intTypePromotion=1

Hợp chất siêu dẫn sắt từ UCoGe

Chia sẻ: Tho Tho | Ngày: | Loại File: PDF | Số trang:8

0
19
lượt xem
0
download

Hợp chất siêu dẫn sắt từ UCoGe

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Bài báo trình bày sự xuất hiện đồng thời của trật tự sắt từ và trạng thái siêu dẫn trong hợp kim UCoGe tại áp suất khí quyển. Số liệu thực nghiệm thu được từ các phép đo cơ bản đặc trưng của các tính chất từ, nhiệt, điện cho thấy UCoGe là chất sắt từ yếu với nhiệt độ Curie của chuyển pha sắt từ - thuận từ TC = 3 K, và là vật liệu sắt từ với nhiệt độ chuyển pha siêu dẫn Ts = 0.8 K.

Chủ đề:
Lưu

Nội dung Text: Hợp chất siêu dẫn sắt từ UCoGe

TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 14, SOÁ K2 - 2011<br /> ON DISCOVERY OF THE FERROMAGNETIC SUPERCONDUCTOR UCoGe<br /> Nguyen Thanh Huy(1), Dao Duc Cuong(1), Vu Thanh Thu(2), Bui Tu An(1)<br /> (1) PetroVietnam University; (2) VNU-HaNoi<br /> (Manuscript Received on October 21th, 2010, Manuscript Revised January 21st, 2011)<br /> <br /> ABSTRACT: We report the coexistence of ferromagnetic order and superconductivity in UCoGe<br /> at ambient pressure. The data obtained from the basic thermal, magnetic and transport properties on<br /> the macro and microscopic scale show that UCoGe is a weak ferromagnet with a Curie temperature TC<br /> = 3 K, and also, is a superconductor with a resistive transition temperature Ts = 0.8 K. Those present<br /> evidence that UCoGe is an unconventional superconductor and argue that superconductivity is<br /> mediated by critical ferromagnetic spin fluctuations.<br /> Keywords: Ferromagnetic superconductor, Ferromagnetic quantum critical point, Critical spin<br /> fluctuations.<br /> In the year 2000, the discovery of the first<br /> <br /> 1. INTRODUCTION<br /> In ferromagnetic (FM) state below the<br /> <br /> superconducting ferromagnet UGe2 came as a<br /> <br /> Curie temperature, TC, the electron spins align<br /> <br /> big surprise [2]. In UGe2, superconductivity is<br /> <br /> to produce a net magnetization. For a long time<br /> <br /> realized well below the Curie temperature,<br /> <br /> it was thought that superconductivity (SC) is<br /> <br /> without expelling the ferromagnetic order.<br /> <br /> incompatible with ferromagnetism. This is<br /> <br /> Since<br /> <br /> rooted<br /> <br /> in<br /> <br /> the<br /> <br /> three<br /> <br /> other<br /> <br /> superconducting<br /> <br /> theory<br /> <br /> of<br /> <br /> ferromagnets have been discovered: UIr [3],<br /> <br /> 1957<br /> <br /> by<br /> <br /> URhGe [4], and UCoGe [5]. These materials<br /> <br /> Bardeen, Cooper, and Schrieffer (BCS) [1].<br /> <br /> have in common that ferromagnetic order is<br /> <br /> Within<br /> <br /> a<br /> <br /> due to the uranium 5f magnetic moments and<br /> <br /> superconducting condensate is formed under<br /> <br /> has a strong itinerant character. Moreover,<br /> <br /> the influence of an attractive force due to<br /> <br /> superconductivity occurs close to a magnetic<br /> <br /> lattice vibrations which binds electrons with<br /> <br /> instability. The coexistence of SC and FM in<br /> <br /> antiparallel spins in singlet Cooper pairs ↑↓〉<br /> <br /> these materials can be understood in terms of<br /> <br /> with orbital momentum L = 0 and projection of<br /> <br /> spin fluctuation models [6]: in the vicinity of a<br /> <br /> the spin momentum Sz = 0. When magnetic<br /> <br /> FM quantum critical point, critical magnetic<br /> <br /> impurity atoms are placed in a conventional<br /> <br /> fluctuations can mediate superconductivity by<br /> <br /> superconductor, the local field surrounding the<br /> <br /> pairing the electrons in spin-triplet Cooper<br /> <br /> impurity atom suppresses singlet Cooper pair<br /> <br /> pairs, that is, the equal spin pairing (ESP) ↑↑〉<br /> <br /> formation, which causes a rapid depression of<br /> <br /> with L = 1, Sz = 1, ↓↓〉 with L = 1, Sz = -1, and<br /> <br /> the SC transition temperature, Ts.<br /> <br /> the state (↑↓〉 +↓↑〉)/1.41 with L = 1, Sz = 0.<br /> <br /> superconductivity<br /> the<br /> <br /> microscopic<br /> <br /> then,<br /> <br /> published<br /> <br /> standard<br /> <br /> in<br /> <br /> BCS<br /> <br /> scenario,<br /> <br /> Trang 21<br /> <br /> Science & Technology Development, Vol 14, No.K2- 2011<br /> In recent years ample evidence has been<br /> <br /> spin relaxation of polycrystalline UCoGe<br /> <br /> presented<br /> <br /> pairing<br /> <br /> samples. Magnetization measurements show<br /> <br /> mechanism is at work in superconducting<br /> <br /> UCoGe is a weak itinerant ferromagnet with a<br /> <br /> ferromagnets [2,7].<br /> <br /> Curie temperature TC = 3 K and a small ordered<br /> <br /> that<br /> <br /> such<br /> <br /> an<br /> <br /> unusual<br /> <br /> With the discovery of superconducting<br /> <br /> moment m0 = 0.03 µB, while SC is observed<br /> <br /> ferromagnets a new research theme in the field<br /> <br /> with a resistive transition temperature Ts = 0.8<br /> <br /> of magnetism and SC has been disclosed.<br /> <br /> K. Muon spin relaxation measurements provide<br /> <br /> Research into ferromagnetic superconductors<br /> <br /> unambiguous proof that ferromagnetism is a<br /> <br /> will help to unravel how magnetic fluctuations<br /> <br /> bulk<br /> <br /> can stimulate superconductivity, which is a<br /> <br /> superconductivity on the microscopic scale.<br /> <br /> central<br /> <br /> materials<br /> <br /> Since SC occurs right on the borderline of FM,<br /> <br /> families as diverse as the heavy-fermion<br /> <br /> UCoGe may present a typical example of<br /> <br /> superconductors [2,4,8], high-Ts cuprates [9]<br /> <br /> triplet SC stimulated by critical fluctuations<br /> <br /> and<br /> <br /> associated with a FM quantum critical point<br /> <br /> theme<br /> <br /> the<br /> <br /> running<br /> <br /> recently<br /> <br /> through<br /> <br /> discovered<br /> <br /> FeAs-based<br /> <br /> property,<br /> <br /> which<br /> <br /> superconductors [10]. This novel insight might<br /> <br /> (QCP).<br /> <br /> turn out to be crucial in the design of new<br /> <br /> 2. EXPERIMENT<br /> <br /> superconducting materials.<br /> UCoGe<br /> <br /> belongs<br /> <br /> to<br /> <br /> coexists<br /> <br /> with<br /> <br /> Most experimental methods have been<br /> the<br /> <br /> family<br /> <br /> of<br /> <br /> carried out at the Van der Waals – Zeeman<br /> <br /> intermetallic UTX compounds, with T is a<br /> <br /> Institute<br /> <br /> transition metal and X is Si or Ge, and<br /> <br /> Amsterdam. Polycrystalline UCoGe samples<br /> <br /> crystallizes<br /> <br /> TiNiSi<br /> <br /> were prepared with nominal compositions<br /> <br /> structure (space group Pnma) [11]. From<br /> <br /> U1.02CoGe by arc melting the constituents<br /> <br /> magnetization, resistivity and specific-heat<br /> <br /> (natural U 99.9%, Co 99.9%, and Ge 99.999%)<br /> <br /> measurements, UCoGe is considered as a<br /> <br /> under a high-purity argon atmosphere in a<br /> <br /> paramagnetic ground state down to temperature<br /> <br /> water-cooled copper crucible. The as-cast<br /> <br /> down 1.2 K [12,13]. However, in a search for a<br /> <br /> samples were annealed for 10 days at 875 oC.<br /> <br /> FM quantum critical point induced in the<br /> <br /> Samples with typical dimensions of 1×1×6<br /> <br /> ferromagnetic superconductor URhGe (Ts =<br /> <br /> mm3 for the different experiments were cut by<br /> <br /> 0.25 K, TC = 9.5 K) by doping Rh with Co [14],<br /> <br /> AGIEPLUS spark erosion, after which the<br /> <br /> it was discovered that UCoGe is actually a<br /> <br /> defected surface was removed by polishing.<br /> <br /> weak itinerant ferromagnet below TC = 3 K<br /> <br /> Powder x-ray diffraction patterns at T = 300 K,<br /> <br /> and, moreover, a superconductor below Ts =<br /> <br /> which were verified by a Philips APD-1700<br /> <br /> 0.8 K, firstly reported in 2007 [5].<br /> <br /> diffractometer<br /> <br /> in<br /> <br /> the<br /> <br /> orthorhombic<br /> <br /> (WZI)<br /> <br /> of<br /> <br /> using<br /> <br /> the<br /> <br /> University<br /> <br /> Cu-Kα<br /> <br /> of<br /> <br /> radiation,<br /> <br /> In this paper we review the basic thermal,<br /> <br /> confirmed the TiNiSi structure. The lattice<br /> <br /> magnetic and transport properties and muon<br /> <br /> constants extracted by means of a Rietveld<br /> <br /> Trang 22<br /> <br /> TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 14, SOÁ K2 - 2011<br /> refinement using X’pert HighScore Plus to be a<br /> <br /> Switzerland in the temperature range T = 0.02<br /> <br /> = 6.845 Å, b = 4.206 Å and c = 7.222 Å, in<br /> <br /> - 8 K.<br /> <br /> agreement with literature [11]. The phase<br /> <br /> 3. RESULT<br /> <br /> homogeneity of the annealed samples was<br /> <br /> 3.1.<br /> <br /> investigated by electron microprobe analysis on<br /> 1:1:1<br /> <br /> composition<br /> <br /> and<br /> <br /> all<br /> <br /> Itinerant<br /> <br /> Ferromagnetic<br /> <br /> Order<br /> <br /> JEOL JXA-8621 equipment. The matrix has<br /> the<br /> <br /> Weak<br /> <br /> Magnetization<br /> <br /> data<br /> <br /> taken<br /> <br /> on<br /> <br /> samples<br /> <br /> polycrystalline samples provide solid evidence<br /> <br /> contained a small amount (2%) of impurity<br /> <br /> that UCoGe is a weak itinerant ferromagnet,<br /> <br /> phases.<br /> <br /> see Fig. 1. The Curie temperature TC = 3 K is<br /> <br /> The dc-magnetization was measured for<br /> <br /> deduced from the temperature derivative of the<br /> <br /> temperatures T = 2 K and magnetic fields B = 5<br /> <br /> magnetization dM(T)/dT. A hysteresis loop that<br /> <br /> T in a Quantum Design SQUID magnetometer<br /> <br /> has an S-shape and exhibit visible remnant<br /> <br /> MPMS-XL [15]. Four-point low-frequency<br /> <br /> moments and coercive fields of 0.3 mT<br /> <br /> resistivity and ac-susceptibility data were<br /> <br /> measured at 2 K further corroborates FM order,<br /> <br /> obtained using a Linear Research AC Bridge<br /> <br /> see the inset of Fig. 1. The very small size of<br /> <br /> Resistance model LR700 operating at a<br /> <br /> the ordered moment of 0.03 µB is obtained from<br /> <br /> frequency of 16 HZ and low excitation currents<br /> <br /> a smooth extrapolation of the data to T → 0 K.<br /> <br /> 10 – 100 µA in the range T = 0.02 - 8 K.<br /> <br /> Consequently, the ratio, peff/Ms, of the Curie-<br /> <br /> Thermal expansion data were collected using a<br /> <br /> Weiss effective moment peff = 1.7 µB over the<br /> <br /> capacitance dilatometer for T = 0.23 - 8 K.<br /> <br /> saturation moment Ms is small, which classifies<br /> <br /> Here experiments were carried out in an<br /> <br /> UCoGe as a weak itinerant ferromagnet [16].<br /> <br /> Oxford Instruments HelioxVL<br /> <br /> 3<br /> <br /> He system<br /> <br /> The<br /> <br /> temperature<br /> <br /> dependence<br /> <br /> of<br /> <br /> the<br /> <br /> (Tbase = 250 mK) and an Oxford Instruments<br /> <br /> electrical resistivity of UCoGe samples is<br /> <br /> Kelvinox MX100 dilution refrigerator (Tbase =<br /> <br /> shown in Fig. 2. A broad hump around 3 K<br /> <br /> 20 mK).<br /> <br /> associated with the ferromagnetic transition is<br /> <br /> The specific heat was measured in a home-<br /> <br /> observed. In the FM phase (Ts < T < TC), the<br /> <br /> built set-up using a semi-adiabatic method<br /> <br /> resistivity obeys the relation ρ ~ T 2. In the<br /> <br /> employing a mechanical heat switch in a 3He<br /> <br /> temperature range above TC (TC < T < 3TC) the<br /> <br /> cryostat equipped with a 17 T superconducting<br /> <br /> resistivity is well described by a function ρ ~ T<br /> <br /> magnet in the temperature range T = 0.5 - 8 K.<br /> <br /> 5/3<br /> <br /> Zero-field (ZF) muon spin relaxation (µSR)<br /> <br /> resistivity of UCoGe (~ T<br /> <br /> +<br /> <br /> . The temperature dependence of the<br /> 2<br /> <br /> and T<br /> <br /> 5/3<br /> <br /> for T<br /> <br /> experiments were carried out using the µ SR-<br /> <br /> below<br /> <br /> dedicated beam line on the PSI-600MeV<br /> <br /> characteristic for a weak itinerant electron<br /> <br /> proton accelerator at the Swiss Muon Source of<br /> <br /> ferromagnet. The T<br /> <br /> the Paul Scherrer Institute (PSI) in Villigen,<br /> <br /> scattering at magnons, while for T > TC the T 5/3<br /> <br /> and<br /> <br /> above<br /> 2<br /> <br /> TC,<br /> <br /> respectively)<br /> <br /> is<br /> <br /> term below TC is due to<br /> <br /> Trang 23<br /> <br /> Science & Technology Development, Vol 14, No.K2- 2011<br /> term signals scattering at critical FM spin<br /> <br /> present in the whole sample volume. Moreover,<br /> <br /> fluctuations [17]. The resistivity data provide<br /> <br /> magnetic order persists in the superconducting<br /> <br /> further evidence that UCoGe is near the critical<br /> <br /> state. Interestingly, in the superconducting state<br /> <br /> boundary for magnetic long-range order.<br /> <br /> the precession frequency shows a small<br /> <br /> The thermodynamic signature of the<br /> ferromagnetic transition in the specific heat<br /> <br /> decrease of about 2%, indicating magnetism<br /> and superconductivity interact.<br /> <br /> measured on a polycrystalline sample is shown<br /> <br /> 0.03<br /> <br /> in Fig. 3. Here TC = 3 K is identified by the<br /> <br /> UCoGe<br /> <br /> inflection point in c/T at the high T side of the<br /> <br /> indicates UCoGe is a correlated metal, but the<br /> <br /> M (10-2µB/f.u)<br /> <br /> heat γ amounts to 0.057 J/molK2, which<br /> <br /> M (µB/f.u.)<br /> <br /> peak. The linear term in the electronic specific<br /> <br /> 2<br /> <br /> 0.02<br /> <br /> 0.01<br /> <br /> electron interactions are relatively weak. The<br /> <br /> 0<br /> <br /> -2<br /> -5<br /> <br /> magnetic entropy Smag involved in the magnetic<br /> 0.00<br /> <br /> T, is 0.3% of Rln2 (i.e. the value for a local<br /> <br /> 0<br /> <br /> 2<br /> <br /> 5<br /> <br /> µ0H (mT)<br /> <br /> B = 0.01T<br /> <br /> transition, obtained by integrating cmag/T versus<br /> <br /> T=2K<br /> 0<br /> <br /> 4<br /> <br /> 6<br /> <br /> 8<br /> <br /> 10<br /> <br /> T (K)<br /> <br /> moment S = 1/2 system). Such a small value is<br /> expected for a weak itinerant ferromagnet [18].<br /> <br /> Figure 1. Magnetization of UCoGe as a function of<br /> <br /> In order to investigate the weak itinerant<br /> <br /> temperature in a field B of 0.01 T as indicated. The<br /> <br /> ferromagnetism of UCoGe on a microscopic<br /> scale, the muon spin relaxation experiments<br /> <br /> dashed line is a smooth extrapolation of the data to 0<br /> K. Inset: Hysteresis loop measured at 2 K in the FM<br /> state.<br /> <br /> have carried out [19]. The temperature<br /> 200<br /> <br /> variation ν(T) is shown in Fig. 4 and tracks the<br /> <br /> UCoGe<br /> <br /> macroscopic magnetization M(T). For T ≤ TC,<br /> 150<br /> <br /> = ν0(1 – (T/T*)α)β with values of the<br /> spontaneous frequency ν0 = 1.98 MHz for T →<br /> 0, the critical temperature T* = 3.02 K ≈ TC, α<br /> = 2.3, and the critical value β = 0.4 which is<br /> close the theoretical value predicted for 3D<br /> Ising-like magnet. The frequency ν ≈ ν0 = 2<br /> MHz<br /> <br /> measured<br /> <br /> at<br /> <br /> low<br /> <br /> temperatures,<br /> <br /> ρ (µΩcm)<br /> <br /> the data are well described by the relation ν(T)<br /> <br /> TC<br /> <br /> ~T<br /> <br /> 100<br /> <br /> 5/3<br /> <br /> Ts<br /> 50<br /> <br /> ~T<br /> 0<br /> <br /> 0<br /> <br /> 2<br /> <br /> 2<br /> <br /> 4<br /> <br /> 6<br /> <br /> 8<br /> <br /> T (K)<br /> <br /> corresponds to an internal field Bi ~ 0.0148 T at<br /> <br /> Figure 2. Temperature dependence of the electrical<br /> <br /> the muon localization site. These data provide<br /> <br /> resistivity measured on polycrystalline UCoGe<br /> <br /> unambiguous proof for magnetic order being<br /> <br /> sample. Arrows indicate the Curie temperature TC<br /> <br /> Trang 24<br /> <br /> TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 14, SOÁ K2 - 2011<br /> and superconducting transition temperature Ts. The<br /> solid lines represent fits of the data to ρ ~ T and ~ T<br /> 2<br /> <br /> 5/3<br /> <br /> in the temperature ranges below and above TC,<br /> <br /> out at a low frequency of 16 Hz and in a small<br /> driving field of ~ 10-5 T. Fig. 5 shows the real<br /> <br /> respectively.<br /> <br /> part<br /> <br /> 80<br /> <br /> of<br /> <br /> the<br /> <br /> ac-susceptibility,<br /> <br /> χ′ac,<br /> <br /> of<br /> <br /> polycrystalline UCoGe as a function of<br /> temperature. The weak peak observed at 3 K<br /> <br /> 75<br /> <br /> reveals the ferromagnetic transition. Below 1<br /> <br /> 2<br /> <br /> c/T (mJ/molK )<br /> <br /> Ac-susceptibility measurements are carried<br /> <br /> 70<br /> <br /> K, χ′ac rapidly decreases to a large diamagnetic<br /> <br /> TC<br /> <br /> 65<br /> <br /> value, which reflects the superconducting<br /> transition. The onset transition temperatures<br /> <br /> 60<br /> <br /> Ts,onset is determined at 0.61 K. Tha result is<br /> <br /> 55<br /> <br /> good agreement with the resistivity data,<br /> <br /> UCoGe<br /> <br /> 50<br /> 0<br /> <br /> 2<br /> <br /> 4<br /> <br /> however, the ac-susceptibility χ′ac starts to drop<br /> <br /> 6<br /> <br /> 8<br /> <br /> T (K)<br /> <br /> when the resistive transition is complete. At the<br /> lowest temperature χ′ac reaches a value of 60%<br /> of the ideal screening value χs = -1/(1 - N)<br /> <br /> Figure 3. Temperature dependence of the specific<br /> <br /> (here N ≈ 0.08 is the demagnetizing factor of<br /> <br /> heat of UCoGe divided by temperature c/T in zero<br /> <br /> our samples). This indicates UCoGe is a type II<br /> <br /> field.<br /> <br /> SC which is always in the mixed phase.<br /> Because of the intrinsic FM moments the local<br /> field is nonzero and the magnitude of χ′ac is<br /> reduced.<br /> <br /> 1.5<br /> <br /> 0.0<br /> <br /> 1.0<br /> <br /> TS<br /> <br /> 0.5<br /> <br /> UCoGe<br /> 0.0<br /> 0.0<br /> <br /> 0.5<br /> <br /> 1.0<br /> <br /> -5<br /> <br /> B = 10 T<br /> <br /> TC<br /> <br /> χ′ac (SI units)<br /> <br /> Frequency (MHz)<br /> <br /> 2.0<br /> <br /> -0.5<br /> <br /> 1.5<br /> <br /> 2.0<br /> <br /> 2.5<br /> <br /> 3.0<br /> <br /> 3.5<br /> <br /> T (K)<br /> Figure 4. Temperature dependence of the muon<br /> <br /> precession frequency ν(T) of the polycrystalline<br /> <br /> UCoGe<br /> -1.0<br /> <br /> 0<br /> <br /> 1<br /> <br /> 2<br /> <br /> 3<br /> <br /> 4<br /> <br /> 5<br /> <br /> 6<br /> <br /> T (K)<br /> <br /> sample UCoGe in zero magnetic field.<br /> <br /> 3.2. Unconventional Superconductivity<br /> <br /> Figure 5. Temperature dependence of the real part<br /> <br /> of the ac-susceptibility χ'ac in polycrystalline<br /> UCoGe. Arrows indicate TC and Ts.<br /> <br /> Trang 25<br /> <br />
ADSENSE
ADSENSE

CÓ THỂ BẠN MUỐN DOWNLOAD

 

Đồng bộ tài khoản
2=>2