A system for insitu pressure and ac susceptibility measurements using thediamond anvil cell: T c (P) for HgBa2CuO4+δCharles C. Kim, M. E. Reeves, M. S. Osofsky, E. F. Skelton, and D. H. Liebenberg Citation: Review of Scientific Instruments 65, 992 (1994); doi: 10.1063/1.1144878 View online: http://dx.doi.org/10.1063/1.1144878 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/65/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The scaling analysis on effective activation energy in HgBa2Ca2Cu3O8+δ J. Appl. Phys. 111, 07D709 (2012); 10.1063/1.3673436 Susceptibility measurements at high pressures using a microcoil system in an anvil cell Rev. Sci. Instrum. 74, 4728 (2003); 10.1063/1.1614861 Magnetic susceptibility measurements at high pressure using designer diamond anvils Rev. Sci. Instrum. 74, 2467 (2003); 10.1063/1.1544084 Annealing temperature and O2 partial pressure dependence of T c in HgBa2CuO4+δ J. Appl. Phys. 76, 7127 (1994); 10.1063/1.358052 Pressure–volume measurements by using diamondanvil cells and an image processing system Rev. Sci. Instrum. 57, 500 (1986); 10.1063/1.1138918

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A system for in-Mu pressure and ac susceptibility measurements using the diamond anvil cell: T,(P) for HgBa2CuQ4+6 c

Charles C. Kim, M. E. Reeves, a) M. S. Osofsky, and E. F. Skelton Dynamics of Solid Branch and Materials Physics Branch, Naval Research Laboratory, Washington D. C 203 75-534s

D. H. Liebenberg O#ice of Naval Research, Arlington, Virginia 22217-5000

(Received 14 October 1993; accepted for publication 20 December 1993)

Substantial improvements have been made in the sensitivity of an ac susceptibility measurement system which determines the pressure dependence of the superconducting transition temperature, T,(P). The pressure, P, is determined near T, from the position of the RI and R2 fluorescence peaks from ruby chips. The system was used to determine T,(P) for several high T, single crystals, but was not sufficiently sensitive to determine T,(P) of polycrystalline samples for HgBa2Cu04+,s. This motivated us to improve the sensitivity of our system. Other parts of the system and the data analysis procedures also have been improved. As a demonstration, T,(P) of HgBa,Cu04+6 p 01 cr y y stal to 4 GPa is presented. The improvement of the system using the diamond anvil cell opens the possibility of measuring T, above 10 GPa and on smaller samples.

I. lNTRODUCTlON

Knowing the superconducting transition temperature as a function of pressure, T,(P), is important because that information can guide research directed at improving T,(O). For example, T, of ( IaesBao.l)2C~04--y is 39 K at atmospheric pressure and increases to 52.5 K at 1.2 GPa, at a rate of 11 K/GPa.’ Motivated by this observation, Chu et al. discovered YBazCus07 with T, above 90 K.2 Additionally, T,(P) measurements help in understanding the mechanism of superconductivity for new materials.

Many groups have reported T,(P) of high T, super- conducting materials using various methods.3*4 In our sys- tem, pressure is applied to the sample by the diamond anvil cell and measured near T,. T, is determined from mea- surements of the ac susceptibility, x(T)=x’( T) + ix” ( T) , as a function of temperature. The advantage of our system over others5’6 is that we measure P and X(T) simultaneously near the sample transition temperature. Us- ing this system, we have reported T,(P) of single crystals of T12Ba&aCu,0s+8,7 T12Ba2Ca2Cus010-8,8 and NdBa&usO,-6 (Ref. 9) to 7 GPa.

Recently, the compound HgBa2Cu04+B (Ref. 10) was found to be superconducting with T, around 90 K. Our ac susceptibility measurement, however, was not sensitive enough to determine T,(P) of the small poIycrystalline HgBa2Cu04+s samples. This motivated us to improve the system to obtain T,(P) with better precision and accuracy.

Pressure can be applied to the sample by a number of techniques.3,” The helium-gas cell is one of the techniques, but its highest pressure is limited to 1.5 GPa. In fact, Klehe et aLI reported T,(P) of HgBa2Cu04+s to 1 GPa using the helium-gas cell. To reach higher pressures the diamond anvil cell (DAC) is the apparatus of choice. The DAC

‘IPresent address: Physics Department, George Washington University, Washington, D.C. 20052.

enables one to reach far above 10 GPa, but there is a constraint on the sample size. As the size of the culet and pressure cavity decreases, the maximum pressure produced inside the cavity increases.‘3 Thus, higher pressures will be generated with smaller samples. This decrease in sample volume makes the determination of T, from x(T) more difficult because the amount of flux excluded from the sam- ple decreases. Even for the same size of the sample, the change in ,y( T) due to superconductivity for the polycrys- talline sample is usually smaller than that for the single crystal. It is obvious that the sensitivity of the ,y( T) mea- surement must be improved in order to reach higher pres- sures using a DAC.

The sensitivity of ac susceptibility in our previous sys- tem has been improved by modifying the geometrical con- figuration of the coils and the related circuits. The ther- mometry has been also improved by introducing a thermal radiation shield around the DAC. The determination of onset temperature from the data is now performed system- atically by fitting the processed data with an appropriate function, thereby eliminating human judgments. The im- proved system together with the improved analysis has been used to determine T,(P) of a polycrystalline sample of HgBa2Cu04+s to 4 GPa. The new system opens the possibility of determining T,(P) above 10 GPa with better accuracy.

II. INSTRUMENTATION

A. Overview

Figure 1 shows a schematic diagram of our experimen- tal setup. The cryostat is cooled by the closed-cycle He- refrigerator (CRYOMECH GB220). During operation, a vacuum pressure below 50 p.Torr is maintained to provide thermal isolation. The cryostat cap has two silica windows which allow optical access. The top window allows gath- ering of the ruby fluorescence from the DAC by the use of

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, u” prl ,“---I M$;CO~ 4 r;m PhatomT 1 I a=-- IP*uo

- Diamond anvil cell.

FIG. 1. Schematic diagram of the experimental setup to obtain T,(P).

the microscope. The bottom window enables us to locate the pressure cavity which is illuminated from the bottom. The data are acquired by a Macintosh 11x using Lab View software (National Instruments).

The heart of the system is the DAC which is used to generate a controlled, static pressure in a confined cavity.14*15 Its size (3.5X3.5X3.7 cm) is small enough to be attached to the cold head inside the cryostat. It consists of a pair of diamonds, one stationary and the other mounted on a movable piston, and a gasket with pressure cavity at the center. Each diamond has a girdle diameter of about 4.3 m m and weighs about 0.5 carat (Dubbledee Har- ris Diamond). The culet diameter of each diamond is about 1 mm. The pressure cavity is in the shape of a right circular cylinder and has a diameter of about 700 ym or less. The gasket thickness varies from 200 to 500 pm. The gasket material is either Be-Cu or Inconel (Into Alloys). Any other material can be used as long as it has a high tensile-strength and is non-magnetic. If the gasket material is magnetic, the signal-to-noise ratio in the measurement of x( T) decreases, preventing the accurate determination of T,. The sample must fit into the pressure cavity and allow room for the surrounding fluid and ruby chips to ensure the hydrostatic pressure.

B. In-situ pressure measurement

Pressure is determined by the shift in wavelength of fluorescence from ruby. The DAC is clamped at room tem- perature thus setting the pressure in the sample chamber at room temperature. Due to differences in the thermal ex- pansivities of the various components of the cell, the pres- sure normally increases as the cell is cooled. In our system, P can be measured at any temperature. This is possible by comparing the R, and R2 fluorescence peaks from ruby chips inside and outside the pressure cavity.g The pressure in the cavity is determined from the shift in the peak po- sition, h/z. Noack and Holzapfeln’ demonstrated that h/z/ AP is thermally invariant. We use 0.365 nm/GPa for h/z/ AP from Jayaraman” to determine the pressure at all tem- peratures. This value of AUAP is valid up to 30 GPa.18

Figure 2 shows a typical relationship between the pres- sure measured at room temperature and near T,. One set

of data was obtained using methanol:ethanol::4:1 as pres- sure medium and the other, using silicone oil. The results indicate that the pressure fixed at room temperature is roughly doubled near T, regardless of the pressure me- dium. This relationship may be useful if pressure cannot be measured near T,. Such extrapolation, however, is not rec- ommended, since the relationship is not quite linear in our system and may vary from setup to setup. The error in determining A,% is of the order of *0.02 nm which corre- sponds to an uncertainty in pressure of +0.06 GPa. This is due to the uncertainties of wavelength of the monochro- mator.

C. Thermometry

Temperature is determined by monitoring the voltage across a calibrated Si thermodiode (Lake Shore DT-470- SD-13) whose accuracy and precision are ho.5 K and ho.01 K, respectively, in the range between 2 and 100 K. The thermodiode is too big to be placed inside the pressure cavity and therefore is located as close to the pressure cav- ity as possible. Nevertheless, a thermal hysteresis of about 2 K between T, obtained from x’(T) upon cooling and that from x’ (T) upon warming was observed.g Moreover, an absolute value of T, could not be determined accurately and had to be adjusted using the values obtained from other systems. The error was clearly due to the thermal gradient inside the DAC, a result of the lack of a thermal radiation shield. Our primary interest was to measure pres- sure at all temperatures, which prevented us from install- ing the radiation shield around the fist stage of the cry- ostat and the DAC in the past.

The system has been modified to cover the first stage with a radiation shield tube and the DAC with a radiation shield cap. Most of the upper part of the DAC still re- mained uncovered, since the distance between the upper part of the DAC and the optical window had to be main- tained within 2 m m to allow optical access. In order to establish the accuracy of the T, measurement, x(T) for the same sample was measured several times as tempera-

‘.‘.‘,.“‘.“‘.“““““,,I 0 0.5

Pressure at’ room ZimperaLre (GEI) 3

FIG. 2. Relationship between pressure at room temperature and pressure near T, inside the DAC whose pressure is fixed at room temperature. The solid line is a guide to the eye assuming 100% increase.

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TABLE I. T, and its standard deviation, U, for the same sample without the radiation shield cap around the DAC and with it.

cooling Warming

Without the cap With the cap TC fs TC c7

91.78 0.06 91.28 0.11 92.06 0.11 91.22 0.14

ture decreased or increased. From each measurement, T, at the onset temperature was evaluated following the pro- cedure described in Sec. III. Table I summarizes the results with the radiation shield around the DAC and without it. In both cases, the first stage was enclosed by a radiation shield. Notice the effect of the radiation shield cap on the T, determination. The reproducibility is similar in both cases; the absolute values of T, differ by less than 1 K. When the first stage was enclosed by the radiation shield, the thermal hysteresis dropped from about 2 K to below 0.3 K. When the DAC was also enclosed by the radiation shield, the thermal hysteresis was further reduced to below 0.1 K. With the radiation shield cap, the absolute value of T, could be determined well within the accuracy of the thermometer.

D. ac susceptibility

x(T) is obtained from measurements of the mutual inductance between a primary and compensated secondary coils. For convenience, we designate one of the compen- sated secondary coils containing the sample as the second- ary and the other, the compensating. x(T) is proportional to the voltage amplitude, V(T) = V’ ( T) + W” ( T) of the secondary. V(T) is measured by a lock-in-amplifier (Stan- ford Research SRSSO) with the voltage of the primary coil as a reference. Figure 3 shows the result of V’(T) at P= 1 atm for the HgBa&!u04+6 polycrystal before the system was improved. The detailed configuration of the previous system was reported elsewhere.g In particular, the mag- netic field per current at the center of the primary is about 200 G/A, producing 2.0 G rms for 10 mA rms through the primary. The sample volume was about 0.5 x 0.5 X 0.4 mm. The large linear change in V’(T) came from the change of mutual inductance due to the temperature variation. The portion of V’ ( T) due to superconductivity, ViJ T), was so small that T, was difficult to determine from the raw data. By extracting VlJT) from V’(T) (see Sec. III), T, at P=l atm was determined. But, as pressure increased, V’,(T) decreased and the transition width broadened. Then, the slight nonlinear variation of the background made the separation of V&(T) from V’(T) impossible. Therefore, V’,(T) at P= 1 atm had to be sufficiently large to determine T,(P).

ViJ T) can be increased by completely filling the pres- sure cavity with the sample. This causes several problems. When the pressure cavity is completely filled, the ruby chips are often buried underneath the polycrystalline ma- terial. As a result, the determination of pressure becomes difficult. More importantly, the condition for hydrostatic pressure inside the cavity is not met. As pressure is applied

to the sample, the wall of the pressure cavity directly pushes the sample, producing either shear stresses or a large pressure gradient. Thus, tilling the pressure cavity with sample is not a proper solution to increase V&(T).

Increasing the current through the primary increases I$( T). The overall signal-to-noise of the system, however, is not improved, because the background also increases proportionally. For clarity, we define AI’& as the total amount of change in VlJ T) due to the shielding effect around T, and define the sensitivity,

s= A cc V’(T/J-V’(T/) ’ (1)

where Th and Tl are temperatures well above and below Tc. For example, S=3.1% between Th=80 and Th= 100 K for V’(T) with the old system. S, not AV& alone, should increase for a better determination of T,.

S has been improved by changing the location of the coils. In the previous system, the secondary was located inside the cryostat which was subjected to the temperature change during the measurement. The compensating coil, however, remained outside the cryostat at room tempera- ture. The change of temperature occurring only in the sec- ondary coil induces the large change of mutual inductance, being the source of the large linear background seen in Fig. 3. To achieve better compensation of the mutual induc- tance between the secondary and the compensating, all of the coils are now located inside the cryostat.

S also depends on the filling factorlg which is inversely proportional to the radius of the coil. In our system, the filling factor has been improved by designing new primary, secondary, and compensating coils. The geometrical con- figuration of the previous system is compared with that of the new system in Fig. 4. In the previous system, the pri- mary and secondary, were wound together to conform the contour of the diamond as closely as possible. The ultimate limit was the size of the gasket, since the coils could not be placed closer to the sample than the boundary of the gas- ket. The diameter of the gasket was typically 3.2 mm and the smallest diameter of the coil had to be limited to 5 mm

0.4 0.4

By new system 0.3 0.3

: 0.2 0.2

: 0.1 0.1 5 5

-0 0 52 52 < <

-0.1 - -0.1 -

; -0.2 -0.2

.’ -...._ $ -0.3 -0.3 ,_..” :

I,, , . t *;-0.4 -0.4 70 70 80 80 90 90 100 100 110 110

Temperature (K) Temperature (K)

FIG. 3. Comparison of V’(T) at P= 1 atm for the polycrystalline HgBa$uO,+, by the old system with that by the new system.

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EB Primary and secondary coil !SSd Primary coil lpmmm Secondary coil I Compensated coil

Old configuration New configuration

FIG. 4. Comparison of the geometrial configuration of the previous coils with that of the new coils.

to accommodate the gasket. The limit can be overcome by setting the coils only on one diamond, not across both dia- monds. Figure 4 shows the design of the new coils. The coils are wound directly on a Lucite form which closely approximates the contour of the diamond. The wire diam- eter for the coils is 0.025 mm allowing 1600 turns per 1 mm2 cross section. The secondary and compensating have 67 and 71 turns, respectively. The number of turns is dis- tributed in such a way that the calculated mutual inductance”’ of each coil is nearly equal, making the total mutual inductance of two coils nearly zero. The calculated mutual inductance of each coil is on the order of 35 ,uH. After finishing the winding of the coils, the Lucite form was dissolved by methylene chloride to leave a free stand- ing coil set. The separated coils were installed around the diamond, leaving a slight gap between the coil and the surface of the diamond culet for the gasket indentation. The radius of the secondary changed from approximately 5 mm to 1 mm, increasing the filling factor by a factor of about 5.

The new system uses a different bridge circuit from the previous system. The circuitry is simpler than that of the previous system which used a Hartshorn ac bridge. Figure 5 shows the circuit diagram. The internal oscillator of the lock-in-amplifier supplies 5 V rms for the primary. The frequency of the voltage is typically set at 5 kHz. The magnetic field per current at the center of the primary is 900 G/A, producing 0.45 G rms for 0.5 mA rms through the primary. Figure 3 shows V’( r> at P= 1 atm obtained with the new system. The sample was again the HgBa,CuO+s polycrystalline sample with similar volume

0 Sample

Internal oscillator of Lock-in-amplifer

I

FIG. 5. Circuit diagram to obtain V(T) for the new system.

-0.12 ’ ” ’ ’ I ’ I ” ’ ’ ’ ‘1’ 8 ’ 8 ’ ’ * ” ’ 8 ’ 8 t ” ” 1 -I 70 75 80 85 90 95 100 105

Temperature (K)

FIG. 6. The dots show the processed data after subtracting the back- ground and the residual linear portion above T, . The solid line is the fit to the processed data.

used for V’(T) at P= 1 atm with the previous system. S=33 % between Th=80 and Th= 100 K for V’(T) with the new system. Between two measurements, S increased by a factor of about 10, indicating that the sensitivity of our system improved by a factor of about 10. The circuit can be further improved by introducing the variable inductance which balances V”(T) and the transformer which sepa- rates the primary and the compensated secondary coils from the source and the detector to improve the signal-to- noise ratio.

111. ANALYSIS

T, is determined by analyzing V’(T). Determining T, accurately from the raw data of Y’ ( r> is difficult, since V’,(T) is much smaller than V’(T). Further processing of the raw data is necessary to extract V&(T), from which T, can be determined. The background data, taken without the sample, are subtracted from the raw data with the sample. This was necessary because the background data are not exactly linear and have a slight curvature around 90 K which happens to be close to T,. Then, a small residual linear temperature dependence above T, is sub- tracted from the processed data. The dots in Fig. 6 show the processed data of ViC( T).

T, is frequently defined as the intersection of two straight lines,” normally designated as onset temperature T,. As the signal-to-noise ratio decreases, drawing these lines can be also a source of errors. If the variation of T, between two successive pressures is small, the method of drawing the lines along the data can critically affect the determination of T,. In order to determine T, from our processed data more systematically, we fitted the data by the parameterized function,

I

-AVIC if T<Tb

f(T; AViC,T,,T,)= a+bT if T,<T<T,, (2) 0 if T&T

where a=-A?$TJ(T,-Tb) and b=AViJ(T,-TT,). For example, the solid line in Fig. 6 shows the resultant fit to J$( T) at P=2.14 GPa. Table II lists the values of

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TABLE II. The resultant parameters obtained from fitting EZq. (2) to V’,,(T) at different pressures.

P (GPa) -AVk (pv) Tb (K) To (K) AT, (K)

0.00 0.069 82.79 88.97 0.16 0.11 0.08 1 82.75 89.46 0.08 0.71 0.088 83.05 90.83 0.10 1.34 0.097 83.51 91.68 0.10 2.14 0.097 84.5 1 91.92 0.10 2.47 0.100 83.98 92.39 0.10 2.77 0.099 83.14 93.12 0.10 3.04 0.085 81.49 93.68 0.13 3.70 0.070 80.15 93.92 0.15 3.95 0.033 80.48 94.50 0.29

parameters determined from fitting Eq. (2) to the pro- cessed data V&(T) at different pressures. AT, is deter- mined from the variances using the root mean squared value between the fit and data as the standard deviation for each data point.21 The uncertainty due to this numerical analysis is similar to the precision of our temperature mea- surement. Several pressure dependent trends on ViJ T) are seen in this table. First, the transition width (T,,-- TJ,) gradually increases from 6.18 K at 0.0 GPa to 14.02 K at 3.95 GPa. Second, AV’, increases from 0.070 ,uV at 0.0 GPa to 0.100 PV at 2.47 GPa and decreases to 0.034 ,uV at 3.95 GPa. It increases due to the advancement of the sam- ple toward the secondary under pressure. It decreases due to the fragmentation of the sample as the pressure cavity shrinks under pressure. The values of Tb fluctuate due to the broadening of the transition width, whereas the values of To monotonically increase. This indicates that Tb may not be a good representation of T, for this type of study.

T, is chosen to represent T, for HgBa&uO,+,. Figure 7 shows T, as a function of pressure. T, increases mono- tonically as pressure increases. The solid line in Fig. 7 shows the linear fit to the data fixing the value of T, at P-O GPa. The initial slope turns out to be 1.46 K/GPa. On the other hand, the initial slope becomes 1.88 K/GPa when the data are fitted by the quadratic function of P. The diamond signs in this figure show T,(P) for HgBazCuOb+B as reported by Klehe et al. l2 T, at P=O GPa is higher than ours. The pressure effect on T, is al- most linear to 1 GPa, producing 1.8 K/GPa as an initial slope.

IV. DISCUSSION

For the measurements reported here, silicone oil was used as the pressure medium. In the past, the methanol: ethanol::4:1 was used because it solidifies at a pressure of about 10 GPa at room temperature and is widely used for higher pressure research.14 It was suitable for the T,(P) measurements of most single crystals by our system. I-’ This mixture, however, destroyed the supercon- ducting properties of our polycrystalline samples of HgBa2Cu04+6. It is likely that Hz0 in the mixture reacts with the sample and decomposes it.22 In contrast, the sili- cone oil does not affect the superconducting properties. It also remains transparent near T, to make ruby fluores-

cence measurements possible. Vacuum grease was also tried as a pressure medium, but did not allow in-situ mea- surements of the ruby fluorescence. Helium is another good candidate as a pressure medium, but requires a spe- cial facility to load into the pressure cavity. The silicone oil is easy to load and appears to serve our purpose.

Several factors limited the maximum pressure to 4 GPa. The culet diameter was 1 mm. The thickness of the Be-Cu gasket was about 0.5 mm. The tensile strength of the Be-Cu was about 1.5 GPa after annealing. The diam- eter of the pressure cavity was 0.7 mm. These parameters can be changed to increase the pressure above 10 GPa. For example, DACs which reach above 10 GPa have usually a culet diameter less than 0.6 mm, a cavity diameter less than 0.2 mm and a gasket thickness less than 0.2 mm. They also use materials with higher tensile strength for the gas- ket. The maximum pressure will increase when our current parameters for the DAC change to such values. The re- duced size of the sample may not prevent us from deter- mining T,(P), since the sensitivity has been improved.

Many have reported T,(P) far above 10 GPa using ac susceptibility measurements together with the DAC. Bi- reckoven and Wittig5 reported T,(P) for Pb up to 50 GPa. They introduced a SQUID sensor to take advantage of the higher sensitivity. Like our design of the coils, their sec- ondary and compensating coils were also located on one diamond. Unlike our system, the pressure was measured at room temperature rather than near the sample transition temperature. Braithwaite et al. 6 reported T,(P) for the sin- gle crystal and polycrystal YBa2Cus07--6 to 14 GPa. Their geometrical configuration of the primary, secondary, and compensating coils is very similar to ours, except that the compensating was outside the primary rather than inside the primary. This geometrical difference may not affect the sensitivity much. In their system, the pressure was also measured at room temperature rather than near T,. They mentioned that the pressure decrease due to cooling was less than 0.5 GPa. Based on their observation, the pressure measurement at room temperature is justified to represent. the pressure near T,. Their observation differs largely from

66t”“‘““‘““‘““‘““‘“““““““’ 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Pressure (GPa)

FIG. 7. T,(P) for the polycrystal HgBa,Cu0,+6 obtained from the new system. The solid line is the linear fit to the data. The diamond signs represent the work by Klehe et al.”

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our observation as reported in this paper. The difference search Associate at the Naval Research Lab. D. H. Lie- may be due to the specific DAC design and materials, but benberg acknowledges the support of the Scientific Office indicates the importance of in-situ pressure measurements. Research Program at the Office of Naval Research.

It is obvious that the sensitivity of ac susceptibility will increase as the coils get closer to the sample. In light of this, the limitation of our approach is the culet size which determines the smallest radius of the coils. An approach to overcome this limit is to put the coils inside the pressure cavity. The idea has been demonstrated by the metal- gasket cell” and Widalox anvil cell3 which reached the maximum pressure of about 10 GPa. Incorporating such an idea into the DAC, however, is quite a challenge, since the coils as well as the sample must be smaller than the size of the culet and yet the connections to the outside of the pressure cavity must be maintained at high pressure. More- over, it is not clear whether such an approach will give more sensitivity than one can achieve with the approach shown in this paper. Putting the coils inside the pressure cavity increases the filling factor, but limits the number of turns. Our approach is simpler and easier to adopt in the practical measurement, keeping the highest possible sensi- tivity;

Our system has been improved to measure T,(P) with a better accuracy. The major improvement is in the sensi- tivity of ac susceptibility measurements. S has increased by the factor of 10 compared with the previous system. These improvements are demonstrated by determining T,(P) for a HgBa2Cu04,6 polycrystalline sample. Specifically, the modification of the coil geometry improved the filling fac- tor by about 5. This modification also makes it much easier to mount the gasket and center the pressure cavity with respect to the diamond culet. The better thermal radiation shielding reduced the thermal hysteresis below 0.1 K. A new analytical procedure.has improved the consistency in determining T,(P).

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ACKNOWLEDGMENTS

The polycrystal samples of HgBa2Cu04+s were pro- vided by W. Chang and E. Helstrom in the Applied Su- perconductivity Center at the University of Wisconsin- Madison. The authors are grateful to A. R. Drews who suggested the silicone oil as the new pressure medium. C. C. Kim is the National Research Council Postdoctoral Re-

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