WO3 interface

Accepted Manuscript

Metastable superconductivity of W/WO3 interface A.V. Palnichenko, O.M. Vyaselev, A.A. Mazilkin, I.I. Zver‘kova, S.S. Khasanov PII: DOI: Reference:

S0921-4534(16)30283-0 10.1016/j.physc.2017.02.002 PHYSC 1253129

To appear in:

Physica C: Superconductivity and its applications

Received date: Accepted date:

19 January 2017 9 February 2017

Please cite this article as: A.V. Palnichenko, O.M. Vyaselev, A.A. Mazilkin, I.I. Zver‘kova, S.S. Khasanov, Metastable superconductivity of W/WO3 interface, Physica C: Superconductivity and its applications (2017), doi: 10.1016/j.physc.2017.02.002

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Highlights • W/WO3 interface was formed by surface oxidation of W. • Magnetic susceptibility and electrical resistance of the interface was studied. • Superconductivity at 35 -75K of the W/WO3 interfaces was observed.

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• The superconducting interface is instable under normal conditions.

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Metastable superconductivity of W/WO3 interface A. V. Palnichenko∗, O. M. Vyaselev, A. A. Mazilkin, I. I. Zver‘kova, S. S. Khasanov Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow region, 142432, Russia

Abstract

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Metastable W/WO3 interface has been formed at the surface of a tungsten metal bar using a solid state redox reaction of W with powdered WO3 . Superconductivity at 35 ≤ T ≤ 75 K in the W/WO3 interfacial layer has been observed by means of the ac magnetic susceptibility and electrical resistance measurements. Comparative analysis of the experimental results infers that the W/WO3 interfacial layer consists of weakly linked superconducting regions. Keywords: High temperature interfacial superconductivity, Tungsten-tungsten oxide interface, Dynamic magnetic susceptibility, Electrical resistance, Electron microscopy preparation of nanoparticle tungsten bronze samples M x WO3−y (M = Na, Li) incorporated into porous nanostructured host matrices in order to increase the volume fraction of the superconducting surface phase. In these samples, the localized superconductivity has been detected in a temperature range 125 132 K [17]. Later on, a local non-percolated two-dimensional superconductivity with T c ' 120 K has been observed in WO3 crystals doped with hydrogen on the sample surface [18]. Another exotic feature of WO3 -based superconductors is a sheet superconductivity which arises at T c ' 3 K along the twin boundaries in the non-superconducting tetragonal crystals of the reduced WO3−x [19]. Subsequently, a transient superconductivity with T c enhanced up to ' 7 K has been observed in granular WO3−x films [20]. This marginal superconductivity arising in the twin or grain boundaries, or in the surface doped WO3 samples, may be related to the interfacial superconductivity [20–22], which appears within the boundary layer located between two neighboring non-superconducting phases. Here we report on the superconductivity at relatively high temperatures 35 ≤ T ≤ 75 K, which arises in the W/WO3 interfacial layer formed by surface oxidation of metallic tungsten. Comparison of the results of the ac and dc magnetic susceptibility and the electrical resistance measurements infers that the W/WO3 interface consists of weakly linked superconducting regions incorporated into the non-superconducting matrix.

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1. Introduction

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Tungsten oxide WO3 is an intensively investigated material due to its application in a wide variety of advanced technologies and devices, including electrochromic, gasochromic, solar energy, optical modulation, writing-reading-erasing optical devices and photo catalysts [1–5]. WO3 is a semiconductor with an optical band gap of 2.75 eV [6]. Structurally it is described as a three-dimensional network of corner-sharing WO6 octahedrons, which can connect together in various ways resulting in a large multiplicity of the WO3 crystal structures with various kinds of structural distortions [7–9]. The structural interstices located in between the interconnected WO6 octahedrons allow the intercalation of WO3 , resulting in formation of so-called tungsten bronzes M x WO3 , where M is an intercalant ion that donates electrons to the WO3 host matrix [10]. Since the discovery of superconductivity in tungsten bronzes [11], these superconductors are under study up to the present time. In particular, tungsten bronzes are attractive due to similarity of their features to those of the high temperature superconducting oxides, such as a relatively low density of electronic states at the Fermi surface and the highest temperatures of the superconducting transition, T c , for the compositions close to a structural phase boundary [12, 13]. Although in the bulk all the known tungsten bronzes exhibit superconductivity at relatively low temperatures, T ≤ 5.5 K [12, 13], the high temperature superconductivity with T c ∼ 90 K has been reported for the WO3 single crystals doped with sodium at the sample surface [14–16]. Scanning tunneling microscopy and X-ray photoemission spectroscopy measurements [15, 16] revealed that the superconductivity occurs within metastable two-dimensional Na-rich islands of 20150 nm lateral size and 10-20 nm thick, which are randomly arranged on the WO3 sample surface. These results motivated the ∗ Corresponding author. Tel.: +7 906 095 4402; fax: +7 496 524 9701. E-mail address: [email protected] (A. V. Palnichenko)

Preprint submitted to Physica C: Superconductivity and its Applications

1.1. Sample preparation The W/WO3 interfaces have been prepared using a solid state redox reaction at the surface of tungsten metal with powdered WO3 . 1.5 × 1.5 × 5 mm3 bars of sintered 99.99%-pure W and powdered, 7 – 14 µm grain size, 99.98%-pure WO3 were used for preparing the samples. Initially the W bar was annealed in a 4 mm-bore quartz tube at ' 570 K for about 1 h under the dynamic rotary pumping. The tube was then removed from the furnace, let cool down to room temperature, disconnected from the pumping line, and the WO3 powder was added in February 10, 2017

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the amount sufficient to envelop the tungsten bar. Then, in Method 1, the W/WO3 sample was sealed-off in an evacuated quartz tube using a blowtorch, annealed at 570 K for about 1 h, and rapidly cooled (quenched) in liquid nitrogen. In Method 2 the W/WO3 -sample was annealed under the same conditions but with dynamic rotary pumping. The evacuated tube was then rapidly removed from the furnace, sealed-off and quenched in liquid nitrogen. Taking into account that the prepared samples are unstable under the normal conditions, ampoules with the samples were stored in liquid nitrogen at T = 77 K between the measurements, thus conserving the samples properties for infinitely long time.

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diffractometer, CuKα radiation, as well as an Oxford Diffraction Gemini R diffractometer, MoKα radiation. For microstructure studies, a VERSA 3D dual beam machine and a JEM-2100 high-resolution transmission electron microscope (TEM) have been used. For TEM observations, a microscopic lamella of the sample was prepared using a focused ion beam technique. In order to prevent damaging of the sample surface during the microscopic lamella preparation, a protecting platinum layer was deposited on the sample surface, initially by the electron beam, subsequently by the ion beam induced deposition. Elemental composition of the sample was determined in a scanning transmission (STEM) mode by means of an energydispersive X-ray spectroscopy (EDXS) facility of the VERSA 3D microscope.

1.2. Measurement techniques The ac magnetic susceptibility, χ = χ0 − iχ00 , was studied using a mutual inductance ac susceptometer [23, 24], operating at the amplitude of the excitation magnetic field, Hac , 0.9 - 450 mOe, the excitation frequency, ν, ranged from 1.5 to 15 kHz, and the superimposed dc magnetic field, Hdc , up to 250 Oe. In the measurements, the long edges of the sample were directed along the ac and dc magnetic fields. In order to prevent the sample degradation, the quartz ampoule containing the evacuated sample was removed from liquid nitrogen and immediately placed in the pickup coil of the measurement insert, which was then dipped into a cryostat precooled by liquid helium. After the sample was cooled down to the equilibrium state at T ' 4.5 K, the χ(T ) measurements were performed upon heating. The static magnetic moment of the samples sealed in the evacuated ampoules was studied by a Quantum Design SQUID magnetometer in the temperature range 5–80 K and static magnetic fields 30–300 Oe. Although the standard routine of loading the sample to the SQUID magnetometer can take up to 15 minutes until the sample is re-cooled in the magnetometer cryostat, the subsequent ac susceptibility measurements have proved that the properties of the sample survive such exposure to room temperature. The electrical resistance, R, was measured using an ac lockin technique with the excitation current amplitude Iac = 24 mA at frequency ν = 23 Hz. In order to prevent the sample degradation during the electric contacts mounting, a spring-clamp four-probe fixture was used for the measurements, which ensured the sample temperature was never let above 80 K before the R(T ) measurements. Schematically, the spring-clamp fixture is shown in the inset of Fig. 5. Voltage clamps were stainless steel pins (1) fed through insulated coaxial holes in 6 mmdiameter copper disk-shaped current clamps (2). The contact clamps clutched flat 1.5 × 1.5 mm2 faces of the sample. For the contacts mounting, the ampoule with the sample was placed into a foam plastic box filled with liquid nitrogen, where the sample was retrieved from the ampoule and spring-clamped to electric leads of the measurement insert. During transportation of the insert into the precooled cryostat, the sample was kept in the ambience of liquid nitrogen, which was then evacuated from the cryostat sample space by heating to ' 80 K and pumping. Crystal structure of the samples was investigated by Xray powder diffraction measurements using a Siemens D-500

2. Experimental results

2.1. Dynamic magnetic susceptibility and electrical resistance

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In total, more than a hundred samples were studied. The 4πχ0 (T ) dependencies shown in Fig. 1 by Curves I to V for the five distinctive W/WO3 samples generalize the whole set of the prepared samples. Curve VI demonstrate 4πχ0 (T ) of the pristine tungsten bar. A step-like anomaly in χ0 (T ) is clearly visible in Curves I to V, in contrast to Curve VI.

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Figure 1: Temperature dependencies of the real part of 4πχ for the five selected W/WO3 samples (Curves I to V) and for the pristine tungsten bar (Curve VI). The measurements were performed at ν = 4 kHz in Hac = 450 mOe.

The samples prepared by both methods described in Section 1.1, demonstrated qualitatively similar properties. However, statistically Method 2 yielded a more prominent and sharp anomaly in χ(T ). For all the W/WO3 samples studied, the temperature of the anomaly, T c , arbitrarily varied from sample to sample in the range 35 – 75 K . Fig. 2 shows 4πχ(T ) dependencies measured in several static magnetic fields, Hdc , for one of the samples (III in Fig. 1). A step-like anomaly in χ0 (T ) at T c ≈ 56 K for Hdc =0 and Hac = 450 mOe, accompanied by a peak in χ00 (T ), is clearly visible in Curves 1. As Hdc increases (Curves 2-3), the anomaly in χ(T ) is suppressed. The increase of the driving ac magnetic field amplitude, Hac , gives a similar effect, as shown in Fig. 3. 3

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Figure 2: Temperature dependencies of the real (a) and imaginary (b) parts of 4πχ for the W/WO3 sample III measured under the dc magnetic field Hdc = 0, 122 Oe and 244 Oe (Curves 1 to 3, respectively). Curves 4 correspond to the pristine tungsten bar. The measurements were performed at ν = 4 kHz in Hac = 450 mOe.

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Figure 3: Temperature dependencies of the real (a) and imaginary (b) parts of 4πχ for the W/WO3 sample III measured with the amplitude of the ac driving field Hac = 0.9, 27, 90 and 450 mOe (Curves 1 to 4, respectively). Curves 5 correspond to the pristine tungsten bar. The measurements were performed at ν = 4 kHz in Hdc = 0.

Fig. 4 shows temperature dependencies of χ0 and χ00 measured at frequencies from 1.5 to 15 kHz. One can see that the position of the anomaly (T c ) in χ0 (T ) and χ00 (T ) is essentially frequency independent, while the shape of χ(T ) curves changes with the frequency. Upon completion of the χ(T ) measurements, the sample was mounted onto the resistance measurement insert, as described in Sec. 1.2. Curve 1 in Fig. 5 depicts the resistance of the sample recorded upon heating. One can see that as the temperature increases, the weak increase in R(T ) at low temperatures switches to a steep increase in the range 58 – 60 K, followed by a more gradual monotonic increase of R upon further warmup. Next, the sample was kept in the measurement insert at room temperature for ≈ 24 hours under a residual helium pressure ∼ 100 Pa, whereupon the R(T ) dependence was re-measured. The result is shown by Curve 2 in Fig. 5. No step-like anomaly is visible in Curve 2; instead, a monotonic increase of R with temperature exhibits the metallic behavior of the sample. Finally, the χ(T ) dependence measured for this sample was found to coincide with that of the pristine tungsten bar, shown in Fig. 3 by Curves 5 and Fig. 2 by Curves 4, as well as in Fig. 1 by Curve VI. Therefore, these results suggest that the phase state responsible for the observed anomalies in χ(T ) and R(T ) is unstable at room temperature.

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2.3. Sample structure

2.2. dc Magnetic moment In the zero-field-cooled and field-cooling regimes, the dc magnetic moment of the W/WO3 samples measured in static magnetic fields Hdc = 30–300 Oe have revealed the value of the dc magnetic susceptibility ∼ 10−7 cm3 /g, which have exhibited no anomaly in the temperature range 5–80 K. 4

In contrast to WO3 powder heat-treated separately in an evacuated quartz ampoule, the powder that was in contact with the metallic tungsten changed its color from the original brightlemon to dark-bluish after annealing, thus signifying a partial reduction of the tungsten oxide by the metallic tungsten. X-ray powder diffraction patterns of the pristine and partially reduced powdered tungsten oxides shown in Fig. 6, are essentially identical. All the observed diffraction peaks originate from the monoclinic (P21 /n, reference: JCPDS No. 71-2141) and triclinic (P1, reference: JCPDS No. 71-0305) WO3 phases. The coincidence of the diffractograms means that the oxygen vacancies formed in the partially reduced tungsten oxide powder, are randomly distributed with no influence on its crystal symmetry. A typical X-ray diffraction pattern of the surface-oxidized tungsten bar is shown in the inset of Fig. 6. All the observed diffraction rings in the diffraction pattern are related to the W polycrystalline structure, (Im3m, reference: JCPDS No. 040806). No crystal structures additional to tungsten have been revealed in the sample, apparently due to relatively thin tungsten oxide surface layer imperceptible to the the X-ray diffraction measurements. The tungsten and oxygen EDXS elemental profiles corresponding to the STEM image of the near-surface region of the W/WO3 sample are shown in Fig. 7. In the STEM image, the rightmost and leftmost areas correspond, respectively, to the protective Pt layer deposited during the microscopic lamella preparation, and the W interior of the sample. The oxygen and tungsten EDXS profiles measured along the dashed horizontal line on the STEM image, exhibit a broad peak and drop, respectively, signifying the existence of ' 55 nm-deep tungsten

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Figure 6: X-ray powder diffraction patterns of the pristine (a) and partially reduced (b) tungsten oxide powders (CuKα1 radiation). All the diffraction peaks originate from the monoclinic (P21 /n, reference: JCPDS No. 71-2141) and triclinic (P1, reference: JCPDS No. 71-0305) WO3 phases. Inset: X-ray diffraction pattern of the surface-oxidized tungsten bar. All the diffraction rings correspond to the W polycrystalline structure (Im3m, reference: JCPDS No. 040806).

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Figure 4: Temperature dependencies of the real (a) and imaginary (b) parts of 4πχ for the W/WO3 sample III measured at frequencies 1.54, 2.6, 4, 10 and 14.7 kHz (Curves 1 to 5, respectively). Hac = 450 mOe, Hdc = 0.

clinic (reference: JCPDS No. 71-0305) WO3 phases, which are shown by the bar charts 2 and 3, respectively.

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Figure 5: Temperature dependencies of the electrical resistance, R(T ), for the W/WO3 sample measured before (Curve 1) and after (Curve 2) the ≈ 24 h exposure to room temperature (Iac = 17 mA, ν = 23 Hz). Inset: a schematic view of the spring-clamp voltage (1) and current (2) contacts mounting to the sample.

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oxide layer, which is outlined by white lines in the STEM image. According to the EDXS profiling results, the W content in the WO x oxide layer decreases steadily towards the sample surface within the depth of ' 15 nm (as x goes from 0 to 3), until it reaches the WO3 composition. One of the high resolution TEM images of the W/WO3 interfacial region is shown in Fig. 8. Detailed analysis of the TEM images reveals that the tungsten oxide layer consists of crystallites of about 4 nm in size. Curve 1 in Fig. 9 shows the diffraction intensity radial distribution versus scattering wave vector derived from the fast Fourier transform (FFT) pattern shown in inset a of Fig. 9. The FFT pattern is calculated for the tungsten oxide region marked in inset b of Fig. 9 by the dashed line. Positions of the intensity peaks in Curve 1 are consistent with those of the X-ray powder diffraction patterns, corresponding to the monoclinic (reference: JCPDS No. 71-2141) and tri5

3.1. Evidence of superconductivity The collection of the above experimental facts is entirely consistent with superconducting transition that occurs in the W/WO3 interfacial layer formed between the surface of metallic tungsten and the WO3 exterior. This layer consists of superconducting grains embedded into the non-superconducting host matrix and interconnected by weak links. First of all, the superconductive nature of the transition becomes apparent from the effect of suppression of the anomaly in χ(T ) by magnetic fields (Figs. 2, 3), which is a characteristic feature of superconducting materials [25, 26]. Moreover, a relatively small increase of Hdc or Hac suppresses the anomaly in χ(T ) (Curves 1–3 in Fig. 2 and Curves 1–4 in Fig. 3), which implies penetrability of the sample to low magnetic fields, typical for weakly linked granular superconductors [26]. Besides, one can see in Figs. 1- 4 that 4πχ0 (T ) > −1 and χ00 (T ) > 0, even at temperatures well below T c . This infers that the sample is not enveloped by a closed superconducting surface capable of trapping the magnetic flux and providing ideal diamagnetism with 4πχ0 (T ) = −1, χ00 (T ) = 0. Rather, there are weak-linked superconducting regions embedded into a non-superconducting host matrix. This conception explains also the absence of superconducting response in the dc magnetometry of the W/WO3 samples. As the dc magnetic field is applied in the zero-field-cooled regime, the intergranular shielding current through energy dissipative weak links decays rapidly, while the intragranular superconducting currents give negligible contribution to the mag-

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Figure 7: Tungsten and oxygen EDXS elemental profiles (left), taken along the direction indicated by the dashed line in the STEM image of the near-surface region of the surface-oxidized tungsten bar (right). The tungsten oxide layer is outlined white. The vertical dashed lines in the EDXS elemental profiles indicate the phase distribution at the sample surface layer.

netic field shielding, either due to the small grain size (compared to the London penetration depth, λ) or because of an imperceptible superconducting phase fraction volume. In contrast to the dc magnetometry, the alternating magnetic field used in the ac magnetic susceptibility measurements, maintains the intergrain shielding current. This enables the superconductivity to be detected even in discontinuous, energy dissipative loops, consisting of superconducting sub-λ-grains weakly coupled either by Josephson junctions or by narrow non-superconducting bridges. Of course, the diamagnetism and electrical resistance of such structures are far from those of an ideal superconductor (4πχ0 (T ) = −1, χ00 (T ) = 0 and R = 0) and, due to the arbitrary arrangements of the superconducting grains in the samples, are different from one sample to another, as shown in Fig. 1. For the temperature range reported, neither the pristine tungsten bar nor the partially reduced tungsten oxide powder surrounding the bar, were found to be superconductors. We conclude therefore that the observed superconductivity is formed in the interfacial layer located between the metallic tungsten interior and the WO3 exterior of the W/WO3 sample. Thus we attribute this phenomenon to the interfacial superconductivity [20–22]. The dynamic magnetic susceptibility of a conducting sample is a function of the skin depth, r ρ c δ= , (1) 2π µν

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Figure 8: High resolution TEM image of the tungsten oxide layer (right), consisting of the crystallites of ∼ 4 nm in size, adjacent to the tungsten interior (left) of the W/WO3 sample.

even at out experimental temperature range bottom, T ' 4.5 K. This agrees with the assumption of granular energy dissipative structure of the superconductive W/WO3 interface. Moreover, a highly conductive metallic interior of the sample shunts the W/WO3 interface, thus contributing to energy dissipation in the measured sample even at T  T c . As it was mentioned above, the superconducting W/WO3 interface is unstable at room temperature. A possible reason for such instability is the tungsten/oxygen ionic diffusion process activated at room temperature in the W/WO3 interface, which destroy the superconductivity in the sample.

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3.2. Frequency dependence of the susceptibility The change of χ(T ) with frequency is shown in Fig. 4. One can see in Fig. 4a that the real part curves, χ0 (T ), shift down with increasing ν even for T  T c . This signifies that the applied ac magnetic field is incompletely shielded by the surface supercurrents and induce normal eddy currents in the metallic interior of the sample, that contributes significantly to the diamagnetism with increasing ν. Moreover, the drop in χ0 (T ) at T c ≈ 56 K for ν = 4 kHz (Curve 3) is more significant than that for ν = 1.54 kHz (Curve 1) and ν = 14.7 kHz (Curve 5). This non-monotonic frequency dependence of the drop in χ0 (T ) at T c is unusual for bulk metallic superconductors; instead, a gradual decrease of the drop in χ0 (T ) at T c with increasing frequency, due to the decrease of the skin depth in the normal state, is observed for the regular superconductors. As for the imaginary part of χ(T ) (Fig. 4b), upon cooling χ00 (T ) exhibits a step-like rise at T c ≈ 56 K at low frequencies (Curves 1–2), a broad maximum at intermediate frequencies (Curves 3–4), and a step-like drop at high frequency (Curve 5). The observed frequency effect on χ0 (T ) and χ00 (T ) can be interpreted merely involving classic electrodynamics [28]. The

where ρ and µ are, respectively, the electric resistivity and magnetic permeability of the material, ν is the ac magnetic field frequency and c is velocity of light in vacuum [27]. Since the dc magnetic moment measurements of the W/WO3 samples have exhibited a nonmagnetic (µ ' 1) character of the susceptibility without any magnetic anomalies in the temperature range 5– 80 K, the only reason of the observed anomaly in χ(T ), shown in Figs. 1–4, is a sudden change of ρ. This is proved also by the anomaly in R(T ) at 58 K< T < 60 K, shown by curve 1 in Fig. 5. The resistance of the sample, however, does not fall to zero 6

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Figure 9: Curve 1: Diffraction intensity radial distribution versus the scattering wave vector, corresponding to the FFT pattern shown in inset a. The FFT pattern is calculated for the tungsten oxide region marked in inset b by the dashed line. Bar charts 2 and 3 correspond, respectively, to the powder X-ray diffraction intensity distribution for the monoclinic (reference: JCPDS No. 71-2141) and triclinic (reference: JCPDS No. 71-0305) WO3 phases.

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Figure 10: Temperature dependencies of the skin depth, δ, extracted from the χ0 (T ) data in Fig. 4a using Eq. (2). Curves 1 to 5 correspond to the ac drive frequencies of 1.54, 2.6, 4, 10 and 14.7 kHz, respectively. Arrows indicate the transition region T c ± ∆T , where ∆T is the half-width of the peak in dχ0 (T )/dT at T c . Inset: Temperature dependencies of the resistivity for the W/WO3 sample, calculated for each δ(T ) curve in the main panel and averaged (Curve 1), and for the pristine tungsten bar (Curve 2) calculated from the corresponding χ0 (T ) (Curve VI in Fig. 1).

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ac magnetic susceptibility of a long conducting solid cylinder in the presence of an ac magnetic field directed along the cylinder axis can be expressed through zeroth and first order Bessel functions, J0 (u) and J1 (u), as 2J1 (u) − 1, uJ0 (u)

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where u = (1 + i)r0 /δ and r0 is the cylinder radius [27]. Equating the real part of Eq. (2) to the experimental χ0 (T ) data in Fig. 4a and considering the W bar a long solid cylinder with r0 = 0.75 mm, we have extracted T -dependencies of the skin depth δ for each χ0 (T )–curve, as shown in Figure 10. Using Eq. (1), we calculated the resistivity of the W/WO3 sample for each δ(T ) curve in Fig. 10. The resulting ρ(T ) curves coincide fairly well, with the scatter (rms) of ∼ 25%. Curve 1 in the inset of Fig. 10 shows the mean ρ(T ), which was obtained by averaging over the five experimental datasets obtained from the corresponding δ(T ) curves. For comparison, the resistivity calculated from χ0 (T ) of the pristine tungsten bar (Curve VI in Fig. 1), is presented in the inset of Fig. 10 by Curve 2. Remarkably, the values of the resistivity determined this way are very reasonable: at 65 K we obtain ρ ' 0.7 µΩcm, while the tabulated value for tungsten at this temperature is ' 0.45 µΩcm [29]. Fig. 11 demonstrates the plot of the real and the imaginary parts of 4πχ in function of r0 /δ, calculated using Eq. (2). The values of r0 /δ at T = T c determined from δ(T ) curves in Fig. 10, are shown by vertical dashed lines labelled with the corresponding frequency values. Horizontal bars indicate the corresponding superconducting transition regions, r0 /δ(T c ± ∆T ), where ∆T is the half-width of the peak in dχ0 (T )/dT at T c , marked on δ(T ) curves in Fig. 10 by arrows.

One can see in Fig. 11 that depending on frequency, the transition regions r0 /δ(T c ± ∆T ) occupy sections with different slopes on the 4πχ curves. Consequently, the drop in 4πχ0 corresponding to the change in r0 /δ at T c , is evidently smaller for 1.54 and 14.7 kHz compared to that for 4 and 10 kHz. That is why the change in χ0 (T ) at T c appears non-monotonic in frequency (Fig. 4a). Furthermore, the superconducting transition measured at 4 and 10 kHz occurs around r0 /δ = 1.77 where 4πχ00 has a maximum; at lower frequencies 4πχ00 grows while at higher frequencies it diminishes with r0 /δ at the transition region. This explains the maxima in χ00 (T ) at 4 and 10 kHz and the change of the χ00 (T ) slope with the frequency, visible in Fig. 4b. 3.3. Nature of the Observed Superconductivity

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The ‘marginal’ superconductivity previously discovered in various tungsten oxide systems such as surface doped layer of WO3 [14–16, 18] and twin boundaries in the reduced WO3−x [19, 20], has been ascribed to some exotic rearrangements of the WO6 octahedrons at the sample surface, twin boundaries or interfaces due to the reduced atomic coordination [12]. The superconductivity in amorphous WO3−x films [20] has been attributed to the electron doping due to de-oxygenation of WO3 and considered as a possible example of a Fr¨ohlich-type sliding charge density wave superconducting system [30]. However, many other metal/metal-oxide interfaces, besides W/WO3 presented in this paper, have been reported to reveal superconductivity. These are Na/NaO x [31], Mg/MgO [24, 32], Al/Al2 O3 [28, 33], Cu/CuO x [34, 35] and Fe/FeO x [36]. De7

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such clusters are expected to represent a narrow, partially filled energy band at the Fermi level, resulting in a new hypothetical family of high-Tc (> 150 K) superconductors [39–41]. Finally, the high-temperature interfacial superconductivity observed in the variety of metal/metal oxide interfaces may be considered within the framework of a mechanism which is applicable to any materials where the superconducting order parameter competes with another order parameter (e.g. charge- or spin-density wave) [42]. In such materials, local superconductivity appears in structural defects, interfaces or even surface of the sample, where the rival order parameter is only slightly suppressed.

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Metallic tungsten bars were subjected to special surface oxidation process including their redox reaction with powdered WO3 followed by quenching of the product to liquid nitrogen. The ac magnetic susceptibility and electrical resistance measurements have revealed metastable superconductivity of the samples at 35 ≤ T ≤ 75 K. Analysis of the experimental data reveals that the superconductivity arises in the interfacial layer, between the external WO3 layer and the metallic W interior of the samples. The superconducting interfacial layer consists of superconductive regions embedded into a non-superconducting host matrix. The nontrivial frequency behavior of χ(T ) is well described by the classic electrodynamics of a conducting solid cylinder in the ac magnetic field.

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Figure 11: The plot of the real (4πχ0 ) and the imaginary (4πχ00 ) parts of the ac magnetic susceptibility calculated using Eq. (2), in function of r0 /δ. The vertical dashed lines and the horizontal bars denote, respectively, r0 /δ(T c ) at the labelled frequency and the corresponding transition regions, r0 /δ(T c ± ∆T ), as determined from δ(T ) curves in Fig. 10.

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spite the variety of metals from different groups involved, superconducting properties of all these systems have quite distinct features: (i) Incomplete shielding of the rf field; (ii) Vulnerability to low magnetic fields; (iii) Absence of superconducting response in the dc magnetometry, which together with (i) and (ii) implies a surface island-type superconductivity; (iv) Room temperature instability of the superconducting metal/oxide interfaces. This similarity of different metal/oxide interfacial superconductors suggests the generality of the phenomenon, which is apparently not related to some unique properties of tungsten or other chosen metals and/or their oxides, but rather deals with specific oxygen states located at the superconducting metal/metal oxide interface. From this point of view, the superconductivity observed in the metastable two-dimensional Na-rich islands located on the WO3 sample surface [15, 16] may be explained by formation of the superconducting Na/NaO x interfaces [31] within the Na/WO3 islands, due to partial reduction of WO3 by sodium at the WO3 sample surface. In this context, an important role of oxygen in the formation of the metal/oxide interfacial superconductivity has been emphasized in Refs. [37, 38]. Within this concept, the observed room temperature instability of the superconducting metal/oxide interfaces [24, 28, 31–36] is caused by thermally activated diffusion process which makes the interfacial oxygen state O1− , responsible for the metal/oxide interfacial superconductivity, to approach the equilibrium O2− state typical for nonsuperconducting metal oxides. Otherwise, metastable nanometer-sized metallic clusters may spontaneously arise in the metal-oxide interfacial layer during either the surface oxidation process of the bulk metallic samples [28, 31, 32, 34, 36] or the shock-wave pressure treatment of the metal-oxide mixtures [24, 33, 35]. Electronic structure of

Acknowledgements We gratefully acknowledge useful discussions of the results with V. V. Ryasanov. The work has been supported by RAS Presidium Programs ”Actual problems of Low Temperature Physics” and ”Thermal physics and mechanics of extreme energy impacts and physics of strongly compressed matter”. References

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