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Register a new userThree interaction energy scales in single-layer high-T$_C$ cuprate HgBa$_2$CuO$_{4+delta}$
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The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures (T$_C$) and, after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa$_2$CuO$_{4+delta}$ (Hg1201) is a single-CuO$_2$-layer cuprate with a particularly high optimal T$_C$ and a simple crystal structure; yet there exists little information from ARPES about the electronic properties of this model system. Here we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales -a sub-gap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping-dependence in several single-layer cuprates, and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material, and in terms of connecting comparable spectral features in multiple structurally similar cuprates.
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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.
We present an inelastic neutron scattering study of the structurally simple single-layer compound HgBa$_2$CuO$_{4+delta}$ close to optimal doping ($T_c approx 96$ K). A well-defined antiferromagnetic resonance with energy $omega_r = 56$ meV ($approx 6.8 k_BT_c$) is observed below the superconducting transition temperature $T_c$. The resonance mode is energy-resolution limited and exhibits an intrinsic momentum width of about $0.2 mathrm{mathring{A}^{-1}}$, consistent with prior work on several other cuprates. However, the unusually large value of the mode energy implies a non-universal relationship between $omega_r$ and $T_c$ across different families of cuprates.
Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa$_2$CuO$_{4+delta}$. Here we report neutron scattering results for HgBa$_2$CuO$_{4+delta}$ (superconducting transition temperature T$_c$ $sim$ 71 K, pseudogap temperature T* $sim$ 305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped hourglass response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped, and significantly enhanced below T*, and hence a prominent signature of the pseudogap state.
The charge-density-wave (CDW) instability in the underdoped, pseudogap part of the cuprate phase diagram has been a major recent research focus, yet measurements of dynamic, energy-resolved CDW correlations are still in their infancy. We report a high-resolution resonant inelastic X-ray scattering (RIXS) study of the underdoped cuprate superconductor HgBa$_{2}$CuO$_{4+delta}$ ($T_c = 70$ K). At $T=250$ K, above the CDW order temperature $T_mathrm{CDW} approx 200$ K, we observe significant dynamic CDW correlations at about 40 meV. This energy scale is comparable to both the superconducting gap and the previously reported low-energy pseudogap. At $T = T_c$, a strong elastic CDW peak appears, but the dynamic correlations around 40 meV remain virtually unchanged. In addition, we observe a new feature: dynamic correlations at significantly higher energy, with a characteristic scale of about 160 meV. A similar scale was previously identified in other experiments as a high-energy pseudogap. The existence of three distinct features in the charge response is highly unusual for a CDW system, and suggests that charge order in the cuprates is closely related to the pseudogap phenomenon and more complex than previously thought. We further observe the paramagnon dispersion along [1,0], across the two-dimensional CDW wavevector $boldsymbol{q}_mathrm{CDW}$, which is consistent with magnetic excitations measured by inelastic neutron scattering. Unlike for some other cuprates, our results point to the absence of a discernible coupling between CDW and magnetic excitations.
The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. The tetragonal cuprate HgBa$_2$CuO$_{4+delta}$, with only one CuO$_2$ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility within the CuO$_2$ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy sets in below the pseudogap temperature $T^*$, which provides thermodynamic evidence for a nematic phase transition with broken four-fold symmetry. Most surprisingly, the nematic director orients along the diagonal direction of the CuO$_2$ square lattice, in sharp contrast to the bond nematicity reported in various iron-based superconductors and double-layer YBa$_2$Cu$_3$O$_{6+delta}$, where the anisotropy axis is along the Fe-Fe and Cu-O-Cu directions, respectively. Another remarkable feature is that the enhancement of the diagonal nematicity with decreasing temperature is suppressed around the temperature at which short-range charge-density-wave (CDW) formation occurs. This is in stark contrast to YBa$_2$Cu$_3$O$_{6+delta}$, where the bond nematicity is not influenced by the CDW. Our result suggests a competing relationship between diagonal nematic and CDW order in HgBa$_2$CuO$_{4+delta}$.
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