2-1. Preparation of -FeSe
Iron granules (99.98%, Alfa Aesar) and Selenium shot (99.999%, Alfa Aesar) were mixed with a proper molar ratio (1+x:1, x = 0.005~0.012) and loaded into a one-end-sealed home-made quartz crucible (outer diameter of 10 mm, length of 30mm and wall thickness of 1.25 mm) and fill with argon. Then the crucible was sealed in a 90 mm long quartz tube with inner diameter of 15 mm. The quartz tube was slowly heated in a high-temperature furnace from room temperature to 750oC in 8 hours (93.75 oC /h) and kept for about 2.5 days to ensure complete reaction of selenium and iron. The sample was further heated to 1075 oC in 4 hours (81.25 OC/h) and kept about 1 day for proper melting and mixing. The sample was then quenched to 400 oC in 3~5 minutes and annealed for 4~6 days. Finally, the quartz tube was quenched in liquid nitrogen.
To prepare high quality -FeSe samples, the first soaking duration is vital that it must be long enough to ensure the Fe-Se reaction is complete. The completeness of reaction can be easily distinguished from the color of the quartz tube. If there is un-reacted selenium vapor, the tube will show a light-red-wine color. As shown in Figure 2-1, there is a wide temperature range for the Fe-Se reaction. However, different choice of temperature will change the FeSe solid-Se vapor equilibrium, and further change the final product properties.
Another key point is the annealing temperature, see Figure 2-2, that it should not be too high to crystallize into -FeSe phase.
In order to prepare high quality -FeSe with high superconducting Tc and no hexagonal non-superconducting hexagonal -FeSe phase or other impurities, some details of procedures were worth to be marked. The most important one is that the cooling rate of reacted FeSe from 1075oC to 400oC was much slower than the quartz tube, which makes
the contacting region of the quartz tube with the FeSe bulk to become cracked
samples were prepared by simply load the ingredients into a single walled quartz tube, there will be air leaking through the crack and resulted in formation o
FeOx and SeO2 (dark red on the quartz wall) impurities 2-4. To avoid cracking of the quartz tube, home
10 mm, length of 30mm and wall thickness of 1.25 ingredients inside the evacuated quartz tube.
crucibles is that their bottom should have as less contact area with the outer tube as possible(see fig. 2-4, fig. 2
Fig. 2
the contacting region of the quartz tube with the FeSe bulk to become cracked
samples were prepared by simply load the ingredients into a single walled quartz tube, there will be air leaking through the crack and resulted in formation of unwanted
(dark red on the quartz wall) impurities as shown as figure 2
To avoid cracking of the quartz tube, home-made quartz crucibles (outer diameter of 10 mm, length of 30mm and wall thickness of 1.25 mm) were used to hold the reaction samples were prepared by simply load the ingredients into a single walled quartz tube, there f unwanted hexagonal-FeSe, as shown as figure 2-3 and figure made quartz crucibles (outer diameter of mm) were used to hold the reaction One more important thing about the quartz crucibles is that their bottom should have as less contact area with the outer tube as
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Fig. 2-2. Binary phase diagram of Fe1+xSe from temperature T = 280 oC to 480 oC [Phys.
Rev. B 79 (2009) 014522]
Fig. 2-3 Selenium oxides glued to the wall in Left quartz tube. Right quartz tube is quite clear.
Fig. 2-4 In the begin, the samples were sealed in single quartz tube. The quartz tube was crack finally.
Fig. 2-5 (a) Smaller quartz
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Fig. 2-5(b) Smaller quartz
Fig. 2-6 -Fe1+xSe in Double quartz tube no crack.
2-2. Preparation of A
x(NH
3)Fe
2Se
2(A = Ba, Sr or Ca)
The alkaline earths metal (Ba, Sr or Ca) and ammonia molecule intercalated Ax(NH3)Fe2Se2 (A = Ba, Sr or Ca) superconductors were prepared by ammonothermo-reaction. The previous prepared -FeSe and alkaline earths metal were placed into the autoclave with desired ratio (2:0.8 in molar ratio) with magnetic stirrer.
Little amount of fresh n-hexane was used to avoid unwanted oxidation of the alkaline earth metal and -FeSe powder. The autoclave was then evacuated carefully for no residual n-hexane before the introduction of ammonia into the autoclave. Gaseous ammonia (99.9 %) was then injected into the autoclave to a pressure slightly greater than atmosphere pressure.
After closing the valve, the autoclave was cooled by merging into liquid nitrogen for ammonia condensation. The ammonia injection valve was opened when the pressure inside the autoclave was lowered to allow ammonia gas flowing in for condensation. The injected amount of ammonia gas was monitored by a flow meter until desired volume. The filled autoclave was left in room temperature for 3.5~4 days for reaction with continuous stirring using magnetic stirrer.
The volume of ammonia was determined by the dissolved concentration of alkaline earth metal to liquid ammonia to be between 0.1~0.3 at%. In addition, to ensure complete intercalation reaction, the ammonia liquid inside autoclave should be at least of 5 mm deep (A + -FeSe : ammonia ~ 1 g : 8.75 ml, A = Ba, Sr or Ca).
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Fig. 2-7. Ammonothermal Method Schematic diagram
Fig. 2-8 Equipment for ammonalthermal methods.
2-3. Analysis of structure by X-ray diffraction
The powder X-ray diffraction patterns of the prepared samples were be performed partially in the Physics Institute of Academia Sinica and others were measured in Department of Physics in National Taiwan University. The former is made by a Rigaku XXXX diffractometer with a Cuα (λ = 1.54187 Å) anode under 45 kV accelerating voltage and 40 mA electron beam current. The later was carry out by a Rigaku Rotating Anode XXX diffractometer with 50 kV accelerating voltage and 100 mA electron beam current.
The scanning range of the 2 is from 10o to 50o with a 0.02o step, in the cases using NTU diffractometer a 0.025o step for continuous mode and 0.01o step for incontinuous mode were used. The schematic diagram in Fig. 2-9 shows the operating concepts of the instruments.
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2-4. Magnetism measurements
The magnetization and magnetic susceptibility measurements were carried out by a QUANTUM DESIGN MPMS2 superconducting quantum interference device (SQUID) magnetometer with a temperature control module that provides an active-regulated, precision thermal environment over range from 2 to 350 K and applied magnetic field from 5 gauss to 1 tesla.
The superconducting magnet system provides a reversible field operation ±1 T using a non-overshoot technique for irreversibility of magnetic susceptibility measurement. The SQUID detector system that includes model 2000 SQUID amplified control electronics, sensing pick-up and special filter with full computer control via the HP-150 interfaced computer. The block diagram of SQUID detector system is shown in Fig. 2-10 and Fig.
2-11. The sample was loaded in a capsule and attached to the sample holder by a clear plastic straw. After putting into the sample chamber, the sample position was carefully calibrated to make it site at the center of SQUID detecting coil array. The data were measured and stored automatically by the MPMSR2 software.
For zero-field cooled (ZFC) measurements, the “Magnetic reset” option was used to quench the superconducting magnet before cooling procedure to reduce the residual or remnant field to less than 0.2 G. Due to the temperature control mechanism, whenever the temperature was raised from lower than 4.4 K to higher than 4.5 K the sample was heated to a temperature higher than its Tc and field was set to zero before cooling down to the measurement temperature. These additional operations were added under the consideration of the limitation of the SQUID temperature control processes, to ensure all cooling processes are truly under ZFC mode.
Fig. 2-10. Block diagram of SQUID detector model SPMS for magnetization and Magnetic susceptibility measurement system of SQUID (QUANTUM DESIGN,
MPMS/MPMS-7 systems)
MPMS response to Dipole Point Source
Fig. 2-11. MPMS Response to Dipole Point Source of SQUID (QUANTUM DESIGN, MPMS/MPMS-7 systems)
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2-5. Electric Resistance measurement
The system will separate three parts to description in this section:
Measurement system
Resistance of sample is measured by four-probe technique. The schematic diagram show as this below:
The power of circuit comes from lock-in amplifier then connects to the resistance switching box. On the other hand, the Rreg not only can prevent the total current overload but control the total current during measure. The different temperature will caused the total resistance changed dramatically only if this Rreg is 10 to 100 times than sample which is measured. The work of Rref helped us to calculate the total current in this circuit. Two volts of Rref and Rsample are measured by two lock-in amplifier which are connected to computer and controlled by LabVIEW.
Rreg: total six resistance can be switched (1, 10, 100, 1K, 10K, 100K Ω) Rref: convert this volt for current
Resistance switching box
I + V + V - I - Sample
Rreg: prevent the total current overload
Rref: total six resistance can be switched (0.1, 1, 10, 100, 1K, 10K Ω)
Fig. 2-12 Schematic circuit of electrical resistance measurement
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Closed-cycle refrigerator system
Basic principle:
The principle of closed-cycle refrigerator is called Gifford-McMahon (GM) refrigerator. Here have 4 steps to cool the metal which is chose.
Fig. 2-13 Schematic diagram of closed-cycle refrigerator
Step1 (a) to (b): while high pressure enters the space, the displacer is moved to the left. High pressure gas is set at the right.
Step2 (b) to (c): the valve is switched from high to low pressure. The high pressure gas expends then absorbs heat from TL state.
Step3 (c) to (d): the displacer is reset to the right and let the displacer pass through low temperature gas.
Step3 (d) to (a): through the high pressure in the left space and repeat Step1 again.
TH TL
(a) (b)
(c) (d)
The 4 Kelvin Closed Cycle Refrigerator Systems from Janis and SHI
Sumitomo (SHI) 4 K Refrigerator - RDK-408D2 have two GM refrigerator stages and the working gas is helium. The system can cool down the cold finger (sample holder) to 4.2 Kelvin at second stage. The connection between lock-in, resistance switching box, heater and closed cycle system will show in next paragraph.
PL
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Instrument setup:
The R-T measurement system is connected by total 22 enameled insulated wires from sample holder space to thermometer and resistance switch box. From the fig, the sample space is desired to place 3 samples which are measured in the same while (12 enameled wire need to be used) for convenience and advanced hardware in the future.
The two sensors are placed in different position. The sensor I is contacted well to the cold finger in order to detect the temperature right at the place and the sensor II is placed to the sample holder because the distance between sensor I and sample holder which is removable provides the temperature which have a gradient, also, the sensor II can avoid the different temperature from sensor II to sample when they are closed (8 enameled wire need to be used).
Last one thing is heater which is placed at near the cold finger and is used to control temperature at the second between the measurements from this moment to next one. This heating controller usually needs couple, one of them offers a main power to resist the power of cold head and another one maintain the temperature which is set. But now this system has one heater to control for desire. Because the well two layer shielding can resist the radiation and offer a vacuum space to defend the thermal convection from gas, the system can be consider a thermal equilibrium state during measurement from 4 K to 280 K (4 enameled wire need to be used).
Fig. 2-14 Schematic diagram of closed-cycle refrigerator with resistence box and sample
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measurement control by LabVIEW
This program is desired to calculate the resistance of sample when the specific temperature is stood still then plot and save data. One of two functions is data selection, another one is desired as a controller for heater in this program.
Data selection:
The basic idea is that we want to clean up the data which isn’t need, so the data is selected in a specific range. For temperature, the condition of the data which is decided to save is how different from the last point that have been saved to newest one. Constrain of decision is not the same for every range (for instance, form 273 K to 200K, per 1 K saved one point).
In order to save the phase transition in R-T diagram precisely, for the resistance part, the condition is that the newest one value of resistance is different from the last one about 5
%. If the condition is agreed, the data will be saved and become the last one data.
Heat Controller:
This heat controller relies on the temperature which is detected by Sensor I to optimize the value of PID and power limit. The first point about temperature controller is that PID of thermometer is needed to be found for each range of specific temperature and every PID accompany different power limit must to fix one.
Due to the data will out of control during the sensitive of PID and power limit switching, the program has to by pass the unreal data about combination of data selection and heat controller by case selection.