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Field, temperature, and concentration dependences of the magnetic susceptibility
of bismuth–antimony alloys

B. Verkin Institute for Low Temperatures Physics and Engineering, National Academy of Sciencesof Ukraine, pr. Lenina 47, 310164 Kharkov, Ukraine ͑Submitted April 9, 1999; revised August 11, 1999͒
Fiz. Nizk. Temp. 26, 54–64 ͑January 2000͒
In the framework of the McClure model, which describes the electronic energy spectrum ofbismuth and its alloys in the neighborhood of the L point of the Brillouin zone, an expression isobtained for the electron energy levels in a magnetic field. This expression is used tocalculate the magnetic susceptibility of bismuth alloys at arbitrary magnetic fields. It is shownthat the theoretical results are in good agreement with the entire set of publishedexperimental data on the field, temperature, and concentration dependences of the magneticsusceptibility of bismuth–antimony alloys. 2000 American Institute of Physics.
͓S1063-777X͑00͒00501-6͔ INTRODUCTION
limit H→0 were done in Ref. 8–10. The models of the elec-tronic band structure11,12 used in Refs. 8 and 9 would later be The electronic band structure of bismuth and its alloys found to give a poor description of the spectrum of bismuth with antimony has been the subject of many papers ͑see, e.g., alloys in the neighborhood of the L point. In Ref. 10 the Refs. 1 and 2 and the references cited therein͒. It has been magnetic susceptibility was calculated using a spectrum established that the Fermi surface of bismuth and its alloys which is intermediate in accuracy between those proposed in ͑at low concentrations of antimony͒ consists of one hole el- Ref. 13 and in Refs. 14 and 15; both of these last provide a lipsoid, located at the T point, and three closed electron sur- good description of the entire set of experimental data on faces of nearly ellipsoidal shape, centered at the L points of oscillation and resonance effects in bismuth alloys. However, the Brillouin zone. Another circumstance that is extremely in Ref. 10 the theoretical and experimental results were com- important for understanding many of the properties of bis- pared only for the dependences of the magnetic susceptibility muth is that in the neighborhood of the L point the conduc- ␹ on ␨ and x, and the comparison was done using values16 of tion band is separated by only a small energy gap from an- other, filled band. The detailed study of the energy spectra of considerably.2 In Ref. 17 the same model of the spectrum as the charge carriers near the L and T points is done mainly by in Ref. 10 was used to calculate the field dependence of the methods based on oscillation and resonance effects. By now magnetic susceptibility, but only in low magnetic fields. For the values of the main parameters characterizing the band high magnetic fields a calculation of ␹ was done in Refs. 6 structure of bismuth and its alloys with antimony have been and 9, but with the use of unrealistic, oversimplified models of the spectrum.11,12 Thus, at the present time there is no The smooth ͑nonoscillatory with respect to the magnetic complete quantitative description of the experimental curves field H͒ part of the magnetic susceptibility of the solid solu- of the magnetic susceptibility of bismuth alloys as a function tions Bi1ϪxSbx exhibits noticeable ͑and often nonmonotonic͒ of H, T, ␨, and x.
changes upon variations of H, the temperature T, the anti- It was shown in Ref. 18 that under conditions of degen- mony concentration x, and the admixture of dopants that eracy of the electronic energy bands of the crystal in a weak shift the level of the chemical potential ␨ of the alloy.3–7 magnetic field (H→0) there can be giant anomalies of the These changes in the susceptibility are due to electronic magnetic susceptibility, and the types of degeneracy of the states located near the L points and belonging to two bands bands which can lead to such anomalies were listed. In Ref.
separated by a small energy gap.8–10 The rest of the elec- 19 the problem of the electron energy levels in a magnetic tronic states all give a contribution to the magnetic suscepti- field was solved exactly for two of these types ͑those most bility that is practically independent of T, ␨, H, and x and often encountered in crystals͒, and the special contribution to represents a constant background. The study of the ‘‘vari- the magnetic susceptibility was calculated for arbitrary val- able’’ contribution to the magnetic susceptibility ͑i.e., its de- ues of H. As expected, this contribution depends strongly on pendences on T, ␨, H, and x͒ will make it possible to check H, ␨, and T. The spectrum of bismuth–antimony alloys in and refine the data on the electronic band structure in the the neighborhood of the L point of the Brillouin zone is close neighborhood of the L point as obtained from investigations to degenerate and is characterized by the circumstance that of oscillation and resonance effects.
for a nonzero gap in the spectrum, the type of degeneracy is Calculations of the special ͑or ‘‘variable’’͒ contribution intermediate between those considered in Ref. 18. This is to the magnetic susceptibility of bismuth and its alloys in the what accounts for the strong field, temperature, and concen- Low Temp. Phys. 26 (1), January 2000
tration dependences of ␹ in these alloys. However, a detailed 1ϪxSbx alloys the dependences of the parameters q i , ␣ comparison of the theoretical and experimental results must and ⌬ on the antimony concentration x are well described by be done with allowance for the aforementioned feature of the spectrum of bismuth alloys. Therefore, generalizing the re- sults of Ref. 19, in Sec. 1 of the present paper we give a solution to the problem of the energy levels of an electron in a magnetic field for the McClure spectrum,13 and in Sec. 2 we obtain the corresponding expressions for the magnetic susceptibility, valid for arbitrary H. In Sec. 3 we use these expressions to compare the theoretical and published experi- are given in atomic units, a.u.͒. In addition, as x mental results for the field, temperature, and concentration increases, the parameter q2(x) generally acquires a real dependences of ␹ in Bi1ϪxSbx alloys. We conclude with a part.10 A nonzero Re(q2) causes the long direction of the electronic isoenergy surfaces to deviate from the axis 2 by anangle ␦␸ϳ(Re(q2)/q3). Such a deviation was actually ob-served in Ref. 16, and it follows from the data of that studythat 1. SPECTRUM
As we said in the Introduction, the dependences of the magnetic susceptibility on the field and on temperature, im- The band energies ␧c(k) and ␧v(k) are found from the equa-
purity concentration, and other external parameters are gov- erned mainly by the electronic states located in the neighbor-hoods of the L points of the Brillouin zone and belonging to ␧Ϫ ͑␣c Ϫ␣v ͒k2ͬ2ϭE2, two bands which lie close to each other and to the level of the chemical potential. These electronic states are described using several models of the energy spectrum which havedifferent degrees of accuracy in terms of the parameter E2ϭͫ⌬ϩ ͑␣c ϩ␣v ͒k2ͬ2ϩq2k2ϩ͉q ͉2k2 where ␧0 is the characteristic energy scale for the two nearby The relative position of these bands as a function of the bands, and E0 is the energy distance from these bands to the antimony concentration x is shown in Fig. 1.
nearest of the remaining bands. The most completemodels10,14,15 have an accuracy of order ␦. However, atpresent the values of the parameters of the spectrum have allbeen determined for the simpler McClure model,13 whichdescribes the spectrum with an accuracy of order ␦1/2. Wewill use the McClure model here. In it the Hamiltonian of theelectrons in the neighborhood of an L point has the form Here and below the energy and chemical potential ␨ are reck-oned from the center of the energy gap 2⌬ ͑here ␧0 ϳ2⌬,͉␨͉͒ which separates the two bands, denoted c and v,which are nearly twofold degenerate at this point. The quan-tities t, u, Kc , and Kv are given by the formulas uϭq2k2 q3k3 , FIG. 1. Diagram of the changes in the electronic energy spectrum of Bi1ϪxSbx alloys at the L and T points of the Brillouin zone. The dashed lines indicate the path of the band edges ␧ 2 is a complex number. The origin of coordinates for c( 0 ) and ␧ v( 0 ) at the L points and the wave vector k is at the L point. The axis 1 is along the
T(0) at the T point as x is changed. The lines were constructed usingformulas ͑3͒ and ͑10͒. At xϷ0.04 the gap in the spectrum at the L point binary axis, and axis 2 is along the length of the Fermi sur- goes to zero, and for xϾ0.07 the alloy undergoes a transition to a semicon- face of pure bismuth at the L point, i.e., at an angle ␸Ϸ6° to ducting state. The solid curves show a schematic illustration of ␧c(k),
the bisector direction. For pure bismuth Re(q2)ϭ0. In ␧v(k), and ␧T(k) at the respective points.
Low Temp. Phys. 26 (1), January 2000
The spectrum of electrons in a magnetic field H directed
along the k2 axis can be obtained from the generalexpression19 The magnetic susceptibility of bismuth and its alloys can be written as the sum of a special contribution due to the electronic states near the three L points and a background term due to all the remaining states. The background term ispractically independent of the magnetic field and temperature where e is the absolute value of the electron charge, and even remains constant upon variations of the chemical S(␧n ,k2) is the cross-sectional area of the isoenergy surface potential ͉␦␨͉ϳ͉⌬͉. The special contribution to the magnetic const, and n is a nonnegative integer. Here it susceptibility consists of a sum of three terms due to the should be kept in mind that the energy levels ␧n with nϾ0 states near the respective L points. Each of this terms can be are twofold degenerate. In the derivation of ͑6͒ we neglected obtained from the following expression for the ⍀ potential the direct interaction of the electron spin with the magnetic field, since the purely spin contribution to the magnetic sus-ceptibility is of order ␦ ͑but the spin–orbit interaction is taken into account in all the formulas given above͒. We note that, although the quantization condition ͑6͒ has the quasi- classical form, in this case it gives the exact eigenvalues for the energy of an electron with the Hamiltonian ͑1͒, ͑2͒. From where the prime on the summation sign means that in taking the sum over n the terms with nϾ0 must be doubled; H␪ isthe projection of the magnetic field on the k2 axis at thegiven L point. In an experiment one measures the quantity 1q 3 / c ប . If the magnetic field is directed at where hϭH/H is a unit vector in the magnetic field direc-
2 axis, then, as was shown in Ref. 19, to an accuracy of ␦ tan2 ␪ the eigenvalues ␧c,v(k tion, and the differential magnetic susceptibility ␹ij is given scribed, as before, by formula ͑7͒ but with H cos ␪ substi- Besides the electronic states in the neighborhoods of the L points of the Brillouin zone, bismuth also has hole states in the neighborhood of the T point. These states have the en-ergy spectrum1 Since the ⍀ potential ͑12͒ depends on H only through H␪ , inour approximation ͑to accuracy ␦1/2͒ we have T k͒ ϭ E T
Here the values of the effective masses mh and mh are where ␪l are the angles between the magnetic field H and the
k is reckoned from the T point, the axes 1 and 2 coincide
2 axis for the three L points.
In the case of weak magnetic fields, for which the char- with the binary and bisector axes, respectively, and ET is the acteristic distance between energy levels in the magnetic energy of the band edge, which in Bi1ϪxSbx alloys falls off field obeys ␦␧ ӶT, we integrate ͑12͒ by parts, use the linearly with increasing x ͑see Fig. 1͒: Euler–Maclaurin summation formula, and differentiate with respect to the magnetic field to obtain for the susceptibilityan expression of the form ␹ϭ␹ ϩ␹ The contribution to ␹ from the hole states at the T point is sions for the H-independent terms ␹ small compared to the contribution from the electronic states those obtained previously in Refs. 10 and 17.
near the L points and is of order ␦. This is because of the Let us now analyze ␹22 in the case of high magnetic relatively large masses mh and, accordingly, the small dis- fields, ␦␧ ӷT. The contribution of the electrons in the con- tances between energy levels ␧T in a magnetic field: duction band to the magnetic susceptibility can be calculated directly using formula ͑12͒, since the number of filled levels ␧ is finite. To calculate the contribution of the filled band v to ␹22, we once again integrate ͑12͒ by parts as many times However, while neglecting the contribution of these states to as necessary, use the Poisson summation formula, and set the susceptibility, one must take into account their influence T). The resulting formula includes one summa- on the position of the chemical potential of the electrons in tion and integrations over n and k 2 . If the quantity ( d ␧ / dn ) in this formula ͓where ␧v is defined in Eq. ͑7͔͒ is written as Low Temp. Phys. 26 (1), January 2000
of the bands rapidly deviates from linearity and approaches a quadratic law. This leads to a more complicated dependence of ␹(H) than in Ref. 19 ͓see Eq. ͑13͔͒. The limiting expres- then the summation and integration over n and k sion ͑16͒ corresponds to the case when the initial ͑linear in done in explicit form. As a result, we obtain for ͉␨͉Ͻ͉⌬͉ part of the band splitting can be neglected, and one can c( k 2) Ϫ ␧ v( k 2) ͉ ϰ k mation is justified even for ⌬ 0͒. Thus formula ͑16͒ actu- ally describes the behavior of ␹(H) for the third type of band degeneracy,18 for which a giant anomaly of the magnetic t2 ͪe͑Q2Ϫ2͒t2K ͑ susceptibility can occur and which was not considered in Ref. 19. Here Eq. ͑15͒ corresponds to the condition when where Q is the following dimensionless combination of pa- c( k 2) and ␧ v( k 2) have different signs. If ␧ c( k 2) and ␧ v( k 2) had the same sign, i.e., if ␥Ͻ1, then, as one can show, for HӷHQ2␥2/(1Ϫ␥2) the magnetic susceptibility is de- scribed as before by formula ͑16͒ but with a different con- Qϭsgn͓⌬͑␣c ϩ␣v ͔͒ͩ 1ϩ ⌬ is the characteristic magnetic field, at which ␦␧ ϳ͉⌬͉ 1/4( x ) is a modified Bessel function, and where F is the hypergeometric function. In the limiting case 0͒ we would arrive at a line of degeneracy of the bands, i.e., at the second case according to the classi- In the derivation of expression ͑13͒ we have assumed that fication of Ref. 18. Then expression ͑16͒ with the factor A from ͑18͒ agrees with the expression obtained in Ref. 19.
Finally, we note that in the case of band degeneracy at an L point or for small ⌬ the parameter Qӷ1, and there is a region of magnetic fields H⌬ӶHӶQ2H⌬ in which the part We note that this condition is satisfied for Bi of the band splitting that is linear in k2 plays the governing for any antimony concentrations x.
role in ␹(H). Then it follows from Eq. ͑13͒ that If the magnetic fields are such that HӶH⌬ , then the magnetic susceptibility ͑13͒ is independent of the field, and it 6␲2 cប 2͉Im͑q ͉͒ ln H is described by the same expression as that given in Ref. 10 for T→0. On the other hand, if HӷQ2H⌬ ͑for bismuth– With an accuracy up to the background constant, this result antimony alloys Qӷ1 for xϳ0.04, while for other antimony agrees with that obtained in Ref. 19 for the first type of band concentrations Qу1 in the region xϽ0.2͒, then degeneracy. Thus the strong field dependence of the mag-netic susceptibility of bismuth alloys is a manifestation of the fact that the spectrum of these alloys is close to those cases cប ͉␣c ϩ␣v ͉1/2 of band degeneracy which lead to a giant anomaly of the The chemical potential ␨ of the electrons in the crystal, generally speaking, itself depends on the magnetic field. This dependence is determined from the condition that the total (x) is the Riemann zeta function, and ⌫(x) is the gamma function. Formulas ͑16͒ and ͑17͒ agree with those obtained In Ref. 19 the field dependence of the magnetic suscep- tibility of electrons was investigated for two of the three To evaluate the magnetic susceptibility at constant ␯, it is types of degeneracy of the energy bands of crystals leading necessary to go over from the ⍀ potential to the free energy.
to strong field dependence. According to Eqs. ͑3͒–͑5͒, in As a result, for ␹ij(H,␯) we have19 Bi0.96Sb0.04 alloys there is band degeneracy of the first type according to the classification of Ref. 18, i.e., a band splitting ␹ij͑H,␯͒ϭͫ␹ij͑H,␨͒Ϫ ץ that is linear in the wave vector k in the neighborhood of the
degeneracy point L. However, bismuth alloys are character-ized by relatively small values of the matrix element q2 re- When obtaining the function ␨(H,␯) using formula ͑19͒ it is sponsible for this linear splitting along the k2 axis. That is necessary to take into account the contributions to the ⍀ why we took terms quadratic in k2 into account in the Hamil- potential not only from the electronic states near the L points tonian ͑1͒–͑3͒. According to Eqs. ͑3͒–͑5͒, as the point k
but also the states near the T point, and also the influence of moves away from the L point along the k2 axis, the splitting donor and acceptor impurities. The states at the T point give Low Temp. Phys. 26 (1), January 2000
a term in the ⍀ potential which is determined by formula ͑12͒ with the energy levels from ͑11͒. Impurities, first, causescattering of the charge carriers and, second, give an addi-tional impurity contribution to the ⍀ potential in semicon-ducting alloys. The scattering of charge carriers can be takeninto account in a simple way by the introduction of a Dingletemperature TD , i.e., by replacing T by TϩTD in all theformulas. In semiconducting alloys of Bi1ϪxSbx (xϾ0.07)we consider the impurity contribution to the ⍀ potential, ⍀imp , in the limiting case of lightly and heavily dopedn-type semiconductors. The case of light doping is charac-terized by the presence of carrier–impurity bound states, theenergies of which form a narrow impurity band lying in thegap of the spectrum. In bismuth–antimony alloys these en-ergies ␧i practically coincide with the band edge, i.e., ␧i FIG. 2. Low-field magnetic susceptibility ␹ as a function of the antimonyconcentration x in Bi 1ϪxSbx alloys. The magnetic field is applied in the imp is the density of doping impurities. As we basal plane of the crystal. Tϭ4.2 K. ␹ is normalized to a unit volume; know,20 the main condition for the existence of impurity lev- ᭺—experimental data of Ref. 7; solid curve—calculation according to the els is that the average size d of the carrier–impurity bound formulas of Ref. 10 with the use of the parameter values given in Eqs. ͑3͒, state be small compared to the distance between impurities, ͑9͒, ͑10͒; dashed curve—calculation done in Ref. 10 using the spectrumparameters given in Ref. 16.
i.e., the condition d␯1/3 Ӷ1. The dimension d is of the order of the ‘‘Bohr’’ radius dϳa*ϭ␬ប2/e2m*, where ␬ is the dielectric constant of the crystal and m* is the effective massof a charge carrier. For a heavily doped semiconductor pressions for the magnetic susceptibility in low fields were d␯1/3 у1, and carrier–impurity bound states do not arise. In obtained previously.10 In the present paper, however, the cal- culations using these expressions were done with the new values of the parameters ͑3͒, ͑9͒, ͑10͒. In comparing the the- oretical and experimental results we chose the constant back- i.e., the semiconductor is transformed into a ‘‘poor’’ metal ground in the susceptibility so as to obtain coincidence with with an intrinsic electron density ␯imp . If the semiconductor the corresponding values for pure bismuth. In the calculation is in a magnetic field H, then we must take into account the it is necessary to find the dependence of the chemical poten- dependence on H of the average size d of a localized state.
tial ␨ on x for the semimetallic alloys Bi1ϪxSbx (xϽ0.07) In a weak magnetic field we have dϳa* , as before. How- from the condition that there be equal numbers of electrons ever, when the magnetic length ␭Х(បc/eH)1/2 becomes and holes at the L and T points, respectively. In the region of smaller than a* , the size of the localized state in the direc- semiconducting alloys (xϾ0.07) the chemical potential is tions perpendicular to H is determined by the value of ␭, and
assumed to lie in the gap of the spectrum between the va- the average size dϳ(␭2a*)1/3 falls off with increasing H.
lence band and conduction band, and the impurity concen- tration ␯imp is taken equal to zero. From the results presented there occurs a magnetic ‘‘freeze-out’’ of the in Fig. 2 it follows that the use of the parameter set ͑3͒, ͑9͒, electrons,21 and the heavily doped semiconductor is trans- ͑10͒ provides a better description of the experimental data for the semiconducting alloys than does the set from Ref. 16.
In addition, we have calculated the dependence of ␹ in aweak field H on the level of the chemical potential ␨ for the 3. COMPARISON OF THE RESULTS OF THE CALCULATION
0.92Sb0.08 and Bi0.97Sb0.03 . The results of the calcu- lation with the new parameter values agreed with the results In Refs. 3–7 significant changes in ␹ were observed in of Ref. 10 to within the limits of experimental error.
bismuth–antimony alloys upon variations in the magnetic Figure 3 shows the field dependence of the magnetiza- field, temperature, antimony concentration, or chemical po- tion M of pure bismuth in magnetic fields so high that the tential, the level of the last being regulated by the introduc- only the lowest Landau level in the conduction band remains tion of doping impurities in the alloy. Our theoretical analy- occupied, and there are no de Haas–van Alphen oscillations.
sis of the dependence of the susceptibility on H, T, x, and ␨ In accordance with Eqs. ͑13͒ and ͑16͒, this curve is nonlinear will be done on the basis of the formulas obtained in Sec. 2, in H. Here for a detailed comparison of the results of the using the values in ͑3͒, ͑9͒, and ͑10͒ for the parameters of the calculation with the experimental data of Ref. 6, we took into consideration that ␨Ͼ⌬ in bismuth, and we added to Eq.
Let us first consider the dependence of ␹(H→0) on the ͑13͒ the contribution due to the conduction electrons. The antimony concentration x in Bi1ϪxSbx alloys ͑Fig. 2͒. Ex- expression for this contribution was obtained directly from Low Temp. Phys. 26 (1), January 2000
FIG. 5. Magnetic susceptibility ␹ as a function of magnetic field H for fieldsgreater than 3 kOe, for the same alloy as in Fig. 4. The calculation was doneusing formula ͑13͒ for two orientations of the magnetic field—along thebinary axis and along the bisector direction. The results of the calculationfor the two cases practically coincide ͑solid curve͒; ᭝,᭺—the experimental FIG. 3. Magnetization M of pure bismuth as a function of the magnetic field data of Ref. 7 for the first and second of the indicated directions of H,
H, directed along the binary axis, for Tϭ20 K and Hу20 kOe; ᭝—the respectively. The values of x, ␯imp , T, and TD are the same as in Fig. 4.
experimental data of Ref. 6; solid curve—the calculation of the presentpaper.
weak (HϽ50 Oe) that the characteristic distance between Eq. ͑12͒. We see that the agreement of the theoretical and electronic energy levels at the L points is much less than the experimental results is quite good, and it is achieved without temperature (Tϭ4.2 K), the aforementioned curve is ap- the use of any adjustable parameters.
proximated by the expression ␹(H)ϭ␹ ϩ␹ The results of the calculations of the field dependence of values of ␹0 and ␹1 agree with those calculated using the the magnetic susceptibility of the semiconducting alloys formulas in Refs. 10 and 17. As the magnetic field is in- Bi0.92Sb0.08 with a concentration of donor impurities ␯imp creased a transition to the case of light doping occurs on ϭ1015 cmϪ3 are presented in Fig. 4. The two ␹(H) curves account of the magnetic freeze-out of the electrons, and, ac- shown differ in that they correspond to the dependence of ␨ cordingly, in the region HϾHcr the agreement with experi- on H obtained for heavily and lightly doped semiconductors.
ment is better for the other curve. As the magnetic field is For the given value of ␯imp an estimate of the field Hcr gives increased further, the chemical potential of the electrons 1 kOe. In accordance with the arguments set forth in comes to lie in the gap of the spectrum, and the field depen- Sec. 2, at fields much smaller than Hcr the theoretical curve dence of ␨(H) ceases to influence the magnetic susceptibil- corresponding to the case of heavy doping gives a good de- ity; then the theoretical curves in Fig. 4 practically coincide.
scription of the experiment. For magnetic fields that are so Here one can find ␹(H) directly using formula ͑13͒. The
results of this calculation are shown in Fig. 5. We see that, in
complete agreement with experiment, the magnetic suscepti-
bility is practically independent of the direction of the mag-
netic field H in the basal plane.
Figure 6 shows the results of calculations of ␹(H) for the alloy Bi0.92Sb0.08 with admixtures of the dopant tellurideat concentrations ␯ first of these concentrations H ϳ than this, the difference in ␹ for the heavily and lightlydoped semiconductor practically vanishes. For the second ofthese concentrations H ϳ heavily doped throughout the magnetic field region consid-ered. Thus for an analysis of the ␹(H) curves it suffices touse the formulas corresponding to a heavily doped semicon-ductor. The introduction of the donor impurity Te raises thelevel of ␨ significantly, and the first few de Haas–van Alphenoscillations appear; these, however, cannot be described bythe quasiclassical formulas. We see that, although the mag-netic susceptibility is a nonmonotonic function of H, the FIG. 4. Magnetic susceptibility ␹ as a function of the magnetic field H for


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