Cd$_3$As$_2$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying Cd$_3$As$_2$ with topologically trivial Zn$_3$As$_2$. We observe the expected topological phase transition around a Zn concentration of $x\sim 1$ while the carrier density monotonically decreases as $x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase of the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow to infer that the enhanced thermoelectric performance is due to a flattening of the band structure in the higher-$x$ region in Cd$_{3-x}$Zn$_x$As$_2$.
We found that a high mobility semimetal 1T'-MoTe2 shows a significant pressure-dependent change in the cryogenic thermopower in the vicinity of the critical pressure, where the polar structural transition disappears. With the application of a high pressure of 0.75 GPa, while the resistivity becomes as low as 10 μΩcm, thermopower reached the maximum value of 60 μVK-1 at 25 K, leading to a giant thermoelectric power factor of 300 μWK-2cm-1. Based on semiquantitative analyses, the origin of this behavior is discussed in terms of inelastic electron-phonon scattering enhanced by the softening of zo…
Cr1+dTe2 is a self-intercalated transition metal dichalcogenide that hosts tunable electronic filling and magnetism in its semimetallic band structure. Recent angle-resolved photoemission spectroscopy (ARPES) studies have unveiled a systematic shift in this semimetallic band structure relative to the chemical potential with increased Cr doping. This report presents the temperature and magnetic field dependence of the longitudinal thermopower Sxx for different Cr1+dTe2 compositions. We show that as doping increases, the sign of Sxx changes from positive to negative at the critical doping level …
Optimal doping and entropic origin of giant thermopower in doped Mott insulators
这篇论文已被作者撤回，因为结果是不正确的，由于使用的杂质求解器，在大的相互作用强度和以上的半填充失败。这首先导致了新的杂质求解器的开发（arXiv：1202.5814或Phys.Rev. B 86，085133（2012）），其次导致了具有不同结论的热电势结果的完全修订。因此，提交了一篇关于热电的新论文（arXiv：1209.4349），并撤回了目前的论文。
Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe$_{\bf 2}$
具有自发电极化的强度的铁电体在当今的器件工程中发挥着重要作用，例如电容器和存储器。它们的物理性质通过抑制长程极性顺序而进一步丰富，例如在低温下具有巨大压电和介电响应的量子顺电体。同样，在金属中，理论上预测极性晶格畸变会产生各种不寻常的物理性质。然而，到目前为止，在金属中的“铁电”样的转变很少被控制，因此它对传输现象的可能影响仍然未被探索。本文报道了在1 T '-Mo $_{1-x}$ Nb $_{x}$ Te $_2$中发现的极性可调的金属相和非极性金属相之间临界区附近的热电势异常增强现象。第一性原理计算和磁输运测量结果表明，电荷输运过程中散射率与能量密切相关，临界地向非极性相的边界演化，导致低温热电势很大.这种结构不稳定性对输运现象的显著影响可能来自波动或异质极性金属态，这将为提高热电效率铺平一条新的途径。
The thermoelectric effect is the generation of an electrical voltage from a temperature gradient in a solid material due to the diffusion of free charge carriers from hot to cold. Identifying materials with large thermoelectric response is crucial for the development of novel electric generators and coolers. In this paper we consider theoretically the thermopower of Dirac/Weyl semimetals subjected to a quantizing magnetic field. We contrast their thermoelectric properties with those of traditional heavily-doped semiconductors and we show that, under a sufficiently large magnetic field, the the…
We study possible Weyl semimetals of strongly-correlated electrons by investigating magnetotransport properties in pyrochlore R2Ir2O7 (R=rare-earth ions), choosing three types of R ions to design the exchange coupling scheme between R 4f and Ir 5d moments; non-magnetic Eu (4f6), isotropic Gd (4f7), and anisotropic Tb (4f8). In the doping-induced semimetallic state, distinctive features of magnetoresistance and Hall effect are observed in R=Gd and Tb compounds due to the effects of the exchange-enhanced isotropic and anisotropic Zeeman fields, respectively, exemplifying the double Weyl semimeta…
Perovskite-type iridates SrIrO$_{3}$ and CaIrO$_{3}$ are a Dirac line node semimetal protected by crystalline symmetry, providing an interesting playground to investigate electron correlation effects on topological semimetals. The effect of Sn doping was examined by growing SrIr$_{1-x}$Sn$_{x}$O$_{3}$ and CaIr$_{1-x}$Sn$_{x}$O$_{3}$ thin films epitaxially on SrTiO$_{3}$(001) substrate using pulsed laser deposition. Upon Sn doping, the semimetallic ground state switches into an insulator. As temperature is lowered, the resistivity, $ρ(T)$, of SrIr$_{1-x}$Sn$_{x}$O$_{3}$ above a critical doping …
Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal {\Ksub} confirm the three dimensional (3D) C-type antiferromagnetic (AFM) ordering of the Mn$^{2+}$ moments at $T_{\rm N}=305 \pm 3$ K which is slightly higher than that of the parent SrMnSb$_2$ with $T_{\rm N}=297 \pm 3$ K. Susceptibility measurements of the K-doped and parent crystals above $T_{\rm N}$ are characteristic of 2D AFM systems. This is consistent with high temperature neutron diffraction of the parent compound that display persisting 2D AFM correlations well above $T_{\rm N}$ to at least $\sim …
The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion (DF) system at the quantum limit is discussed to verify the prediction of high-performance thermopower in an organic conductor α-(BEDT-TTF)2I3. Because of relatively large Zeeman splitting in α-(BEDT-TTF)2I3, the boundless increase of thermopower at high magnetic fields, predicted without the Zeeman effect, is hardly expected, whereas there appears to be a broad local maximum. This is characteristic of 2D DF systems with Zeeman splitting and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl semimeta…
Upon chemical substitution of oxygen with fluor, LaMnAsO has been electron-doped in experiments, resulting in samples with remarkably high Seebeck coefficients of around -300$μVK^{-1}$ at room temperature and 3% doping. Within the framework of density functional theory plus dynamical mean-field theory we are not only able to reproduce these experimental observations, but can also provide a thorough investigation of the underlying mechanisms. By considering electronic correlations in the half-filled Mn-3d shells, we trace the high Seebeck coefficient back to an asymmetry in the spectral functio…
We present a study of Cu-substitution effects in 4f-Ce intermetallic compound CeAu1-xCuxGe, with potentially unusual electronic states, in the whole concentration range (x = 0.0 - 1.0). The parent CeAuGe compound, crystallizing in a non-centrosymmetric hexagonal structure, is a ferromagnetic semimetal with Curie temperature 10 K. Cu-doping on Au-site of CeAuGe, CeAu1-xCuxGe, changes the crystal structure from the non-centrosymmetric (P63mc) to centrosymmetric (P63/mmc) space group at the concentration x ~ 0.5, where the c-lattice constant has a maximum value. Magnetic susceptibility and electr…
We present a study on magnetotransport in films of the topological Dirac semimetal Cd$_{3}$As$_{2}$ doped with Sb grown by molecular beam epitaxy. In our weak antilocalization analysis, we find a significant enhancement of the spin-orbit scattering rate, indicating that Sb doping leads to a strong increase of the pristine band-inversion energy. We discuss possible origins of this large enhancement by comparing Sb-doped Cd$_{3}$As$_{2}$ with other compound semiconductors. Sb-doped Cd$_{3}$As$_{2}$ will be a suitable system for further investigations and functionalization of topological Dirac se…
Contrasting electron and hole doping effects on the spin gap of the caged type Kondo semimetal CeOs$_2$Al$_{10}$: A muon spin relaxation and inelastic neutron scattering investigation
用磁化强度、μ子自旋弛豫（$μ^+$ SR）和非弹性中子散射研究了Ce（Os $_{1-x}$ Ir $_x$）$_2$ Al $_{10}$（$x$ = 0.08和0.15）和CeOs $_{1.94}$ Re $_{0.06}$ Al $_{10}$多晶样品中电子（Ir）和空穴（Re）掺杂对杂化能隙和反铁磁有序的影响. $μ^+$ SR谱清楚地揭示了磁有序低于20和10 K的$x$ = 0.08和0.15样品分别与一个非常弱的签名的μ子初始不对称性的振荡在很短的时间尺度。我们的重要发现是，少量的电子掺杂（i）完全抑制了非弹性磁激发附近11 meV下降到2K，这是在未掺杂的化合物中观察到的，并转换为一个广泛的准弹性响应和（ii）在相应的μ子网站的内场是显着增强了约10倍，与母体化合物相比。另一方面，当空穴（3% Re）掺杂量较小时，11 meV附近的非弹性磁激发强度显著降低。观察到的掺杂效应的主要来源是与Os原子相比，Ir携带的额外5个$d$电子和Re携带的空穴。所获得的结果表明，载流子掺杂的灵敏度很高，并提供了额外的方法来研究它们的异常磁性。
The Seebeck coefficient (thermopower) $S$ of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ was measured across its doping phase diagram (from $p = 0.12$ to $p = 0.25$), at various temperatures down to $T \simeq 2$ K, in the normal state accessed by suppressing superconductivity with a magnetic field up to $H = 37.5$ T. The magnitude of $S/T$ in the $T=0$ limit is found to suddenly increase, by a factor $\simeq 5$, when the doping is reduced below $p^{\star} = 0.23$, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large …
We have measured the electrical resistivity and the thermopower of the electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$ = Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically suppressed. We find that, despite the electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly dis…
ThTaN$_3$, a rare cubic perovskite nitride semiconductor, has been studied using {\it ab initio} methods. Spin-orbit coupling (SOC) results in band inversion and a band gap of 150 meV at the zone center. In spite of the trivial $Z_2$ indices, two pairs of spin-polarized surface bands cross the gap near the zone center, indicating that this system is a topological crystalline insulator with the mirror Chern number of $|{\cal C}_m|=2$ protected by the mirror and $C_4$ rotational symmetries. Additionally, SOC doubles the Seebeck coefficient, leading to a maximum of $\sim$400 $μ$V/K at 150 K for c…
We calculate the optical conductivity of type-I and type-II multi-Weyl semimetals(m-WSMs) in time-reversal(TR) broken case at finite doping. It is found that both longitudinal and Hall conductivities are renormalized by J(monopole charge) perpendicular to Weyl nodes separation and longitudal conductivity along nodes has non-trivial dependence on J. The Kerr rotations in thin films and bulk have different dependence on system parameters. The Kerr rotation in ultra-thin films is independent of monopole charge J and depends only on Fermi velocity v F while in bulk it dependent on monopole charge …
The electronic band structure of correlated Ca3Ru2O7 featuring an antiferromagnetic as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of Ca3Ru2O7 such as the bulk spin valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved even though a FS consisting of electron pockets was found experimentally. Here we report magneto electrical transport and thermoelectric measurements with the electric current and temper- ature gradient dir…
Weyl semimetal (WSM) feature tilted Dirac cones and can be type I or II depending on the magnitude of the tilt parameter ($C$). The boundary between the two types is at $C=1$ where the cones are tipped and there is a Lifshitz transition. The topological charge of a WSM is one. In multi-Weyl it can be two or more depending on the value of the winding number $J$. We calculate the absorptive part of the AC optical conductivity both along the tilt direction ($σ_{zz}$) and perpendicular to it ($σ_{xx}$) as a function of the tilt ($C$) and chemical potential ($μ$). For zero tilt there is a discontin…
We provide a review of recent experimental results on the chiral anomaly in Dirac/Weyl semimetals. After a brief introduction, we trace the steps leading to the prediction of materials that feature protected 3D bulk Dirac nodes. The chiral anomaly is presented in terms of charge pumping between the chiral Landau levels of Weyl fermions in parallel electric and magnetic fields. The related chiral magnetic effect and chiral zero sound are described. Current jetting effects, which present major complications in experiments on the longitudinal magnetoresistance, are carefully analyzed. We describe…
Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta$_2$NiSe$_5$ using $in$-$situ$ potassium deposition. Combining with ang…
Dirac/Weyl semimetals hosting linearly-dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder effects from the substituted ions. Here it is shown that the Fermi energy is tunable across the Dirac point for layered magnet EuMnBi$_2$ by partially substituting Gd$^{3+}$ for Eu$^{2+}$ in the insula…
We report on the evolution of the average and depth-dependent magnetic order in thin film samples of biaxially stressed and electron-doped EuTiO$_3$ for samples across a doping range $<$0.1 to 7.8 $\times 10^{20}$ cm$^{-3}$. Under an applied in-plane magnetic field, the G-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the dopin…
Due to strong spin-orbit coupling, charge carriers in three-dimensional quadratic band touching Luttinger semimetals have non-trivial wavefunctions characterized by a pseudospin of 3/2. We compute the dielectric permittivity of such semimetals at finite doping, within the random phase approximation. Because of interband coupling, the dielectric screening shows a reduced plasma frequency and an increased spectral weight at small wavevectors, compared to a regular quadratic band. This weakens the effective Coulomb potential, modifying the single-particle self-energy for which we present both ana…
We investigate the superconductivity of 3D Luttinger semimetals, such as YPtBi, where Cooper pairs are constituted of spin-3/2 quasiparticles. Various pairing mechanisms have already been considered for these semimetals, such as from polar phonons modes, and in this work we explore pairing from the screened electron-electron Coulomb repulsion. In these materials, the small Fermi energy and the spin-orbit coupling strongly influence how charge fluctuations can mediate pairing. We find the superconducting critical temperature as a function of doping for an s-wave order parameter, and determine i…
Signatures of quantum criticality in hole-doped and chemically pressurized EuFe_2As_2 single crystals
通过测量热电势$S(T)$和电阻率$ρ(T)$，研究了$_x$ Eu $_{1-x}$ Fe $_2$ As $_2$和EuFe $_2$（As $_{1-y}$ P $_y$）$_2$单晶中空穴掺杂和化学压力（等价掺杂）的效应.掺杂后$S(T)$的演变表明，在临界值$x_{\mathrm{cr}}=0.3$和$y_{\mathrm{cr}}=0.21$，分别电子配置的急剧变化，自旋密度波跃迁被完全抑制和超导性（SC）出现。对于化学压力的情况下，与已发表的ARPES测量结果的比较表明，在$y_{cr}$的Lifshitz转变。温度依赖性$S(T)/T\propto \log T$和$Δρ\propto T$观察到的正常状态以上的SC过渡建议在这两个系统中的量子临界。
We compute the effects of electronic interactions on gapless spin-3/2 excitations that in a noninteracting system emerge at a bi-quadratic touching of Kramers degenerate valence and conduction bands, known as Luttinger semimetal. This model can describe the low-energy physics of HgTe, gray-Sn, 227 pyrochlore iridates and half-Heuslers. For the sake of concreteness we only consider the short-range components of the Coulomb interaction. By combining mean-field analysis with a renormalization group (RG) calculation, we construct multiple cuts of the global phase diagram of interacting spin-3/2 fe…
We study superconductivity driven by screened Coulomb repulsion in three-dimensional Luttinger semimetals with a quadratic band touching and strong spin-orbit coupling. In these semimetals, the Cooper pairs are formed by spin-3/2 fermions with non-trivial wavefunctions. We numerically solve the linear Eliashberg equation to obtain the critical temperature of a singlet s-wave gap function as a function of doping, with account of spin-orbit and self-energy corrections. In order to understand the underlying mechanism of superconductivity, we compute the sensitivity of the critical temperature to …
Weak Topological Insulators and Composite Weyl Semimetals: $β$-Bi$_{4}$X$_{4}$ (X=Br, I)
虽然强拓扑绝缘体（STI）在理论预测后不久就在实验上实现了，但弱拓扑绝缘体（WTI）尚未得到明确的证实。一个主要的障碍是缺乏明显的天然卵裂表面来测试WTI的表面选择性标志。通过一个新的方案，我们发现Bi $_{4}$ X $_{4}$（X=Br，I），无论是稳定的还是以前合成的，都可以是具有两个自然解理面的WTI，其中两个各向异性的Dirac锥分别稳定和湮灭.我们进一步发现电荷掺杂下的四个表面态Lifshitz转变和单轴应变下的两个体拓扑相变。在WTI-STI转变附近，出现了一种新的外尔半金属相，即费米弧一般出现在两个解理面上，而费米圆只出现在一个选定的解理面上。
Time-reversal invariant (TRI) Dirac and Weyl semimetals in three dimensions (3D) can host open Fermi arcs and spin-momentum locking Fermi loops on the surfaces. We find that when they become superconducting with $s$-wave pairing and the doping is lower than a critical level, straight $π$-flux vortex lines terminating at surfaces with Fermi arcs or spin-momentum locking Fermi loops can realize 1D topological superconductivity and harbor Majorana zero modes at their ends. Remarkably, we find that the vortex-generation-associated Zeeman field can open (when the surfaces have only Fermi arcs) or e…
