Method for synthesizing cage-mesh calcium boride superconductor at high temperature and high pressure

Abstract

The invention relates to a method for synthesizing a cage-type calcium boride superconductor at high temperature and high pressure, and belongs to the technical field of preparation of high-temperature superconducting materials. According to the method, elementary substance B and elementary substance Ca are taken as initial raw materials, and a diamond anvil cell device is adopted to perform in-situ laser heating on a sample under target pressure, so that the cage-mesh type calcium boride superconductor P6 / mmm CaB3 is synthesized. The prepared calcium boride has a kagome B layer structure, and the superconducting transition temperature (Tc) can reach 22K, which is the highest superconducting transition temperature in known Kagome materials. The method has important scientific significance for understanding the complex relationship between the electronic structure of the metal boride and the macrophysical characteristics of the metal boride, and also provides a direction for searching a light element superconductor with a novel structure.

Description

Technical Field

[0001] The invention belongs to the technical field of high-temperature superconducting material preparation, and particularly relates to a cage-type calcium boride superconducting material and a preparation method thereof. Background Art

[0002] Light element superconductors characterized by light elements such as H and B have always been the focus of research in the field of condensed matter physics, because according to the classical BCS theory, they have high Debye temperatures and may exhibit higher superconducting transition temperatures, making them strong candidates for achieving high-temperature superconductivity. Since the discovery of the traditional superconductor MgB2, its superconducting transition temperature of 39K is close to the Mcmillan limit, which has inspired extensive research on metal boride superconductors. In the past two decades, various metal boride superconductors with different structures have been experimentally synthesized one after another. Such as P6 / mmm MoB2 containing a B layer structure, and Fm-3mYB containing a B-based cage structure. 12 At the same time, theoretical calculations have predicted various new metal boride superconductors, especially two-dimensional kagome metal borides, such as P6 / mmm MgB6, P6 / mmm CaB6, and P31m KB9. However, experimental research on stable new high-temperature superconducting metal borides still needs to continue. Two-dimensional kagome materials have unique crystal structures and electronic structures, and many exotic phenomena have emerged in recent years, including charge density waves, superconductivity, spin density waves, quantum spin liquids, etc. Following similar principles, calcium borides with kagome lattice structures have become the focus of attention due to their unique crystal structure and potential for high-temperature superconductivity. In addition, theoretical predictions have proposed many calcium boride superconductors with different structures, including P6 / mmm CaB2 with honeycomb B layers and P4 / mmm CaB5 and Cmcm CaB7 with different B cage structures. However, high-temperature superconductivity has not been observed experimentally so far. Therefore, the high-pressure synthesis of calcium boride superconductors with new structures (such as kagome B layers) is of great scientific significance for filling the research gap of calcium boride superconductors and deciphering the complex relationship between the electronic structure of metal borides and their macroscopic physical properties. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the problems and defects of the background technology and provide a new method for preparing a new calcium boride superconducting material at high temperature and high pressure.

[0004] The method uses elemental B and elemental Ca as starting materials, and uses a diamond anvil cell (DAC) to perform in-situ laser heating on the sample under a target pressure, thereby synthesizing a cage-type calcium boride superconductor P6 / mmm CaB3. The calcium boride prepared by the present invention has the following excellent properties: superconducting transition temperature (T c) can reach 22K and has a kagome B layer structure.

[0005] The specific technical solutions of the present invention are as follows:

[0006] A method for synthesizing a kagome-type calcium boride superconductor at high temperature and high pressure comprises the following steps: firstly, filling a DAC pressure chamber with single substance B in an argon-protected glove box, and then installing metal Ca on the B layer; subsequently, pressurizing the starting material to 49 GPa by using a DAC device; and then uniformly performing high-pressure in-situ laser heating on the sample by using a 1070 nm infrared laser, wherein the heating temperature is 1500-2000 K, the single heating time is controlled within 1 minute, the time interval between two heatings is 1 minute, the heating is repeated 6-8 times, the pressure after heating is 51 GPa, and a P6 / mmm CaB3 superconducting material is obtained, namely, the kagome-type calcium boride superconductor.

[0007] Preferably, the single substance B fills the DAC pressure cavity with a thickness of 15 μm and a diameter of 50 μm; the metal Ca has a thickness of less than 5 μm and a diameter of 50 μm.

[0008] Beneficial effects:

[0009] The calcium boride prepared by the present invention has a kagome B layer structure and a superconducting transition temperature (T c ) can reach 22K, which is the highest superconducting transition temperature among known Kagome materials. The superconductor prepared by the present invention is of great significance for understanding the relationship between the electronic structure and macroscopic physical properties of metal borides, and also provides a direction for finding light element superconductors with novel structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a four-electrode microphotograph of the raw materials before reaction and the products after reaction in the DAC used in the present invention.

[0011] Figure 2 It is a curve diagram showing the change of resistance of the P6 / mmm CaB3 superconductor prepared by the present invention at 51 GPa as a function of temperature.

[0012] Figure 3 The present invention discloses a curve of resistance variation with temperature of the P6 / mmm CaB3 superconductor prepared by the present invention under different magnetic fields at 61 GPa, and an upper critical magnetic field obtained by fitting the GL and WHH models.

[0013] Figure 4 It is a curve diagram showing the change of resistance of the P6 / mmm CaB3 superconductor prepared by the present invention with current at different temperatures at 54 GPa.

[0014] Figure 5It is a synchrotron radiation XRD refinement spectrum of the P6 / mmm CaB3 superconductor prepared by the present invention under a pressure of 53 GPa.

[0015] Figure 6 This is the crystal structure of the P6 / mmm CaB3 superconductor prepared by the present invention. DETAILED DESCRIPTION

[0016] The present invention is now described in more detail with reference to the following examples. Unless otherwise specified, all reagents used are commercially available products and are not further purified.

[0017] Example 1 High temperature and high pressure preparation of cage-type calcium boride superconductor P6 / mmm CaB3

[0018] The preparation of calcium boride superconductors needs to rely on non-magnetic Be-Cu diamond anvil device (DAC). Ordinary type Ia diamond is selected, and the anvil diameter is 100μm. This device can easily realize laser heating and sample synthesis under million atmospheres. The preparation process of the DAC electrical device is as follows: First, the tungsten sheet with an initial thickness of about 250μm is pre-pressed to about 20GPa, and the indentation is about 20μm thick. Then, a hole with a diameter of 120μm is made in the center of the indentation using a laser drilling device as a cavity for loading the insulating layer. The part of the tungsten sheet except the indentation is coated with black glue, and a layer of cubic boron nitride mixed with a small amount of epoxy resin is first filled in the indentation, and the pressure is applied to 20GPa. Then, a hole larger than the anvil surface is made using a laser drilling device, and magnesium oxide mixed with a small amount of epoxy resin is pressed into it to act as an insulating layer, which can more effectively protect the electrode and improve the light transmittance of the anvil surface. Finally, a hole with a diameter of 50μm is made in the center of the insulating layer as a sample cavity for encapsulating the reactants. Four 5μm thick Pt electrodes were manually laid on the diamond, and then connected to it with external copper wires and conductive silver glue. In a glove box with a water and oxygen content of less than 0.01ppm, the DAC sample cavity was first filled with elemental B (purity 99.9%), and then metal Ca (purity 99.9%) was pre-pressed into a thin sheet less than 5μm thick and about 50μm in diameter and installed on the B layer. The diamond gauge pressure method is used here to determine the pressure. The sample is pressurized to 49GPa, and then a 1070nm infrared laser is used to perform high-pressure in-situ laser heating on the sample. The heating temperature is 1500-2000K, and the single heating time is controlled within 1 minute. The time interval between two heatings is 1 minute. The heating is continued for 6-8 times. The pressure after heating is 51GPa, and the calcium boride superconductor P6 / mmm CaB3 is obtained. The photos of the sample cavity before and after heating are shown in the figure. Figure 1 shown.

[0019] Example 2 Electrical properties of cage-type calcium boride superconductor P6 / mmm CaB3

[0020] The four-electrode method was used to measure the electrical transport properties of the samples. Figure 2 This is the resistance variation curve of the P6 / mmm CaB3 superconductor prepared in Example 1 at 51 GPa, indicating that the superconducting transition temperature of the sample is about 22K. Figure 3 The resistance curve of the P6 / mmm CaB3 superconductor prepared in Example 1 under different magnetic fields at 61 GPa shows that the superconducting transition temperature of the sample gradually decreases with the increase of the magnetic field, further confirming the superconducting behavior of the sample. The upper critical magnetic field μ0H of the P6 / mmmCaB3 superconductor is obtained by fitting the Ginzburg-Landau (GL) equation and the Werthamer-Helfand-Hohenberg (WHH) equation. c2 (0), and their values ​​are 9.8T and 11.5T respectively. Figure 4 The figure is a curve diagram showing the change of resistance of the P6 / mmmCaB3 superconductor prepared in Example 1 at different temperatures and at 54 GPa as a function of current, which further proves the superconductivity of the P6 / mmm CaB3 superconductor.

[0021] Example 3 Structural Characterization of the Kagome-type Calcium Boride Superconductor P6 / mmm CaB3

[0022] After the sample reaction is complete, the crystal structure properties of the sample can be characterized by (synchrotron radiation) X-ray diffraction. Figure 5 This is the X-ray diffraction spectrum of the sample collected at BL15U1 beamline of Shanghai Synchrotron Radiation Light Source at 53GPa. In the figure, it can be seen that in addition to the peaks of reactants Ca, B and electrode Pt, there is a newly generated unknown X-ray diffraction signal (later identified as the target product P6 / mmm CaB3). Combined with theoretical calculation methods, it is determined that the new product is the calcium boride superconductor P6 / mmm CaB3. The crystal structure of P6 / mmm CaB3 is a three-dimensional structure composed of sequentially stacked kagome B layers and Ca layers. The three-dimensional structure is as follows Figure 6 shown.

Claims

1. A method for synthesizing a cage-type calcium boride superconductor at high temperature and high pressure, firstly, in an argon-protected glove box, a single substance B is filled into a DAC pressure chamber, and then metal Ca is loaded onto the B layer. Subsequently, the starting material is pressurized to 49 GPa using a DAC device, and then a 1070 nm infrared laser is used to uniformly perform high-pressure in-situ laser heating on the sample, the heating temperature is 1500-2000 K, the single heating time is controlled within 1 minute, the time interval between two heatings is 1 minute, the heating is repeated 6-8 times, and the pressure after heating is 51 GPa, thereby obtaining a P6 / mmm CaB3 superconducting material, i.e., the cage-type calcium boride superconductor.

2. The method for synthesizing a Kagome-type calcium boride superconductor at high temperature and high pressure according to claim 1, characterized in that: The single substance B fills the DAC pressure cavity with a thickness of 15 μm and a diameter of 50 μm; the metal Ca has a thickness of less than 5 μm and a diameter of 50 μm.

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