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Register a new userHole-doping driven antiparallel magnetic order underlying the superconducting and pseudogap states in high-temperature cuprate superconductor
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Authors
Takashi Uchino
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Unveiling the nature of the pseudogap and its relation to both superconductivity and antiferromagnetic Mott insulators, the pairing mechanism, and a non-Fermi liquid phase is a key issue for understanding high temperature superconductivity in cuprates.We here show that antiparallel magnetic order can be reasonably and naturally predicted in hole-doped CuO2 planes by starting from the ground state of a weakly doped antiferromagnetic insulator, where a Skyrmion-type three-dimensional spin texture is created around the doped hole. The superconducting transition temperature Tc can be understood in terms of the temperature at which long-range antiparallel magnetic ordering is established, resulting in the magnetically mediated superconducting state with phase-coherent Cooper pairs. Upon heating above Tc, long-range phase coherence in the pair state is lost, but the pair condensate still survives on the medium-range length scale, transforming to the pseudogap state with charge and magnetic orders. We believe the present model provides a key initial point to unravel a wide variety of the apparently complex phenomena related to high temperature cuprate superconductors.
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We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors: YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm-1 followed by a featureless continuum between 500 and 1500 cm-1 in the conductivity data and a significant suppression in the scattering rate below 700 - 900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap $Delta_{PG}$. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100 - 200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below $Delta_{PG}$ is solely due to the pseudogap formation whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.
The cuprate high-temperature superconductors (HTSC) have been the subject of intense study for more than 30 years with no consensus yet on the underlying mechanism of the superconductivity. Conventional wisdom dictates that the mysterious and extraordinary properties of the cuprates arise from doping a strongly correlated antiferromagnetic (AFM) insulator (1,2). The highly overdoped cuprates$-$those beyond the dome of superconductivity (SC)--are considered to be conventional Fermi liquid metals (3). Here, we report the emergence of itinerant ferromagnetic order (FM) below 4K for doping beyond the SC dome in electron-doped La$_{2-x} $Ce$_x$CuO$_4$ (LCCO). The existence of this FM order is evidenced by negative, anisotopic and hysteretic magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of which are standard signatures of itinerant FM in metals (4,5). This surprising new result suggests that the overdoped cuprates are also influenced by electron correlations and the physics is much richer than that of a conventional Fermi liquid metal.
The specific heat $C$ of the single-layer cuprate superconductor HgBa$_2$CuO$_{4 + delta}$ was measured in an underdoped crystal with $T_{rm c} = 72$ K at temperatures down to $2$ K in magnetic fields up to $35$ T, a field large enough to suppress superconductivity at that doping ($p simeq 0.09$). In the normal state at $H = 35$ T, a residual linear term of magnitude $gamma = 12 pm 2$ mJ/K$^2$mol is observed in $C/T$ as $T to 0$, a direct measure of the electronic density of states. This high value of $gamma$ has two major implications. First, it is significantly larger than the value measured in overdoped cuprates outside the pseudogap phase ($p >p^star$), such as La$_{2-x}$Sr$_x$CuO$_4$ and Tl$_2$Ba$_2$CuO$_{6 + delta}$ at $p simeq 0.3$, where $gamma simeq 7$ mJ/K$^2$mol. Given that the pseudogap causes a loss of density of states, and assuming that HgBa$_2$CuO$_{4 + delta}$ has the same $gamma$ value as other cuprates at $p simeq 0.3$, this implies that $gamma$ in HgBa$_2$CuO$_{4 + delta}$ must peak between $p simeq 0.09$ and $p simeq 0.3$, namely at (or near) the critical doping $p^star$ where the pseudogap phase is expected to end ($p^starsimeq 0.2$). Secondly, the high $gamma$ value implies that the Fermi surface must consist of more than the single electron-like pocket detected by quantum oscillations in HgBa$_2$CuO$_{4 + delta}$ at $p simeq 0.09$, whose effective mass $m^star= 2.7times m_0$ yields only $gamma = 4.0$ mJ/K$^2$mol. This missing mass imposes a revision of the current scenario for how pseudogap and charge order respectively transform and reconstruct the Fermi surface of cuprates.
The elucidation of the pseudogap phenomenon of the cuprates, a set of anomalous physical properties below the characteristic temperature T* and above the superconducting transition temperature Tc, has been a major challenge in condensed matter physics for the past two decades. Following initial indications of broken time-reversal symmetry in photoemission experiments, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T*. These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point. Here we report inelastic neutron scattering results for HgBa2CuO4+x (Hg1201) that reveal a fundamental collective magnetic mode associated with the unusual order, and that further support this picture. The modes intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter. Its energy of 52-56 meV and its enormous integrated spectral weight render it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies.
Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, voluminous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.
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