SCIENCE CHINA Physics, Mechanics & Astronomyuhp.iphy.ac.cn/article_list/2013_articles/JinCQ_CS.pdf · 2013. 12. 11. · 2340 Jin C Q, et al.Sci China-Phys Mech Astron December (2013)

SCIENCE CHINA Physics, Mechanics & Astronomy

© Science China Press and Springer-Verlag Berlin Heidelberg 2013 phys.scichina.com www.springerlink.com

*Corresponding author (email: [email protected])

• Review • December 2013 Vol.56 No.12: 2337–2350

85th Anniversary for the Institute of Physics, Chinese Academy of Sciences doi: 10.1007/s11433-013-5356-2

New quantum matters: Build up versus high pressure tuning

JIN ChangQing*, WANG XianCheng, LIU QingQing, ZHANG SiJia, FENG ShaoMin, DENG Zheng, YU RiCheng & ZHU JingLong

Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190

Received September 16, 2013; accepted November 6, 2013; published online November 18, 2013

In this article we briefly review new quantum functional compounds primarily based on our recent works. We will highlight the effects of pressures on both materials synthesis and quantum tuning. The contents include (I) “111”-type iron based super-conducting system, (II) pressure induced superconductivity in topological insulators and (III) the new diluted magnetic semi-conductors with decoupled spin charge doping.

quantum functional compound, topological insulator, superconductivity

PACS number(s): 03.65.Vf, 61.50.Ks,74.70.Xa, 74.62.Fj, 74.25.Dw, 75.50.Pp, 75.50.-y, 81.40.Vw

Citation: Jin C Q, Wang X C, Liu Q Q, et al. New quantum matters: Build up versus high pressure tuning. Sci China-Phys Mech Astron, 2013, 56: 23372350, doi: 10.1007/s11433-013-5356-2

Quantum compounds cover the multiple spin-charge-orbital interactions on the basis of crystal lattices. Consequently they display rich physical properties such as topological behaviors, insulator metal transition, and diluted magnetic semiconductor phenomena. Understanding those unusual physical phenomena unveils new aspects of the intrinsic physics of quantum compounds. As one of the independent thermodynamic parameters apart from those usually studied – temperature and composition – pressure has been playing an increasingly important role in condensed matter physics. Pressure can be effective in shortening atomic distance, enhancing orbital overlap, and consequently modifying crystalline structures and tuning interactions. Thus we realize novel high pressure states. It is very well established that each material will usually undergo several phase transitions over the pressure range up to 100 GPa, strongly implying that high pressure could generate plenty of new states of materials based only on the available ambient compounds. Hence high pressure is considered to be a

powerful tool to develop novel quantum states. Pressure provides a new dimension for the study of quantum compounds. Pressure more likely will initiate breakthroughs in understanding fundamental scientific problems such as e-e correlation, e-p interaction, charge orbital ordering, etc of quantum functional compounds. High pressure physics based on quantum funtional compounds as an emerging interdisciplinary science is advancing the state of the art in the physical sciences. Here we introduce some examples of new quantum funtional compounds with effects pressure based on our recent works.

1 “111”-type iron based superconductors

1.1 The discovery of “111”type superconductors and their superconductivity

Since the discovery of superconducting LaFeAsO with Tc 26 K [1], great interest has been stimulated in exploring new superconductors and studying the novel superconduct-ing mechanism in the iron arsenide system [2–8]. The

2338 Jin C Q, et al. Sci China-Phys Mech Astron December (2013) Vol. 56 No. 12

RFeAsO (R=rare earth elements, termed “1111”-type) su-perconductor has a layered crystal structure with Fe2As2 layers sandwiched with R2O2 layers. This was followed by another type iron based superconductors AFe2As2 (A: alka-line earth metal, termed with “122”-type) [2], which crys-tallize into a tetragonal ThCr2Si2 type structure and the Fe2As2 layers are intercalated with alkaline earth metal. We found the third type iron arsenide superconductors AFeAs (A: Li,Na) with Tc about 18 K for LiFeAs [3]. When As atoms in LiFeAs are replaced by P atoms, we further found a new “111”type superconductor LiFeP with Tc about 6 K [8]. We initially use high pressure to synthesize the LiFeAs superconductors in order to control the volatile of Li ele-ment. Also if Li atoms are totally substituted with Na atoms, another “111”-type superconductor NaFeAs is discovered [9]. The crystal structure of “111” family belonging to the Cu2Sb type tetragonal structure with space group P4/nmm, is shown in Figure 1, where iron pnictide layer and a Li or Na layer are stacked alternately. The Fe atoms are in a four fold coordination, forming a FeAs(P)4 tetrahedron. The Fe atoms are in a four fold coordination, forming a FeAs(P)4 tetrahedron and the parameters are shown in Table 1. The parameters in the FeAs4 tetrahedron, such as the As––Fe––As bond angular and the anion height i.e. the dis-tance between the anion (As) and the iron layer, are the key factors that tuning the superconducting temperature, which will be discussed in the paper.

Figures 2(a)–(c) presents the temperature dependence of susceptibility and resistance for LiFeAs, NaFeAs and LiFeP. All the parent compounds of the “111” family show super-conductivity without additional carrier doping and the Tc is about 18 K, 9 K and 6 K, respectively. Figure 3 shows the specific heat data for LiFeAs. A very sharp peak at the su-perconducting temperature is observed demonstrating bulk

superconductivity, which is consistent with the susceptibil-ity measurements in Figure 2(a) showing the complete dia-magnetic shielding signal. For LiFeAs and LiFeP no evi-dence has been observed for spin density wave (SDW) re-lated phase transition that usually results in resistance drop at the transition temperature. Incommensurate spin fluctua-tions along the direction transverse to the AF ordering wave vector Q = (1,0) were found in LiFeAs single crystal by inelastic neutron scattering measurement. The transverse incommensurate spin fluctuations result from the mis-matched hole and electron Fermi surfaces due to the self-electron-doping, so LiFeAs can be considered as an electronic overdoped superconductor. However, from the resistance curve of NaFeAs, it can be clearly seen that the structural transition and the magnetic transition happen in sequence before the superconducting transition. The super-conducting volume fraction of NaFeAs is very small and in fact it is not a bulk superconductor. The structural and magnetic transition temperature can be suppressed by carri-er doping and bulk superconductivity appears with a dome like superconducting phase diagram, which is similar to that of “1111” and “122” families.

1.2 The superconductivity of “111” family tuned by high pressure

Pressure is important in the study of iron pnictide super-conductors because it directly tunes the electronic configu-ration as demonstrated for several typical iron pnictide compounds. In Fe based superconductors, the effect of pressure on the superconducting transition temperature is complex and depends sensitively on the composition of the materials. Therefore, the correlation between the pressure tuned superconductivity and the atomic structure at pressure

Figure 1 (Color online) The crystal structure “111” family of iron based superconductors.

Table 1 Crystal structure parameters for “111” type superconductors under ambient pressure and room temperature [10]

a (Å) c (Å) αa) βa) Anion height (Å)

LiFeAs 3.776 6.358 102.8 112.9 1.51

NaFeAs 3.949 7.039 108.3 110.1 1.43

LiFeP 3.692 6.031 108.6 109.9 1.33

a) α and β denote the two-fold and four-fold bond angle of the FeAs(P)4 tetrahedron, respectively.

Jin C Q, et al. Sci China-Phys Mech Astron December (2013) Vol. 56 No. 12 2339

Figure 2 The temperature dependence of susceptibility and the inset is the resistance curve for (a) LiFeAs, (b) NaFeAs and (c) LiFeP [16].

Figure 3 The specific heat data for LiFeAs.

plays a key role in searching for new materials as well as in elucidating mechanism of superconductivity in iron arsenide superconductors [9–21]. The “111” system is the simplest for the iron arsenic superconductors. Therefore, the knowledge about this system would more directly help un-derstand the superconducting mechanism or help in design-ing new iron based superconductors.

Figures 4(a)–(c) shows the temperature dependence of electrical resistance under different pressures for LiFeAs [10], NaFeAs [9] and LiFeP [11], respectively. For LiFeAs and LiFeP, Tc decreases monotonously with increasing pressure, while the Tc of NaFeAs increases to a maximum value as pressure increases to 3 GPa; further increasing the pressure suppresses Tc. The curves of pressure dependence of Tc for the three superconductors are drawn in Figure 5(a),

Figure 4 (Color online) Temperature dependence of electrical resistance under different pressures for (a) LiFeAs [11], (b) NaFeAs [9] and (c) LiFeP [12].


Figure 5 (Color online) (a) Pressure dependence of Tc for LiFeAs, NaFeAs and LiFeP [10]; (b) the comparison of Tc-P phase diagrams of iron arsenide superconductors. All points are experimental data with reference indicated [9].

where we use T onset c value for NaFeAs and LiFeAs and T

zero c

value for LiFeP. It clearly shows the different characters of Tc tuned by pressure for “111”-type iron-based supercon-ductors [12]. For LiFeAs & LiFeP, the behavior of super-conducting transition temperature Tc decreases monoto-nously with increasing pressure, which is similar to that of SmFeAsO0.85 and NdFeAsO0.85. Pressure suppresses Tc mo-notonously with a pressure coefficient of 1.38 K/GPa and 1.26 K/GPa for LiFeAs and LiFeP, respectively. However the behavior of pressure tuned Tc of NaFeAs is quite differ-ent and shows a non-monotonous curve. Before 3 GPa the Tc of NaFeAs increases with applying pressure. Tc reaches the maximum value about 31 K at 3 GPa and decreases with further increasing the pressure. This parabolic pressure de-pendence of Tc is also observed in LaFeAsO1xFx [15,16] and AFe2As2 (A = Sr, Ba) [17,18]. For comparison, the Tc -P phase diagrams of iron arsenide superconductors are drawn in Figure 5(b).

For iron pnictide superconductors, the [FeAs(P)] layer is considered as the superconducting path. Therefore the crys-tal chemistry parameters, such as the Fe––As bond length or the As(P)––Fe––As(P) bond angle, are critical to supercon-ducting transition temperatures. Mizuguchi et al. [19] estab-lished the plot of Tc dependence on the anion height for the typical Fe-based superconductors. The plotted data points exhibit a unique curve with a peak around 1.38 Å. Tc would decrease when the anion height deviates from the peak val-ue. The data under high pressures for NdFeAsO0.85, SrFe2As2, BaFe2As2 and FeSe also match well with the curve. Mito et al. [20] reported the structure evolution of LiFeAs under pressure up to 17 GPa. The As––Fe––As (two-fold) angle and the Fe–As bond length decrease with increasing pressure. Although the relation between anion height and pressure was not given in that paper, with the data of angle and bond length at different pressures it can be deduced. The anion height can be obtained by the formula: h = d × cos(α/2), where h is the anion height, d the Fe––As bond length and α the As––Fe––As (two-fold) angle. It is

found that the anion height increases with pressure. In providing insights into the pressure behavior of the

111-type NaFeAs, studies on crystal structural evolution as a function of pressure based on in situ high-pressure syn-chrotron X-ray powder diffraction data and Rietveld re-finement are performed [21]. Figure 6(a) illustrates the pressure dependence of the anion height and As––Fe––As bond angles. The As––Fe––As bond angle evidently in-creases with an increase in pressure, resulting in a peak value, and then rapidly decreasing at a high pressure. The pressure dependence of the anion height is contrary to that of the As––Fe––As bond angle. Structural analysis shows that this behavior is attributed to the change in compression of the FeAs4 tetrahedra at different applied pressure values. At a low pressure, FeAs4 tetrahedra present greater com-pression along the c axis direction than in the basal plane and tend to approach an almost regular FeAs4 tetrahedron of 109.4°. Therefore, the compression of NaFeAs is expected to be accommodated by the change in the softest parts of the structure, i.e., at the charge reservoir block of the double Na layers. The change in distance h between the Fe layer and arsenic reflects this tendency. The fast decrease in the c axis over the a axis increases the As––Fe––As angle but at the same time reduces h, which is observed in the low pressure range of Figure 6(a). This evolution results in an isostruc-tural phase transition from one tetragonal phase to a col-lapsed tetragonal phase, which is attributed to the shear movements of the charge reservoir layer. This similar isostructural phase transition was observed previously in NdFeAsO compound [22]. The curves of pressure versus Tc and bond angle α are ploted in Figure 6(b). Obviously, the pressure range at which the maximum Tc as shown in Figure 6(b) is found where Tc reaches the maxima coincides with the onset of pressure where the FeAs4 tetrahedron geometry change and the discontinuous change of the crystal cell volume. Within the transition region, the FeAs4 tetrahedron shape is approaching the regular shape with an average As––Fe––As angle of 109.4°. The anion height is 1.38 Å,


Figure 6 (a) Pressure dependence of As––Fe––As bond angles ( angle) and the anion height from the iron layer; (b) phase diagram of the crystalline structure change of the coordination geometry and superconducting Tc as a function of pressure [21].

which is far less than 1.42 Å at ambient pressure. With an increase in pressure, the As––Fe––As bond angle decreases and dramatically deviates from the ideal tetrahedron value of 109.47°. The anion height also increases with an increase in pressure. This behavior approaches a regular one. At the same pressure, the anion height is close to the optimal value of 1.38 Å reported by Mizuguchi et al. [19]. These behav-iors strongly suggests that the pressure-induced higher Tc superconducting phenomenon is associated with the evolu-tion of the distorted FeAs4 tetrahedron as it approaches the regular tetrahedron and optimized anion height. With an increase in pressure, the negative pressure coefficients of Tc are observed, which are rationalized in terms of the in-creased tetrahedral distortion away from the regular shape at a higher pressure. This is somehow different from those observed in CaFe2As2, where the parent compound becomes superconductive in a certain applied pressure region but with a sharp boundary to the nonsuperconducting high- pressure phase. Assuming that pressure increases Tc of the first tetragonal phase while it suppresses Tc of the second tetragonal phase for NaFeAs, one can however further eventfully correlate Tc with the change of the FeAs4 geome-try as revealed in our studies. Therefore, the FeAs4 geome-try is the key factor that determines the superconducting transition temperature.

2 New type of diluted magnetic semiconductors with independent charge and spin doping

Ferromagnetic systems obtained by doping transition metals into semiconductors have generated extensive studies since the early 1990s because of their potential use for spin- sensitive electronics (spintronics) devices. In proto-typical systems based on III–V semiconductors, such as (Ga,Mn)As and (In,Mn)As, substitution of divalent Mn atoms into tri-valent Ga or In sites leads to severely limited chemical sol-ubility. Because of this, the specimens are chemically meta-stable, available only as thin films, and their material quali-ty exhibits high sensitivity on preparation methods and heat

treatments. This substitution dopes hole carriers together with magnetic atoms, which prohibits electron doping to obtain n-type systems necessary for formation of spintronics p–n junction devices.

We recently synthesized a new system Li(Zn,Mn)As [23] based on the I–II–V semiconductor LiZnAs, showing a Cu-rie temperature (Tc) up to 50 K. In this system, charges are doped via offstoichiometry of Li concentrations, while spins by the isovalent (Zn2+, Mn2+) substitutions. This is different from the simultaneously doping in the classic DMS [24]. Although Li(Zn,Mn)As was a ferromagnetic DMS of a new type having a few distinct advantages over (Ga,Mn)As, the upper limit of currently achievable Tc has been significantly lower than that in (Ga,Mn)As [25]. We discovered a new ferromagnetic DMS (Ba,K)(Zn,Mn)2As2 system [26], which shares the same ThCr2Si2 crystal structure, well known as “122” structure, with (Ba,K)Fe2As2 and (Ba,K)Mn2As2. Via (Ba,K) substitution to dope hole carriers and (Zn,Mn) sub-stitution to supply magnetic moments, samples with 5%– 15% Mn doping exhibit ferromagnetic order with Tc up to 180 K. It is reported that the isostrcutural phosphides are also possible DMS [27].

We further found a DMS in which As has been replaced by nontoxic P as shown in Figure 7(a).We found that Li(Zn,Mn)P exhibits soft ferromagnetic behavior with a relative lower carrier density than those for Li(Zn,Mn)As, (Ba,K)(Zn,Mn)2As2 and (Ga,Mn)As, but with comparable Tc, offering the advantage of much improved semiconduc-tive behavior and the potential for higher Tc via increased carrier concentration [28].

ReFeAsO (where Re denotes a rare earth element) are the known as “1111” type iron-based superconductors [1] with ZrCuSiAs-structure. One more new DMS we found is the “1111” type (LaCa)(Zn1xMnx)SbO (0.05x0.15), which shows ferromagnetic property but retains semiconductive conductivity. We found that the p-type carriers can be in-duced by substituting La3+ with Ca2+. The ferromagnetic magnetic properties of (La0.95Ca0.05)(Zn1xMnx)SbO by sub-stituting Mn2+ for Zn2+ at the doping level x=0.05–0.15 were systemically studied [29].


Figure 7 (Color online) (a) Crystal structure of Li(ZnMn)As and Li(ZnMn)P with space group of F-43m; (b) crystal structure of (Ba,K)(Zn,Mn)2As2; (c) crystal structure of LaZnSbO.

These new DMS materials have the added advantage of being structurally very similar to the recently discovered iron-pnictide high temperature superconductors, opening up the possibility of creating novel junctions between super-conducting, semiconducting, and magnetic materials.

2.1 Li(Zn,Mn)As system

LiZnAs is a direct-gap semiconductor that has a cubic crys-tal structure (Figure 7(a)) [30] and a band gap (1.61 eV) similar to that of GaAs (1.52 eV) [31]. Ferromagnetism was found only in compounds with excess Li. Figure 8(a) shows the temperature dependence of M at 2 kG (no difference between field cooling (FC) and zero-field cooling (ZFC) procedures), whereas Figure 8(b) shows the field depend-ences of M at T=2 K, for Li 1.1 systems with various x val-ues. Clear signatures of ferromagnetic order are seen in these figures. The Tc values show a saturation at ~50 K for x=0.15, whereas the ordered moment values per Mn atom show a maximum for low Mn concentrations and decreases monotonically with increasing x. This is presumably partly due to an effect of the antiferromagnetic coupling for Mn pairs in the nearest neighbour Zn sites. A very thin hystere-sis loop in Figure 8(b) indicates a very small coercive field (<100 Oe), which is promising for spin manipulation. Above Tc exhibits a simple history independent Cu-rie–Weiss behaviour with the effective paramagnetic mo-ment values of around 5.9 Bohr magnetons per Mn, as ex-pected for fully magnetic individual Mn2+ moments.

The Hall effect measurements clearly showed p type car-riers for Li excess systems. Figure 8(c) shows representative Hall results for the Li1.1Zn0.95Mn0.05As system, which exhib-its anomalous Hall effect at H = 0 below Tc and p-type car-rier concentration n~1020 cm3. The p-type carrier for excess Li can be expected when the excess Li occupies substitu-tional Zn sites, and the substituted Zn atoms either escape from the system or remain neutral without ionization. This p type carrier is quite different from those of theoretical pred-

ication [32], In examining the volume fraction and ordered momen size, SR measurements were performed on Li1.1Zn0.95Mn0.05As.The time spectra observed in zero field (Figure 8(d)) clearly exhibit an increase of the relaxation rate below T~25 K, and measurements in longitudinal fields confirm that this is due to static magnetic order. The volume fraction of regions with static magnetism (Figure 8(d)) in-dicates a sharp transition at Tc~30 K, and magnetic order with the full volume fraction achieved below T~10 K.

2.2 (Ba,K)(Zn,Mn)2As2 system

BaZn2As2 is a semiconductor with the tetragonal ThCr2Si2 crystal structure (shown in Figure 7(b)) [31,33]. Figure 9(a) shows temperature dependence of magnetization in ze-ro-field-cooling (ZFC) and field-cooling (FC) procedures under 500 Oe for (Ba1xKx)(Zn0.9Mn0.1)2As2 specimens with x=0.05, 0.1, 0.15, 0.2, 0.25 and 0.3, respectively. Clear sig-natures of ferromagnetic order are seen in the curves, with corresponding critical temperature Tc=5 K, 40 K, 90 K, 135 K, 170 K and 180 K, respectively. Above Tc, the samples are paramagnetic and the susceptibility can be fitted to the Curie-Weiss formula with an effective paramagnetic mo-ment around 5 Bohr magnetons per Mn2+.

As shown in Figure 9(b), the Hall effect measurements indicate that 10% (Ba,K) substitution in (Ba,K)Zn2As2 re-sults in hole concentration of 4.3×1020 cm3, consistent within a factor of two with that obtained by assuming that each K atom introduces one hole to the system. In the fer-romagnetic state below Tc, the anomalous Hall effect is ob-served with a small coercive field ~35 Oe in the temperature region between Tc and the history-dependence temperature THist below which FC and ZFC susceptibility shows devia-tion. The small coercive field above THist will be helpful for spin manipulation.

Using bulk polycrystalline specimens, we also performed positive SR measurements. As shown in Figure 9(c), a


Figure 8 (a) the temperature dependence of M (M(T)) in H = 2 kOe of Li1.1(Zn1xMnx)As; (b) M at T = 2 K in various values of external field H. The grey symbols show a hysteresis loop in x = 0.03 system plotted for small field regions; (c) Hall resistivity of Li1.1(Zn0.95Mn0.05)As at T = 2 K, which exhibits ptype carriers with concentrations of n~1020 cm3 together with the anomalous Hall effect due to spontaneous magnetization at H = 0; (d) the volume fraction of the magnetically ordered region, derived from the amplitude of the fast relaxing signal. Inset is time spectra in zero field that exhibit onset of extra relaxation below T~30 K [23].

Figure 9 (a) Magnetization measured in H=500 G in (Ba1xKx)(Zn0.9Mn0.1)2As2 with several different charge doping levels x, with ZFC and FC procedures. (b) Hall effect results from a sintered specimen of (Ba1xKx)(Zn0.9Mn0.1)2As2 with the charge doping level of x=0.15 having Tc=90 K. Anomalous Hall effect and a very small coercive field is seen at T~50 K near the history-dependence temperature THist, while a large coercive field is seen at T=2 K. (c) Zero Field SR (ZF SR) time spectra obtained in specimen (Ba0.8K0.2)(Zn0.9Mn0.1)2As2. (d) Volume fraction of regions with static magnetic order, estimated by SR measurements in ZF and weak transverse field (WTF) of 50 G [26].

sharp increase of the SR relaxation rate is seen with de-creasing temperature below Tc, and a highly-damped pre-cession signal was observed below T~40 K. The volume fraction of magnetically ordered state was estimated by us-

ing SR data in ZF and weak transverse field of 50G as shown in Figure 9(d). The SR results indicate static mag-netic order developing in the entire volume with a rather sharp onset below Tc.


2.3 Li(Zn,Mn)P system

LiZnP is a direct-gap band gap (2.04 eV) semiconductor that has the same structure with LiZnAs (shown in Figure 7(a)). Robust ferromagnetism was found only in specimens with excess Li. Figure 10(a) shows M(T) for Li1.04(Zn1−x-

Mnx)P specimens, in an applied field of 100 Oe in both ZFC and FC modes. The maximum Tc of Li1.04(Zn1−xMnx)P is up to 34 K when Mn concentration rises to 10%. As shown in

Figure 10 (a) M(T) curves in ZFC and FC modes with H = 100 Oe for Li1.04(Zn1xMnx)P specimens (no visible difference between ZFC and FC procedures for small coercive fields). Inset shows the temperature depend-ence of the inverse susceptibility. (b) Top: Tc and θ for Li1+yZn1−xMnxP. Bottom: Meff and Msat for Li1.04Zn1−xMnxP. Dashed lines indicate extrapola-tion. (c) ρ(T) and the inverse carrier concentration for Li1.04Zn0.9Mn0.1P. Inset shows Hall resistivity, exhibiting p-type carriers with np = 6.1×1016 cm3, 1.82×1017 cm3, and 3.27×1017 cm3 at 100, 200, and 300 K, respec-tively [28].

Figure 10(b), extrapolation of effective paramagnetic mo-ment (Meff) to lower Mn concentrations yields a value of approximately 6 B/Mn, as expected for the fully high spin oriented Mn2+ ion. Figure 10(b) also shows the saturation moment per Mn (Msat) in an applied field of 500 Oe, found to be about 1–2 B/Mn. For ferromagnetic (Ga,Mn)As, Li(Zn,Mn)As, and (Ba,K)(Zn,Mn)2As2, Msat are about 2–4 B/Mn [34], 1–3B/Mn [23], and 1–2B/Mn [26], respec-tively, which are comparable to that of Li(Zn,Mn)P.

Figure 10(c) shows the resistivity and carrier concentra-tion of Li1.04Zn0.9Mn0.1P from 5–300 K. The resistivity value was diverging and too large at a low temperature. As plotted in Figure 10(c), the resistivity obviously increases with de-creasing temperature, whereas the mobile hole concentra-tion decreases. This is indicative of typical semiconducting behavior. Ferromagnetic order was achieved while the sys-tem still shows semiconducting transport behavior. Some-what surprisingly, excess Li concentration systems are found to exhibit p-type behavior, not the n-type behavior that one would expect assuming that excess Li provides additional electrons. This is explained by first-principles calculations, which indicate that the excess Li1+ ions are thermodynamically favored to occupy the Zn2+ sites, there-by rendering Li(Zn,Mn)P a p-type DMS.

2.4 (LaCa)(ZnMn)SbO system

The pattern fitting of step X-ray diffraction indicates that (La,Ca)(Zn,Mn)SbO (0.05x0.15) crystallizes into ZrCuSiAs-type structure with the space group P4/nmm, as shown in Figure 7(c). Figure 11(a) shows the temperature dependence of the magnetization under ZFC and FC for (La0.95Ca0.05)(Zn0.9Mn0.1)Sb. The magnetization intensity increases sharply as temperature decreases to around 25 K, which is denoted as Tc, as a result of the paramagnetic to a ferromagnetic transition [29]. The values of Tc increases with Mn content and up to 40 K at x=0.15. Figure 11(b) shows the representative Hall effect for (La0.95Ca0.05) (Zn0.9Mn0.1)SbO compound, which exhibits anomalous Hall effect below Tc and a p-type carrier nature with concentra-tion of n1020 cm3, in the same order as those for “111” DMS Li(Zn,Mn)As(P) [23,28] or “122” DMS (Ba,K) (Zn,Mn)2As2 [26].

As a conclusion in the section, three new DMS systems were discovered. With isovalent (Zn,Mn) substitution, charge doping decouples from spin doping to overcome several limits in prototypical DMS. “111” type of Li(Zn, Mn)As & Li(Zn,Mn)P, “122” type (Ba,K) (Zn,Mn)2As2, and “1111” type (LaCa)(Zn,Mn)SbO are successfully synthe-sized in bulk materials. Ferromagnetism with a critical temperature of up to 50 K, 34 K, 180 K and 40 K is ob-served in Li(Zn,Mn)As, Li(Zn,Mn)P, (Ba,K)(Zn,Mn)2As2, and (LaCa)(Zn,Mn)SbO respectively. As demonstrated in Figure 12, “122” DMS ferromagnet (Ba,K)(Zn,Mn)2As2, semiconducting BaZn2As2, antiferromagnetic BaMn2As2


Figure 11 (Color online) (a) Temperature dependences of ZFC and FC mode with a DC magnetic field of 2000 Oe for (La0.95Ca0.05)(Zn0.9Mn0.1) SbO. (b) Hall resistivity of (La0.95Ca0.05)(Zn0.9Mn0.1)SbO at different tem-perature, which exhibits p-type carriers with concentrations of n1020 cm3 showing anomalous Hall effect below Tc [29].

Figure 12 (Color online) Crystal structures of (a) superconductor (Ba,K)Fe2As2, (b) ferromagnetic semiconductor (Ba,K)(Zn,Mn)2As2, and (c) antiferromagnetic BaMn2As2. All these structures contain ab plane lattice with perfect matchable constants for each other.

and superconducting (Ba,K)Fe2As2 all share the same crys-tal structure. Moreover, the lattice constants in the a-b plane match within about 5%. These features could open possibil-ities of making junctions between various combinations of these systems through the As layers in efforts to develop spintronics devices showing novel functionalities.

3 Pressure induced superconductivity of topo-logical insulators

In this section we will introduce studies on three dimen-sional topological insulators [35], focusing on pressure in-duced superconducting transitions in those topological quantum compounds [36–39]

Topological insulators (TIs) are a new class of material in condensed matter physics and have thus attracted signifi-cant research interest worldwide [40–44]. Different from conventional insulators, TIs are characterized by a full in-sulating gap in the bulk and gapless edge or surface states in the boundaries which are protected by time-reversal sym-metry. These topological materials have been theoretically predicted and experimentally observed in a variety of sys-tems, including HgTe quantum wells [40], BiSb alloys, and Bi2Te3, Sb2Te3 and Bi2Se3 crystals [35,45]. The first Three-dimensional (3D) TIs experimentally identified are Bi1−xSbx alloy, but the surface structures are rather compli-cated and the band gaps are rather small. This motivated a search for topological insulators with a larger band gap and simpler surface structure. A new 3D TIs Bi2Te3, Sb2Te3 and Bi2Se3 which has a nearly idealized single Dirac cone was theoretically predicted later [35], and the conducting surface states with Dirac cone are confirmed by angle resolved photoemission spectroscopy (ARPES) [45,46].

Similar to TIs, topological superconductors (TSC) have a full pairing gap in the bulk and gapless edge states consist-ing of elusive Majorana fermions [47]. Considering the unique band structure in bulk states and surface/edge states of TI, searching for superconductivity in TI will be a ready way to approach topological superconductors since the sur-face can easily keep gapless topological nature. When TIs are combined with magnetic or superconducting materials, the Majorana state may be realized at their interface. Sever-al applications [48] of interfaces between TIs and super-conductors have been proposed. However, if the TI side and the superconductor side are made from different materials, one may encounter a serious mismatch problem at the in-terface. To overcome this problem needs an ideal design to make the doped TI superconducting and consider the prox-imity effect through the natural “interface” between the bulk and surface regions, without introducing any other super-conducting compound. The superconductivity of Bi2Se3 has been observed by copper intercalation in van der Waals gaps between quintuple layers [49], using a chemical way to introduce carriers that result in superconductivity. Pressure is effective in generating or tuning superconductivity by modifying the electronic structure through directly changing atomic distances without introducing defects or impurities compared with the chemical substitution. So we proposed an alternative way approaching topological superconductor via physical tool: using pressure to tune the electronic structure of Bi2Te3, Sb2Te3 and Bi2Se3 TIs [36–39]. Pres-sure-induced superconductivity via high-pressure resistance


measurement and structural evolution via high-pressure X-ray diffraction experiment of Bi2Te3, Sb2Te3 and Bi2Se3 TIs are obtained and discussed, respectively. Bi2Te3 and Sb2Te3 show superconductivity at pressures before structur-al phase transition, whereas only the high pressure phase is superconducting in Bi2Se3. All of the three materials are transformed from the original rhombohedral structure (am-bient phase) to a monoclinic structure, and further to a dis-ordered body centered cubic structure with a number of reports [50–61] on TIs. The results for some groups are in-consistent, and the discrepancy will be also discussed in this paper.

A2B3 type materials (Bi2Te3, Sb2Te3 and Bi2Se3) share a rhombohedral layered structure with a space group R-3m at ambient pressure, as shown in Figure 13, in which five co-valently bonded atomic layers are grouped into a quintuple layer (QL) and the QLs are weakly bonded along the c axis to form a crystal. The layered structure makes A2B3 a good model to investigate the TI properties.

3.1 Effects of pressures on electrical conductivity

The resistance measurement of Bi2Te3, Sb2Te3 and Bi2Se3

single crystals are performed under pressure. The evolutions of resistance as function of temperature of Bi2Te3, Sb2Te3 and Bi2Se3 at various pressures are shown in Figure 14.

Figure 13 The crystal structure of Sb2Te3 (a), Bi2Te3 (b) and Bi2Se3 (c) topological insulator with space group R-3m.

Figure 14 The resistance as function of temperature at various pressures showing superconductivity and the superconducting transition with applied mag-netic field H perpendicular to the ab-plane for (a) Bi2Te3 [36], (b) Sb2Te3 [37] and (c) Bi2Se3 [38].


As shown in Figure 14(a), a clear superconducting tran-sition was observed at 3.2 GPa for Bi2Te3, and the transition temperature keeps almost constant up to 6.3 GPa. It is noted that the high pressure phase (>9.3 GPa) of Bi2Te3 is also superconducting with higher Tc. Such a superconducting high pressure phase (> 6.3 GPa) has been reported before, and the different transition temperatures (around 2 K re-ported in) may originate from the different carrier type and density of the samples.

Figure 14(b) shows the evolution in resistance as a func-tion of temperature of Sb2Te3 single crystals at various pressures. Below 4.0 GPa, Sb2Te3 does not display super-conductivity at a temperature to 1.5 K. When the pressure was increased beyond 4.0 GPa, a superconducting transition with a Tc of around 3 K was observed. Further increasing pressure to 6.8 GPa, Tc grows rapidly with the resistance drop getting more pronounced and the zero-resistance state being fully realized. A superconducting transition with higher Tc was observed at 7.5 GPa, after which Tc becomes constant up to 30 GPa. The pressure induced superconduc-tivity exhibits more complex behaviors when the pressure was further increased from 16.3 GPa to 30 GPa. When the pressure was higher than 30 GPa, the superconducting tran-sition becomes sharp again, which indicates the good ho-mogeneity of a single superconducting phase with Tc of about 7.3 K.

Figure 14(c) shows the evolution in resistance as a func-tion of temperature of Bi2Se3. When the pressure is below 3.1 GPa, Bi2Se3 displays weak metallic behavior. It be-comes more metallic above 5.1 GPa, which is probably re-lated to the pressure-induced electronic topological transi-tion (ETT) in Bi2Se3 near 5 GPa, as previously reported [61]. A drop in the resistance first occurs at around 12 GPa with a Tc of about 4.4 K. The concentration of Se vacancies can induce variations in the resistance and carrier density by several orders of magnitude at ambient pressure. Different initial carrier densities could be the origin of the different resistance versus temperature curves [58]. The value of Tc

increases rapidly to a maximum of 8.2 K at 17.2 GPa, and decreases to around 6.5 K at 23 GPa, and then remains al-most constant with further increases in pressure.

To assure the resistance drops induced by pressure in Bi2Te3, Sb2Te3 and Bi2Se3 are indeed a superconducting transition we further measured the resistance versus tem-perature as a function of magnetic field, as shown in Figure 14. It is obvious that the drops of resistance shift toward lower temperature with increasing magnetic field, which indicates that the transition is superconductivity in nature. Using the Werthdamer-Helfand-Hohenberg formula [62] of Hc2(0)=0.691[dHc2(T)/dT]T=Tc

×Tc, the upper critical field

Hc2(0) is extrapolated to be 1.83 T, 2.6 T and 4.7 T for Bi2Te3, Sb2Te3 and Bi2Se3 respectively for H||c (here the single crystal is placed inside the diamond anvil cell with magnetic H direction perpendicular to ab-plane).

The high pressure Hall experiments for Bi2Te3, Sb2Te3 and Bi2Se3 are also performed. We compare the Tc as function of pressure with the changes of carrier density, as shown in Figure 15. The superconducting phase diagrams of Bi2Te3 and Sb2Te3 are divided into three regions as follows: region A with no superconductivity and regions B and C with superconductivity. Tc in region B corresponds to the superconductivity of the ambient phase, whereas Tc in re-gion C corresponds to the superconductivity of the high pressure phase. In region B, carrier density increases fast. Only a negligible increase in the carrier density of region C compared with that of region B was observed in both Bi2Te3 and Sb2Te3. Sb2Te3 that is hole dominated at ambient pres-sure assumes an electron-dominated character within the pressure range of 7.2–8.3 GPa, which is similar to Bi2Te3 crystals. This kind of carrier type flip at high pressure was observed in several semiconductor materials, and is ascribed to the change in electronic structure, e.g., the Lifshitz phase transition [63]. The evolution of Tc and the carrier density for Bi2Se3 divide the range of the pressure into two regions: the ambient phase and the high pressure phases. Hall coeffi-cient measurements indicate that the carrier of the Bi2Se3

Figure 15 (Color online) Pressure dependence of the superconducting transition temperature and the carrier density for Bi2Te3 [39], (b) Sb2Te3 [37] and Bi2Se3 [38].


crystals used in the experiments is electron dominated with carrier density to be approximately 2×1018 cm3 at ambient pressure and does not change over the entire pressure range measured, whereas the change of the electron state from the hole-dominated to the electron-dominated type is induced by pressure in Bi2Te3 and Bi2Se3. When the carrier density increases rapidly with increasing pressure at high pressure phase, the Tc increases sharply too. Therefore, the variations in Tc with pressure are closely connected with the change in carrier density.

3.2 High pressure structure

The in situ high-pressure synchrotron radiation experiments of Bi2Te3, Sb2Te3 and Bi2Se3 are performed. The evolution of crystal structure with pressure of Bi2Te3 is shown in Fig-ure 16. No structural transition occurs at a pressure lower than 8 GPa. Some new reflections which are denoted by arrows show the high pressure phases above 8 GPa for HP I (high pressure phase I) and at 14.3 GPa for HP II and HP III, respectively. The new reflections which indicate phase tran-sition are also observed in Sb2Te3 and Bi2Se3.

3.3 The first principle calculation

To confirm the topological nature of superconductivity at ambient phase, we calculated the electronic structures of Bi2Te3 and Sb2Te3 by the first-principles calculations based on density functional theory and the generalized gradient approximation, using the experimental lattice parameters and atomic positions. Figure 17 shows the results of Bi2Te3 at 0.0 GPa and 4.0 GPa. The topologically nontrivial surface state does exist even within 4.0 GPa, although the band gap is reduced. The similar result is also given for Sb2Te3. One possibility is that the topological surface states can maintain their character in the presence of the p type bulk carriers, while the resulting proximity effect with the bulk super-

Figure 16 (Color online) The evolution of X-ray diffraction reflections with pressure for Bi2Te3. The arrows indicate new reflections high pressure phases [39].

Figure 17 The calculated bulk electronic structures (left panels) and surface states (right panels) of the Bi2Te3 at 0.0 GPa [(a) and (b)] and 4.0 GPa [(c) and (d)], respectively [36].

conducting carriers can give rise to Majorana fermions in the surface state. The other more exciting possibility is that the superconductivity of the ambient phase is truly three dimensional in a similar way to the Balian and Werthamer state in He3-B-phase.

The superconducting and structural phase diagrams as a function of pressure for Bi2Te3, Sb2Te3 and Bi2Se3 are ob-tained, as shown in Figure 18. For Bi2Te3, possible phases for intermediated high pressure phases of HP I & HPII are suggested to be of monoclinic sevenfold C2/m structure and eightfold C2/c structure respectively. The X-ray diffraction patterns of HP III above 16 GPa are indexed with BCC structure. Within the BCC structure Bi and Te atoms are disordered to share the bcc lattice sites. This may be ob-tained at high pressure when A & B ion radii are nearly equal caused by charge transfer from Bi to Te [51]. By re-ferring to the high pressure X-ray diffraction experiments for Sb2Te3 at higher pressures at room temperature reported in ref. [57], as well as the isostrutural compound Bi2Te3, four phases are assigned, i.e., a, b, c, d being ambient phase, the high pressure phase I, high pressure phase II, high pres-sure phase III, respectively, as shown in Figure 18(b). Re-ferring to the results obtained from synchrotron radiation in ref. [58], we infer that Bi2Se3 has the same four phases as Bi2Te3, as shown in Figure 18(c). But a different result is given by another group [61]. Bi2Se3 undergoes only two high pressure phases, and the sevenfold monoclinic struc-ture remains in a larger pressure range of 10.4–34.4 GPa than that in Bi2Te3 and Sb2Te3. At about 24.5 GPa, Bi2Se3 is transformed to a body-centered tetragonal structure without an obvious disordered atom array.


Figure 18 (Color online) The superconducting and crystal structural phase diagram of Bi2Te3 (a) [39]; Sb2Te3 (b) [37]; Bi2Se3 (c) [38].

4 Summary

We discovered the “111” type iron based superconductors LiFeAs (Tc ~ 18 K) and LiFeP (Tc ~ 6 K) and obtained the highest Tc ~ 31 K among the “111” family at about 3GPa in NaFeAs superconductor. Further more, we study the role of pressure in tuning superconductivity and found that the su-perconducting transition temperature Tc is intimately related to the anion height and As––Fe––As bond angles, i.e. the maximum Tc occurs at the phase transition boundary with optimized FeAs4 coordination geometry.

We found a new approach to separately doping the local moment and charge carriers into the parent semiconductors to overcome several limits in prototypical DMS. We con-sequently successfully discovered three new types of DMS systems including “111” type Li(Zn,Mn)As, Li(Zn,Mn)P, “122” type (Ba,K)(Zn,Mn)2As2, “1111” type (LaCa)(Zn,Mn) SbO. The highest critical temperature 180 K is achieved in 122 type (Ba,K)(Zn,Mn)2As2 system. Moreover, ferromag-netic (Ba,K)(Zn,Mn)2As2 shares the same “122” crystal structure with antiferromagnetic BaMn2As2 and supercon-ducting (Ba,K)Fe2As2 with nearly the same lattice constants in the a-b plane, which makes them promising for the de-velopment of multilayer functional devices.

We have experimentally observed superconductivity in Bi2Te3, Sb2Te3 & Bi2Se3 induced via pressure. Topological surface states remain to be well defined in both Bi2Te3, Sb2Te3 at the pressure ranges where superconductivity is first observed. The superconductivity pressure induced via pressures in those topological superconductors is assumed to be intimately related to the topological nature.

This work was supported by the National Natural Science Foundation of China and Ministry of Science and Technology (MOST). We would like to thank all collaborators for their great contributions. We are grateful to Prof. Uemura for the collaborations on DMS studies.

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