Magnetically Induced Electronic States in 2D Superconductors

Transcription

1 Magnetically Induced Electronic States in D Superconductors Jongsoo Yoon University of Virginia B Insulator normal metal (linear I-V) Carlos Vicente Yongho Seo Yongguang Qin Yize Li Metal (U) SC T Christine Lyon Chester Rubbo Brian Gross disorder National High Magnetic Field Laboratories (NHMFL) Sponsored by NSF DMR 3153 DMR 3945

2 Magnetically Induced Electronic States in D Superconductors Resistance (ρ) Insulating phase dρ < dt Increasing Magnetic fields (B) Superconducting phase ( ρ = ) T c T c T c Temperature (T)

3 How does the phase change occur? Superconductor Insulator Transition Superconductor Metal Insulator Transition Resistance (ρ) Resistance (ρ) Insulating phase Insulating phase Superconducting phase Metallic phase Superconducting phase T c T c T c Temperature (T) T c T c T c Temperature (T)

4 How does the phase change occur? Superconductor Insulator Transition InO (Bi, Be, MoSi, ) Superconductor Metal Insulator Transition MoGe V. F. Gantmakher et al, LETP 71, 16 () Mason and Kapitulnik, PRL 81, 534 (1999)

5 1. What we are studying material system Material selection (tantalum) Growth Characterization. What we found magnetic field induced metallic phase How to identify the phases transport characteristics Is the metallic phase real? What is the origin? The nature of the phase transitions? 3. Future experiments and related issues 4. Summary

6 1. What we are studying material system Material selection (tantalum) Amorphous vs. Granular Films Amorphous Films (Uniform, or homogenous films) Granular Films (Non-uniform, or inhomogeneous films) Continuous path T c : decreases with decreasing thickness metal-metal interaction wetting < metal-substrate interaction T c : independent of thickness metal-metal interaction > metal-substrate interaction non-wetting Ti Ta : T c (bulk) =.4 K : T c (bulk) = 4.5 K

7 1. What we are studying material system Growth Dc sputtering Chamber cleaning Baking 3-4 days at ~ 11 C Pre-sputtering for ~ 3 minutes growth ~ 4 mtorr Ar pressure.1 nm/sec growth rate Pattering shadow mask patterning (Hall bar shape) growth of 1 samples at one batch Substrates: silicon, glass, quartz,

8 1. What we are studying material system Characterization 1.9 nm 1. nm.1 nm Ta/Si.3 nm.5 nm.8 nm 3.1 nm 3.4 nm nm 4. nm 4.5 nm 5.5 nm ρ (h/4e ) X-ray Intensity (arb. unit) T (K) θ (degree) 15 nm 1 nm 5 nm Bare Substrate 4. K 1.5 K.6 K.1 K. K

9 dv/di (KΩ). What we found magnetic field induced metallic phase M-I boundary (b).46 T.7 T.16 T S-M boundary V (mv) (a) V (mv) 5 How to identify the phases non-linear transport characteristics B =.6 T mk B = mk dv < di dv > di dρ Insulating phase < dt Metallic phase dv/di (KΩ) hysteretic I V ρ (KΩ) ( ρ = finite) Superconducting phase ( ρ = ) T (K).46 T.7 T.1 T.16 T.13 T.6 T T

10 . What we found magnetic field induced metallic phase Is the metallic phase real? Question: Can it be due to electron heating? Joule heating due to the measurements Inefficient electron-phonon coupling electron temperature > sample stage temperature ρ direct S-I transition (no metallic phase) T s T e T T s T e Superconducting phase hysteretic I V Thermal run-away Metallic phase ( ρ = finite) dv > di Insulating phase dv < di

11 . What we found magnetic field induced metallic phase Is the metallic phase real? Question: Can it be due to electron heating? No, it cannot be. The metallic phase is real. 1.5 B = - 3 T V (V) Ta P c = I c V c. T.5 T.1 T (I c, V c ) In the heating scenario, P c should slowly decrease with increasing B. Strong increase of P c Smooth evolution across S-M boundary V (mv) mk Ta P c (nw) nm.5 K T.1 T. T.3 T ρ (kω) B (T)....4 T (K).6.8 P c (pw) 1 36nm.97 K B (G)

12 . What we found magnetic field induced metallic phase What is the origin? 1.5 B = - 3 T V (V) Ta P c = I c V c. T.5 T.1 T (I c, V c ) V (mv) mk Ta T.1 T. T.3 T ρ (kω) T (K).6.8 By tracing the S-M critical fields, we can map the phase diagram in B-T plane.

13 . What we found magnetic field induced metallic phase What is the origin? ρ (Ω) 1 5 V(V) T (K) Hysteresis in the S-phase is likely due to Pinning-depinning transition of vortices. B Phase diagram in B-T plane I M dv > di S dv < di normal metal (linear I-V) T

14 Vortex pinning depinning transition The transition arises from the competition between Pinning force due to disorder potential Lorentz driving force due to the bias current Hysteresis with respect to the driving force Slow relaxation (logarithmic time dependence) Vortex system Irreversible magnetic properties in type II superconductor (thermal activation of magnetic flux lines out of pinning site) Hysteresis : Y. Yashurun et. Al. Rev. Mod. Phys. 68, 911 (1996) Slow relaxation : Vortex motion in the presence of disorder is analogous to the flow of sand grains in a sand pile. Sand pile granular flow under the competition between jamming and driving force Hysteresis : S. G. K. Tennakoon et al. Europhys. Lett. 45, 47 (1999) Slow relaxation : H. M. Jaeger et al. Phys. Rev. Lett. 6, 4 (1989)

15 Vortex pinning depinning transition Slow relaxation is observed in dynamic transport measurements 6.5 K τ (sec) 1 G 15 G G V(µV) Logarithmic dependence Vt () = V + V exp( t/ τ ) t(sec) 6 G τ (sec).984 K.988 K.99 K.99 K.994 K 1. K I (A) Voltage time Current

16 .5 K τ (sec) 1 G 15 G G G τ (sec).984 K.988 K.99 K.99 K.994 K 1. K Vortex pinning depinning transition Slow relaxation is observed in dynamic transport measurements dlog(v)/dlog(i) dlog(v)/dlog(i) I (A) I (A) B I M dv > di S dv < di normal metal (linear I-V) T

17 . What we found magnetic field induced metallic phase What is the origin? Vortex dynamics in the presence of disorder Phase diagram in B-T-disorder space B I M dv < di dv > di normal metal (linear I-V) S T disorder

18 1. What we are studying material system Material selection (tantalum) Growth Characterization. What we found magnetic field induced metallic phase How to identify the phases transport characteristics Is the metallic phase real? What is the origin? The nature of the phase transitions? 3. Future experiments and related issues Homogeneous (amorphous) superconducting films Nonlinear transport It is real! Vortex dynamics in the presence of disorder 4. Summary

19 . What we found magnetic field induced metallic phase The nature of the phase transitions? M-I transition at T? T = 6 mk dv/di (KΩ) ρ (KΩ) 1.46 T.7 T.1 T.16 T.13 T.6 T T T (K) dv/di (KΩ) M I M 6 4 T = mk 18 T 1 T.56 T.36 T.7 T.16 T.6 T T 1. T 1.15 T 1.1 T 1.5 T 1. T.95 T.9 T.85 T Phase change is caused by increasing bias current MIT is percolation-like

20 . What we found magnetic field induced metallic phase The nature of the phase transitions? percolation-like. In this percolation-type picture, it is expected dv/di should be non-monotonic in the insulating phase in the limited range of magnetic fields with I s that increases with B dv/di (KΩ) T = 6 mk I s I s (µa) 1.5 T = 5 mk B (T) ρ (kω/ ) B c =.98 T 6 mk 13 mk 175 mk T 1. T 1.15 T 1.1 T 1.5 T 1. T.95 T.9 T.85 T B (T)

21 3. Future experiments and related issues Measurements down to ~ 1 µk. High B insulating phase, up to ~ 45 T. Effect of parallel magnetic fields. Direct measurements of electron temperature. High temperature superconductivity. Gantmakher et. al. LETP () InO LaSrCuO InO InO Ando et. al. PRL (1995) Steiner et. al. PRL (5) 1 1 Ta 1. Ta R (kω) T (K) R(B)/R(B=9T) mK 13mK 175mK 5mK 3mK 5mK B (T)

22 3. Future experiments and related issues Gantmakher et. al. LETP () InO Measurements down to ~ 1 µk. High B insulating phase, up to ~ 45 T. Effect of parallel magnetic fields. Direct measurements of electron temperature. High temperature superconductivity. Quantum Hall Effect Kravchenko et. al. (1995) Sarachik and Kravchenko (1999) Si-MOSFET ρ (Ω/ ) Plot 1 Plot p-gaas/algaas p-gaas/algaas p = 3.7 x 1 1 cm T (K) B II (T) dv/di(ω/ ) Ta p-gaas/algaas p=3.7x1 1 cm p-gaas/algaas T = 6 mk I (ma) B = 7 T 6. T 5.5 T 5. T 4. T 3.5 T 3. T. T. T Ta ρ (KΩ) 1.46 T.7 T.1 T.16 T.13 T.6 T T T (K) dv/di (K Ω) T 1. T 1.15 T 1.1 T 1.5 T 1. T.95 T.9 T.85 T

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24 1. What we are studying material system Material selection (tantalum) Growth Characterization. What we found magnetic field induced metallic phase How to identify the phases transport characteristics Is the metallic phase real? What is the origin? The nature of the phase transitions? 3. Future experiments and related issues Homogeneous (amorphous) superconducting films Nonlinear transport It is real! Vortex dynamics in the presence of disorder Percolation-like 4. Summary