders. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field B c2 was systematically studied over a wide range of temperatures, which went as low as ??0.5 K, and magnetic fields, which went up to ??38 T along the c axis and in the ab plane. In the high-field region parallel to the ab plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust to disorders. This result suggests that the high-filed SC state in FeSe may not be a FFLO state, which should be sensitive to disorders.

We report theoretical predictions on the pairing symmetry of the newly discovered superconducting nickelate Nd 1−x Sr x NiO 2 based on the renormalized mean-field theory for a generalized model Hamiltonian proposed in [Phys. Rev. B \textbf{101}, 020501(R)]. For practical values of the key parameters, we find a transition between a gapped ( d+is )-wave pairing state in the small doping region to a gapless d -wave pairing state in the large doping region, accompanied by an abrupt Fermi surface change at the critical doping. Our overall phase diagram also shows the possibility of a ( d+is )- to s -wave transition if the electron hybridization is relatively small. In either case, the low-doping ( d+is )-wave state is a gapped superconducting state with broken time-reversal symmetry. Our results are in qualitative agreement with recent experimental observations and predict several key features to be examined in future measurements.