Supplementary material Paramagnetic Meissner effect in electrochemically doped indium-tin oxide films

Ali E. Aliev,11 Monica Jung de Andrade1, and Myron B. Salamon2,3

1 Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX, 750832 Department of Physics, University of Texas at Dallas, Richardson, TX, 750833MPA-CMMS, Los Alamos National Laboratory, Los Alamos, NM, 87545

S1.1. Sample preparation: intercalation of ITO films. The alkali, and alkali earth ions doping of ITO films has been performed via an electrochemical intercalation using a CHI-660B Electrochemical Workstation (CH Instruments, Inc.) employing a three electrode experimental setup. The ITO film coated glass substrate (10x20x0.75 mm2) were connected to the working electrode using silver paste, H20E (Epoxy Technology). A 25x12.5x1 mm3 graphite plate (Highly Ordered Pyrolytic Graphite, Alfa Aesar, 99,999%) and a saturated calomel electrode (Sigma-Aldrich) were used as the counter and reference electrodes, respectively. To avoid the current leakage through open terminals and silver paste coatings they were covered with insulating silicone paste (RTV sealant 732, Dow Corning). The intercalation was performed in aqueous (1M (LiCl, NaCl, KCl, RbCl, CsCl, MgCl2, MgSO4, CaCl2)) and non-aqueous (1M (LiClO4, NaClO4 in Propylene Carbonate, (PC)) electrolytes in constant current mode with current density varying in the range of 0.1-10 mA/cm2 and time durations of 20-5000 s. To avoid the degradation of electrolyte the applied voltage was kept below 0.7 V and 2.5 V, respectively.

S1.2. Reversible electrochromic coloration. The charge injection changes the color of ITO film from transparent to gold-yellow and further to beige-brown hue and dark brown. Table S1 shows the color development along with charge injection time and appearance of superconductivity. On discharge, superconductivity disappears, as the color returns to clear. To improve the spatial distribution of injected ions but leave intact the oxygen stoichiometry all intercalated samples were post-annealed at 160-170 oC for 1 hour in ambient, if not otherwise specified.

Table S1. The transition temperature correlates with electrochromic coloration. 400 m thick LT-G002 ITO film was charged in 2 M NaCl aqueous electrolyte with constant current of 0.1 mA/cm2.

The spectroscopic transparency of pristine ITO extends from the NIR through to the UV with a fundamental band edge at about 315 nm (see blue curve in Fig. S1a), which correspond to the band gap of 3.95 eV. The series of observed oscillations result from the optical interference in 400 nm thick LT G002 film. The PLS spectrum of hydrogen intercalated ITO [1] suggests that the majority electrochemical reaction occurring in the ITO is: In +3 + 2e- → In+. Following to the tungsten trioxide coloration 1 Corresponding author. Tel.: +1 972-883-6543; fax: +1 972-883-6529. E-mail address: Ali.Aliev@utdallas.edu.

* Corresponding author. Tel.: +1 972-883-6543; fax: +1 972-883-6529. E-mail address: Ali.Aliev@utdallas.edu.

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mechanism (W6+↔W5+) the electron exchange between In3+ and In+ results in intensive absorption below 1300 nm which is increased with the number of injected ions (see Fig. S1a).

Fig. S1. (a) The optical transmission spectra of NaxITO films as a function of intercalation time: Ic = 0.2 mA, t = 25; 50; 200; 500; 1000; 1500 s. (b) The temperature dependence of normalized ac resistance (Iexc = 1 µA) for ITO film intercalated in aqueous 1M NaCl electrolyte at fixed current of 0.2 mA for intercalation time increased from 20 s to 3000 s. (c) The change of resistance behavior in normal state for samples shown on the panel (b). The inset shows the increase of room temperature sheet resistance versus injected charge. (d) Other cations show the same behavior shifting the transition temperature towards lower temperatures. All samples were charged under identical conditions (Q = 1000 s x 0.1 mA/sm2) in aqueous solution of 1 M chloride salts.

S1.3. Resistance behavior. Four-probe measurements of pristine ITO film’s resistance in a broad temperature range exhibit metallic behavior below room temperature with smooth minimum around 125-150 K. The further decrease of temperature shows negative dR/dT , i.e. “semiconducting behavior” with the slope depending upon the concentration of tin and oxygen deficiency. The intercalation with light alkali ions gradually decreases the resistance minimum down to ~10 K and introduces instability in electronic subsystem observed at low intercalation levels (see open blue and open cyan curves in Fig. S1(c)). The smooth R(T) behavior changes to step-like anomalies below 125 ±25 K. The height and

(a) (b)

(c) (d)

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number of steps depends upon the cooling rate and the sample thickness. The thinner films (G001-120 nm; DT-100 nm (CB-50IN-1507, Delta Technologies Ltd.)) exhibit more pronounced jumps in resistivity. Eventually, the charge injection brings the AxITO (A= Li+, Na+, K+, Rb+, Cs+, Mg+2, Ca+2) composite to superconducting phase transition which temperature increases up to 5 K upon the charge injection level. Fig. S1b shows the shift of resistance onset for ITO film (400 nm) intercalated in aqueous 1M NaCl electrolyte for increased intercalation time at fixed cationic current of 0.2 mA. The intercalation reveals the maximum of Tc = 5 ± 0.1 K at injected charge of Q = 0.2 mA x 800 s = 0.16 C, which corresponds to x = 2 atomic weight % obtained by EDX analysis. The further increase of intercalation time decreases the transition temperature and increases the transition width ΔT (see brown, green and dark cyan solid circles in Fig. S1b).

S2. Three steps doping level gradient with maximum Tc at the center of disk.

Fig. S2. MgxITO film intercalated in three steps with Tc maximum at the center of the disk: (a) The 400 nm thick ITO film on 10x20x0.75 mm3 glass substrate step by step covered with silicone protective layer and charged with Na+ ions. (b) The resulted three-steps charged sample was cut into a round-shape 6 mm disk and inserted into plastic straw, shown in (c). The rest of rectangular pieces were used for resistance measurements.

(a) (b)

(c)

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S3. Magnetization: Sample size dependence. PME in 5 mm disk disappears at H=10 Oe (see inset to Fig.4(a) in main text), while in 4 mm sample shown below is clearly observable at applied magnetic fields up to ~ 40 Oe.

Fig. S3.1. The temperature dependence of magnetic moment normalized to the applied magnetic field for 4 mm in diameter MgxITO round-shape sample (400 nm ITO film doped for 2000 s at I = 0.2 mA in 1M MgSO4

aqueous electrolyte) measured in FC regime. (dT/dt = 0.1 K/min, L = 1 cm, number of point 64 and one scan per measurements. Tc = 4.4 K). The top inset shows the linear increase of magnetization versus reciprocal applied magnetic field. The bottom inset shows the photo image of 4 mm round shape sample.

The same PME behavior was observed for Na+ doped home-made ITO. Magnetic moment decreases and PME is extend to higher applied magnetic fields for smaller in diameter (D = 4.7 mm) disks.

Fig. S3.2. The temperature dependence of FC magnetic moment in as-intercalated 1.2 m thick NaxITO (home-made) film shown in Fig. S5.2(a), but with reduced diameter, D = 4.7 mm.

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S4. The home-made ITO films. The tin-doped indium oxide (In2-ySnyO3-δ, y~0.1) films were deposited by rf-sputtering (1% O2 in Ar carrier gas, flow rate 12 cm3, gas pressure of 4 mTorr) using commercial target (In/Sn atomic ratio is 16.6 2). The thickness of as deposited ITO films were measured by profilometer and averaged upon three different regions. For sample shown below it was 1.2 ± 0.05 m. The grain size in SEM picture shown in Fig. S4.1 is varying between 150 and 330 nm. The compositional analysis performed on three different regions using EDS revels a reduced Sn concentration (one Sn atom per cell): In/Sn atomic ratio of 16.6 ± 2. The commercial samples LT-G001and LT-G002 have about 2 Sn atoms per cell: In/Sn atomic ratio of 9.77.

Fig. S4.1 The scanning electron microscope images of top (a) and side (b) view of 1.2 m thick ITO film deposited by rf-spattering (before annealing).

S5. The post-annealing reduces the critical field and critical current.

(a)

(b)

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Fig. S5.1. The temperature dependence of four-probe resistance for as-intercalated (top panel) and post-annealed at 350 oC for 30 min (bottom panel) 1.2 m thick NaxITO film measured at increasing H and Iext = 10 A for each plot. The inset shows the increase of critical magnetic field linearly extrapolated to the zero temperature versus Tc. The Tc for each plot was obtained at 0.9 Rn, where Rn is the resistance of normal state.

Annealing in air at 350 oC for 30 min shifts the Tc from 4.8 K to 5.33 K and restores regular diamagnetic FC and ZFC behavior, as shown in Fig. S5.2.

Fig. S5.2. The FC magnetization of 1.2 m thick NaxITO film, (a) as-intercalated, and (b) after annealing at 350 oC for 30 min. D = 5 mm, dT/dt= 0.05 K/min, scan length is 2 cm. Insets for both plots show the FC, FH, and ZFC magnetizations. At the end of FC (T = 3 K) magnetic field was switched to 0 Oe (no-overshot regime) and then back to 1 Oe, and m measured at increasing temperature (FH). ZFC was measured at the same conditions with H = 1 Oe.

(b)(a)

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S6. Magnetization loop for MgxITO. Fig. S6. shows the typical magnetization loops of MgxITO film in parallel (a) and transverse (b) magnetic fields. Note, that the magnetization for perpendicular arrangement is a linear function of the magnetic field (i.e. reversible) only within a small magnetic field range, Hc1 < 0.5 G, and the slope (see dashed red line in inset to Fig. S6(b)) corresponding to apparent susceptibility eff. is a factor of ~ 60 higher than that for parallel sample orientation. This clearly indicates that for such thin films, there is a large demagnetization and substantial penetration of the magnetic field into the disk at relatively small fields. It is possible to estimate the demagnetization correction from the aspect ratio of the sample, R/h. The demagnetization correction to the magnetization is given by

M ZFC=−M ZFC

¿ H4 π (1−N )

=− χ ZFC H,

where N is the demagnetization factor and MZFC* is the diamagnetic response corresponding to complete

shielding. For a flat disk of radius R and thickness h, the correction factor is (1-N)-1 ≈ R/h. For our film, R/h = 6250, which is two orders higher than was experimentally observed. Apparently, thin disk-shaped samples cannot be described within ellipsoids model used above for N.

The PM background of the glass substrate, shown by dashed (blue) line in Fig. S6(b), tilts the symmetrical loop. For such irreversible loop, after the applied magnetic field (1 Oe) is swept down to zero, the induced magnetic field alone contributes to the total magnetic field, which is the trapped magnetic field. Hence, according to Bean's model, switching of applied magnetic field down to zero switches magnetization to positive value close to ZFC magnetization.

Fig. S6. The irreversible magnetization loops at 2 K for MgxITO round-shape sample placed parallel (a) and perpendicular (b) to magnetic field. The ITO film was charged in 1M MgSO4 equeous solution at 0.2 mA current for 2000 s. The blue arrows in panel (b) show the consequent changes of H.

S7. PME in thin (127 m) niobium disk. To further understand the behavior of magnetization in the PME state we tested thin niobium disks cut from sheets of rolled Nb. Niobium is a one of three elemental metals exhibiting type-II superconductivity and showing paramagnetic susceptibility in the normal state. As purchased, 127 m thick niobium foil (Alfa Aesar, 99.97%), cut in a disk-shape of 5 mm, readily shows pronounced PME in magnetic fields perpendicular to the foil surface in the FC regime with magnetization behavior similar to ref. [2-4]. The rise of PM response with typical diamagnetic overshot at 9.26 K is very sharp (ΔT < 0.1 K). After reaching a maximum the magnetization slightly decreases and then remains essentially constant below 9.1 K. The flux compression by an extended superconducting ring does not change the PM magnetization below Tc down to the lowest available temperature, 2 K . At

(a) (b)

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these low temperatures the critical field Hc1 becomes much larger than the applied field and sample should rapidly expel the captured field. Apparently, at low applied fields the Hr/z component of magnetic gradient still overcome the critical current of shielding ring and holds the critical state. However, above 20 Oe the flat PM magnetization turns to decline, with much sharper decline at higher H, (see Fig. S7.2(a)). Kostic' et al speculated that Nb samples cut from rolled sheet apparently have higher Tc on surface than underlying bulk [4]. On the other hand, the earlier nucleation of superconducting state in such thick samples might be induced at the lateral surface at Hc3 , as suggested in Ref [5, 6] for small in diameter samples, without any assumption of different surface composition. Unlike studied here thin doped-ITO films, (h = 400 nm), in Nb foil the observed PM signal (4x10-5 emu) is a very small fraction of ZFC magnetization (9.3x10-3 emu at H = 2 Oe), similar to that of thick home-made ITO films (h = 1.2 m). For this relatively thick Nb disks the demagnetization correction corresponding to complete shielding is (1 N)-1 = 23, which agrees fairly well with the aspect ratio, R/h = 20.

Fig. S7.1. The temperature dependence of magnetization in as-purchased 127 m thick niobium foil, D = 5 mm, h = 127 m: (a) For FC regime and (b) for FC, FH (after OFF-ON switch of the applied field), and ZFC regimes.

Fig. S7.2. (a) The PM response in niobium foil at increasing applied magnetic fields. (b) The expanded view of PM transition close to Tc.

(a) (b)

(a) (b)

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Annealing of sample at 1000 oC for 2 hours in high vacuum (0.1 mTorr) totally restores the diamagnetic response with FC magnetization signal of -1.6x10-4 emu (see S7.3 (a)). The same magnetization behavior with sharp PM jump and further decrease of magnetization was reported in artificially constructed three phase In-Sn alloys with higher Tc on the outer ring [7]. The irreversibility magnetization loop for Nb foil placed perpendicular to the magnetic field shows strong demagnetization effect with enhanced susceptibility comparing to the same sample placed parallel to H, ( ≈ 23II).

Fig. S7.3. The FC magnetization for Nb foil (D = 5 mm, h = 127 m) annealed at 1000 oC for 2 h in in high vacuum (0.1 mTorr). (b) The irreversibility magnetization loop for Nb foil placed perpendicular to the magnetic field. The inset shows the temperature dependence of resistance in pristine niobium strip (1x5 mm2) cut from 127 m thick foil. The RRR from measured temperature dependence of resistance (RRR = R(300 K)/R(9.3 K) = 18.0) is much higher than for bulk material: RRR = 6.2.

S8. Search for PME in Lead, Pb. Interestingly, the pure lead foil (type-I superconductor with diamagnetic susceptibility in normal state, the coherence length, 0 = 83 nm, the London penetration depth, λL = 37 nm) of the same shape and different thicknesses (50, 150, 500 m) does not show PME at all. We tested Pb disk-shaped samples placed perpendicular to the magnetic fields (0.1-100 Oe) with different cooling and heating regimes and rates. A 50 m thin foil with a 3 mm hole fabricated in the center of 5 mm round-shape sample showed pronounced absorption in AC mutual inductance measurements (measured using ACSM option in PPMS) and a wide superconducting transition region, but did not show any PM onset of magnetization.

In fact, the PM response did not show up in our ac susceptibility (mutual inductance) measurements. The skin effect currents are opposite to the internal Jc

PM ring (they support the external shielding ring JcSh)

in moderate frequency range used in ACMS option of PPMS (10 Hz - 10 kHz). The skin effect strongly suppresses the PME at high frequencies. To observe the PME in field modulated regime (and avoid the sample movement) one should use low frequencies in mHz range with sensitive SQUID detectors. S9. Table of materials showing paramagnetic response. We must note that most of studied materials showing PME are paramagnetic in the normal state (see the Table 3 in supporting information). Apparently the PM attraction of external field in normal state (which is extremely small in bulk, m ~ 10-5, but can be enhanced by a factor of (4/)(R/h) ~ 2x103 on the edges of thin round-shaped film placed perpendicular to H [8]), reduces the local flux density at the edges of sample, thereby might facilitate the earlier nucleation of superconducting state at the very edges and flux trapping at transition to superconducting state.

(a) (b)

10

SC material: Tc,K

n, 10-5

emu/cm3Hc1,

OeHc2,T

Ref.

Nb 9.2 23.7 1800 0.4 [10]Al 1.175 2.22 105, 158 [11]YBa2Cu3O7- 95 0.31 800II

8000

140±20

[12,13]

Bi2Sr2CaCu2O8.182 84-95 2x10-7 cm3/g 0.001 200 [14]Bi1.73Pb0.27Sr2Ca2Cu3Oy 110Nd2-xCexCuOy 21La2CuO4+ 32Ba-K-Bi-O 32In-Sn alloy (γ, β) 3.7-

4.7NaxITO 5.3 100 0.57

8.2 II

4 II

3.5

Pb 7.19 -1.8 803 [15]

S.10. WHH formulation [1-3]The WHH prediction for upper critical magnetic field, Hc2(T) is [1-3]

h(T )=( 4π2) Hc 2 (T )

Tc (− dH c 2

dT )T c

which gives for T=0

H c 2(0 )=−0 .69T c ( dH c 2

dT )T c

where h is the magnetic field, T is the temperature.

1. E. Helfand and N. R. Werthamer, Temperature and purity dependence of the superconducting critical field, Hc2, Phys. Rev. Letters 13, 686 (1964).2. E. Helfand and N. R. Werthamer, Temperature and Purity Dependence of the Superconducting Critical Field, Hc2 , II, Phys. Rev. 147, 288 (1966).3. N. R. Werthamer, E. Helfand, and P. C. Hohenberg, Temperature and Purity Dependence of the Superconducting Critical Field, Hc2. III. Electron Spin and Spin-Orbit Effects, Phys. Rev. 147, 295 (1966).

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