A method for preparing a YBCO film with a quantum metallic state

Abstract

The present invention discloses a method for preparing a YBCO film with a quantum metallic state, belonging to the field of superconducting film preparation technology, specifically: preheating the equipment, heating the temperature of the annealing chamber and the deposition chamber to above 800°C; pre-sputtering for 10 to 30 minutes; placing the substrate in the deposition chamber, passing argon gas, passing oxygen when the pressure reaches 10 to 20 Pa, starting sputtering at 30 Pa, and the dual-target sputtering current of 0.5A. The substrate is rotated and grown at a rate of 0.5 to 0.6 nm / min for 5 minutes. The annealing chamber is cooled to below 700°C and the oxygen is stopped. The film is annealed for 30 minutes to obtain an ultra-thin YBCO film with a thickness of 2.5 to 3 nm. The film is placed in a constant magnetic field of more than 6 T and cooled to below 2K to obtain a YBCO film with a quantum metallic state. The present invention realizes superconducting-metal transition on a YBCO film without any other processing. The preparation method is simple and rapid and can be used for large-scale preparation.

Description

Technical Field

[0001] The invention belongs to the technical field of superconducting film preparation, and in particular relates to a method for preparing a YBCO film with a quantum metallic state. Background Art

[0002] In conventional BCS superconductors, when the interaction between electrons in the system changes from mutual attraction to mutual repulsion, the system changes from a superconducting state to a normal metallic state. The above-mentioned superconducting-metal transition process does not have a quantum critical region, and the resulting metallic state is no different from a classical metal, and its properties can be described by the Landau Fermi fluid theory and the Drude model. The superconducting-metal transition is consistent with most experiments, but there are still some resistance saturation phenomena at low temperatures that cannot be explained by classical theory (Fermi fluid theory or Drude model). This state of resistance saturation at extremely low temperatures is called an "anomalous metallic state" or a "quantum metallic state." Compared with conventional metals, the anomalous metallic state has the following characteristics: 1. The resistivity tends to saturate when the temperature approaches 0, and the value is much smaller than the quantum resistance (R Q ≈6.45kΩ), which can even be 3 to 4 orders of magnitude smaller; 2. The emergence of the anomalous metallic state has little to do with the specific type of system disorder (such as sample morphology); 3. It has particle-hole symmetry and the Hall resistance disappears at low temperatures; 4. It has a large positive magnetoresistance effect; 5. There are strong superconducting fluctuations and correlations on finite length scales.

[0003] Quantum materials and quantum phase transitions have been a hot topic in condensed matter physics and materials research this century. Research on two-dimensional superconductor-insulator and superconductor-weakly localized metal phase transitions, textbook examples of quantum phase transitions, won the 2015 Buckley Medal, the highest award in condensed matter physics in the United States. New superconducting quantum devices, such as single-photon detection, superconducting tunnel junctions, and superconducting resistance transition sensors, all operate based on the dramatic change in resistance during quantum phase transitions. Understanding the mechanisms of quantum phase transitions is crucial for designing and fabricating high-performance superconducting quantum devices. Therefore, fabricating thin films in a quantum metallic state is of great significance to fundamental physics research in my country and the development of superconducting quantum devices.

[0004] Currently commonly used to regulate YBCO (YBa2Cu3O 7-δ Methods for achieving superconducting quantum phase transitions in YBCO (yttrium barium copper oxide) thin films include chemical doping, electric field manipulation, magnetic field manipulation, and disorder control. However, chemical doping and electric field manipulation have yet to achieve superconducting-to-metallic phase transitions in YBCO due to process challenges. Furthermore, the upper critical magnetic field of YBCO thin films is very large (the upper critical field Hc for a 50nm thick YBCO film is at least 20T), and the vortex state is very fragile, making it difficult to achieve a quantum metallic state by adjusting the magnetic field without any further processing.

[0005] Attempts have been made to achieve superconducting quantum phase transitions by reducing the thickness of YBCO films to increase their disorder. However, in high-temperature superconducting systems, superconducting properties require very high crystalline quality due to their short coherence length, making it difficult to produce high-quality ultra-thin YBCO films. The thinnest YBCO film currently produced using existing processes is only 10 nm thick.

[0006] In addition, some researchers have found that by attaching a porous aluminum oxide film to a YBCO film as a mask and using a reactive ion etcher to precisely control the etching time, a quantum metallic YBCO film can be obtained. However, this method has a low success rate and is difficult to produce on a large scale.

[0007] Therefore, how to quickly and efficiently prepare YBCO films with quantum metallic states has become the focus of research. Summary of the Invention

[0008] In view of the above problems in the prior art, the present invention provides a method for preparing a YBCO thin film with a quantum metallic state, which does not require mask etching for processing, greatly simplifies the preparation process, and reduces processing time.

[0009] The technical solutions adopted in the present invention are as follows:

[0010] A method for preparing a YBCO film having a quantum metallic state comprises the following steps:

[0011] Step 1: Fix the substrate and YBCO target in the magnetron sputtering equipment, start preheating, and heat the annealing chamber and deposition chamber to above 800°C;

[0012] Step 2: Start pre-sputtering and then turn it off after 10 to 30 minutes.

[0013] Step 3: Place the substrate in a deposition chamber, introduce argon gas, and when the deposition chamber pressure reaches 10-20 Pa, introduce oxygen. When the deposition chamber pressure stabilizes at 30 Pa, start film sputtering, quickly stabilize the dual-target sputtering current at 0.5 A, rotate the substrate, and grow at a growth rate of 0.5-0.6 nm / min for 5 minutes. Turn off film sputtering to obtain a 2.5-3 nm thick ultra-thin YBCO film to be annealed;

[0014] Step 4: Stop the argon flow and adjust the oxygen to a purge state. When the temperature of the annealing chamber drops below 700°C, stop the oxygen flow and place the substrate in the annealing chamber. After annealing for 30 minutes, cool it naturally to room temperature and take out the prepared 2.5-3 nm thick ultra-thin YBCO film.

[0015] Step 5: Place the ultra-thin YBCO film in a constant magnetic field above 6T. When the temperature is lowered to below 2K, a YBCO film with a quantum metallic state is obtained.

[0016] Furthermore, after the 30-minute annealing in step 4, the process of in-situ growing an amorphous YBCO protective layer is also included, specifically: natural cooling, when the temperature drops to below 150° C., argon and oxygen are introduced, when the deposition chamber pressure is stabilized at 30 Pa, thin film sputtering is started, the dual-target sputtering current is stabilized at 0.5 A, the substrate is rotated, and growth is carried out at a growth rate of 0.3 to 0.4 nm / min for 25 minutes, and thin film sputtering is turned off to obtain an amorphous YBCO protective layer with a thickness of 7.5 to 10 nm; after cooling to room temperature, the introduction of argon and oxygen is stopped, the ultra-thin YBCO film with the amorphous YBCO protective layer is removed, and then step 5 is performed.

[0017] Furthermore, the preheating in step 1 is carried out under a gas pressure of 200 Pa. When the temperatures of the annealing chamber and the deposition chamber reach room temperature, 350°C and 600°C, the heating voltages are set to 35V, 50V and 90V respectively, and finally heated to above 800°C.

[0018] Furthermore, when the temperature of the annealing chamber reaches 810° C. and the temperature of the deposition chamber reaches 808° C., pre-sputtering is started.

[0019] Furthermore, the specific process of pre-sputtering in step 2 is: introduce argon gas, and when the pressure in the deposition chamber reaches 20Pa, introduce oxygen gas, and when the pressure in the deposition chamber stabilizes at 30Pa, start pre-sputtering, stabilize the dual-target sputtering current at 0.5A, adjust the sputtering voltage to 220V, pre-sputter for 10 minutes, keep the dual-target sputtering current stable during this period, turn off pre-sputtering, and stop introducing argon and oxygen.

[0020] Furthermore, the substrate is lanthanum strontium aluminate (LSAT) crystal or strontium titanate (STO) crystal.

[0021] The principle process of the YBCO film with quantum metallic state prepared by the present invention is as follows:

[0022] When the carrier concentration of a BCS superconductor is high, a magnetic field can be applied to break up the Cooper pairs, achieving a superconducting-metal transition. However, for copper-based high-temperature superconductors, due to their low carrier concentration and high disorder, a magnetic field should be applied to control the phase coherence between different superconducting islands to achieve a superconducting-metal transition. Under low magnetic fields, the system is still in a superconducting state, the Cooper pairs can move freely, and the vortex state is localized; as the magnetic field increases and exceeds the upper critical field Hc, the vortex state can move freely and the Cooper pairs are localized, and a quantum phase transition occurs. For copper-based superconductors, especially optimally doped copper oxide salts, it is difficult to achieve a quantum phase transition by applying a magnetic field because the upper critical field Hc is extremely large and the vortex state is very fragile.

[0023] The present invention reduces the thickness of the YBCO film to 2.5-3nm to increase disorder, enhance the fluctuation of the superconducting energy gap, and reduce the upper critical magnetic field Hc of the YBCO film, thereby obtaining a YBCO film with a quantum metallic state in a constant magnetic field above 6T.

[0024] The beneficial effects of the present invention are:

[0025] The present invention proposes a method for preparing a YBCO film with a quantum metallic state. By improving the magnetron sputtering process, an ultra-thin YBCO film with a thickness of 2.5 to 3 nm can be prepared. The film is placed in a constant magnetic field of more than 6 T and cooled to below 2 K to obtain a YBCO film with a quantum metallic state. The present invention reduces the thickness of the YBCO film to achieve a superconducting-metallic transition on the YBCO film without any other processing. The preparation method is simple and rapid and can be used for large-scale preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A transmission electron microscope image of a cross section of a YBCO film having a quantum metallic state prepared in Example 1 of the present invention;

[0027] Figure 2 This is a diagram showing the relationship between temperature and resistance of the YBCO film with quantum metallic state prepared in Example 1 of the present invention under magnetic fields of different intensities. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0029] Example 1

[0030] This embodiment prepares a YBCO thin film with a quantum metallic state, which specifically includes the following steps:

[0031] Step 1. Warm up the device

[0032] Turn on the power of the magnetron sputtering equipment, fix the 0.5mm thick strontium tantalum lanthanum aluminate crystal substrate to the deposition fixture, open the argon and oxygen cylinder valves, and turn on the circulating cooling water; evacuate the annealing chamber and deposition chamber, turn on the mechanical pump and pre-pumping valve, pump to 200Pa, close the pre-pumping valve, open the low valve, and turn on the heating power supply at the same time to heat the annealing chamber and deposition chamber. When the temperature reaches room temperature, 350℃ and 600℃ respectively, set the heating voltage to 35V, 50V and 90V.

[0033] Step 2. Pre-sputtering

[0034] When the deposition chamber temperature stabilizes to 808°C and the annealing chamber temperature stabilizes to 810°C, pre-sputtering is started. First, the filling valves of argon (99.99%) and oxygen (99.99%) are opened, and the argon valve is adjusted from the closed state to the valve-controlled state. When the deposition chamber pressure reaches 20Pa, the oxygen valve is adjusted from the closed state to the valve-controlled state. When the deposition chamber pressure stabilizes at 30Pa, the filling is completed; turn on the sputtering unit power supply, adjust the dual-target sputtering current to 0.5A, and the sputtering voltage to 220V. The pre-sputtering process lasts 10 minutes, during which the dual-target sputtering current is kept stable. After the pre-sputtering is completed, turn off the sputtering unit power supply and adjust the argon and oxygen valves to the closed state.

[0035] Step 3. Thin film growth

[0036] The deposition fixture is placed in the deposition chamber through motor control. The temperature of the deposition chamber is kept consistent with the temperature of the deposition chamber during pre-sputtering. The argon valve is adjusted to the valve-controlled state. When the pressure in the deposition chamber reaches 20Pa, the oxygen valve is adjusted to the valve-controlled state. When the pressure in the deposition chamber stabilizes at 30Pa, the sputtering unit power is turned on. The dual-target sputtering current is quickly stabilized at 0.5A, and the deposition fixture is turned on to ensure uniform growth. The growth rate is 0.57nm / min, and the growth time is 5min to obtain an ultra-thin YBCO film.

[0037] Step 4. Film annealing

[0038] Turn off the sputtering current, close the low valve and mechanical pump, adjust the argon valve to the closed state, adjust the oxygen valve to the cleaning state, and then adjust the heating voltage of the deposition chamber and annealing chamber to 40V. After the temperature of the deposition chamber and annealing chamber drops below 700℃, close the argon and oxygen filling valves, and adjust the oxygen valve to the closed state. After maintaining the current annealing state for more than 30 minutes, adjust the heating voltage to 0 and cool down naturally.

[0039] Step 5. In-situ growth of amorphous YBCO protective layer

[0040] When the temperature drops to below 150°C, the argon and oxygen valves are opened, the gas pressure is stabilized at 30 Pa, the dual-target sputtering current is stabilized again at 0.5 A, the fixture rotation is turned on to ensure uniform growth, the growth rate is 0.38 nm / min, and the growth time is 25 min, thereby obtaining a 10 nm thick amorphous YBCO protective layer grown on the ultra-thin YBCO film; after continuing to cool to room temperature, the argon and oxygen are stopped, and the ultra-thin YBCO film with the amorphous YBCO protective layer is taken out.

[0041] The cross section of the obtained ultra-thin YBCO film with an amorphous YBCO protective layer was examined by transmission electron microscopy, and the following results were obtained: Figure 1The transmission electron microscope image shown shows that the thickness of the obtained ultra-thin YBCO film is 2.6 nm and the film has excellent crystal quality.

[0042] Step 6. Apply an external magnetic field

[0043] The ultra-thin YBCO film with an amorphous YBCO protective layer was connected to the test bench through aluminum wire welding. The relationship between the film resistance and temperature was tested using a comprehensive physical property measurement system under the conditions of a magnetic field strength of 0 to 14 T and gradually decreasing temperature. The results are as follows: Figure 2 As shown, it can be seen that when the temperature drops below 2K, for a magnetic field strength of less than 6T, although the vortex is localized, the Cooper pairs can still move freely, so the system is still in a superconducting state; after the magnetic field strength exceeds 6T, the Cooper pairs gradually localize, and then a quantum phase transition occurs. The film resistance at 2K temperature is maintained at 0.1Ω, showing the characteristics of a quantum metallic state, and thus a YBCO film with a quantum metallic state is obtained.

[0044] The above embodiments only illustrate the principles and advantages of the present invention, and are not intended to limit the present invention. They are only for helping to understand the principles of the present invention. The scope of protection of the present invention is not limited to the above configurations and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the disclosed technology without departing from the essence of the present invention, but they are still within the scope of protection of the present invention.

Claims

1. A method for preparing a YBCO thin film having a quantum metallic state, characterized in that: The following steps are involved: Step 1: Fix the substrate and YBCO target in the magnetron sputtering equipment, start preheating, and heat the annealing chamber and deposition chamber to above 800°C; Step 2: Start pre-sputtering and then turn it off after 10 to 30 minutes. Step 3: Place the substrate in a deposition chamber, introduce argon gas, and when the deposition chamber pressure reaches 10-20 Pa, introduce oxygen. When the deposition chamber pressure stabilizes at 30 Pa, start film sputtering, stabilize the dual-target sputtering current at 0.5 A, rotate the substrate, grow at a growth rate of 0.5-0.6 nm / min for 5 minutes, and turn off film sputtering to obtain a 2.5-3 nm thick ultra-thin YBCO film to be annealed; Step 4: Stop the argon flow and adjust the oxygen to a purge state. When the temperature of the annealing chamber drops below 700°C, stop the oxygen flow and place the substrate in the annealing chamber. After annealing for 30 minutes, cool it naturally to room temperature and take out the prepared 2.5-3 nm thick ultra-thin YBCO film. Step 5: Place the ultra-thin YBCO film in a constant magnetic field above 6T. When the temperature is lowered to below 2K, a YBCO film with a quantum metallic state is obtained.

2. The method for preparing a YBCO thin film having a quantum metallic state according to claim 1, wherein: After the 30-minute annealing in step 4, the process of in-situ growing an amorphous YBCO protective layer is also included. Specifically, the process includes: natural cooling, and when the temperature drops to below 150° C., argon and oxygen are introduced. When the pressure in the deposition chamber stabilizes at 30 Pa, thin film sputtering is started, the dual-target sputtering current is stabilized at 0.5 A, the substrate is rotated, and growth is continued at a growth rate of 0.3 to 0.4 nm / min for 25 minutes. The thin film sputtering is then turned off to obtain an amorphous YBCO protective layer with a thickness of 7.5 to 10 nm. After the temperature drops to room temperature, the introduction of argon and oxygen is stopped, the ultra-thin YBCO film with the amorphous YBCO protective layer is removed, and then step 5 is performed.

3. The method for preparing a YBCO thin film having a quantum metallic state according to claim 1, wherein: The preheating in step 1 is carried out under a gas pressure of 200 Pa. When the temperatures of the annealing chamber and the deposition chamber reach room temperature, 350°C and 600°C, the heating voltages are set to 35V, 50V and 90V respectively, and finally heated to above 800°C.

4. The method for preparing a YBCO thin film having a quantum metallic state according to claim 1, wherein: When the temperature of the annealing chamber reaches 810° C. and the temperature of the deposition chamber reaches 808° C., pre-sputtering is started.

5. The method for preparing a YBCO thin film having a quantum metallic state according to claim 1, wherein: The specific process of pre-sputtering in step 2 is as follows: introduce argon gas, and when the deposition chamber pressure reaches 20Pa, introduce oxygen gas. When the deposition chamber pressure stabilizes at 30Pa, start pre-sputtering, stabilize the dual-target sputtering current at 0.5A, adjust the sputtering voltage to 220V, pre-sputter for 10 minutes, keep the dual-target sputtering current stable during this period, turn off pre-sputtering, and stop introducing argon and oxygen.

6. The method for preparing a YBCO thin film having a quantum metallic state according to claim 1, wherein: The substrate is strontium tantalum lanthanum aluminate crystal or strontium titanate crystal.

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