Korringa Law Research Articles

Microscopic properties of Mo4Ga20Sb intermetallic superconductor in normal and superconducting states as evidenced by NMR and NQR spectroscopy

A detailed study of novel intermetallic superconductor Mo4Ga20Sb (Tc = 6.6 K) by means of 69,71Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy was performed to gain insight into the superconducting pairing mechanism and gap characteristics, as well as into the normal-state electronic properties on a microscopic scale. An unexpected step-like increase of the 69Ga1 NQR linewidth was observed at Tc with decreasing temperature, which may indicate coexistence of several superconducting phases on a microscopic scale with slightly different Tc values. Above 140 K, the NQR linewidth decreases rapidly with increasing temperature due to possible oscillatory and/or rotational modes of the gallium cluster network at the elevated temperatures. Nuclear spin lattice relaxation rate 1/T1 of 69Ga nuclei in the Ga1 position follows Korringa law characteristic for the compounds with good metallic conductivity. The calculated Korringa ratio S< 1 indicates the presence of antiferromagnetic correlations in Mo4Ga20Sb, in agreement with the magnetic susceptibility data. Below Tc, a pronounced intensive Hebel-Slichter peak was observed indicating s-wave superconductivity without point or line nodes in the k-space (full gap s-wave superconductivity). We have demonstrated that the best fit of the experimental data is achieved using two s-wave superconducting gaps of 13 K and 6 K with relative weights of 0.8 and 0.2, respectively. The obtained weighted average value of 11.6 K is consistent with the specific heat value of Δ = 12.05 K.

Investigations of Magnetic Properties Hg0.865Mn0.135Te by the Electron Spin Resonance Method

Mercury chalcogenide (Hg 0.865 Mn 0.135 Te) monocrystal was grown by crystallization from a two-phase mixture with replenishment of the melt from a tellurium solution and has n-type conductivity. The electron spin resonance (ESR) spectra of a single crystal were studied before and after X-irradiation. Only one intense anisotropic ESR line was observed at 4.2 K before X-irradiation. The anisotropy parameters were established from the angular dependence of the resonance field of the ESR line. After X-irradiation, three ESR lines were registered in the spectrum. The first and second lines with a large g -factor and unusual temperature dependencies were associated with charge carriers localized on defects with the strong exchange interaction with the manganese ion spins. The third ESR narrowed line was identified as signal from Mn 2+ ions. Estimated anisotropic exchange parameters from angular dependencies of magnetic resonance field depend on temperature. The temperature dependencies of ESR linewidths are well described by the Korringa law and spin-lattice relaxation contributions. The discontinuities in temperature dependencies of resonance field and linewidth at 18 and 65 K were observed.

Phenomenology of 63Cu Nuclear Relaxation in Cuprate Superconductors

Nuclear relaxation is an important thermodynamic probe of electronic excitations, in particular in conducting and superconducting systems. Here, an empirical phenomenology based on all available literature data for planar Cu in hole-doped cuprates is developed. It is found that most of the seemingly different relaxation rates among the systems are due to a temperature-independent anisotropy that affects mostly measured 1/T1∥, the rate with an external magnetic field along the crystal c-axis, while 1/T1⊥ is largely independent on doping and material above the critical temperature of superconductivity (Tc). This includes very strongly overdoped systems that show Fermi liquid behavior and obey the Korringa law. Below Tc, the relaxation rates are similar, as well, if plotted against the reduced temperature T/Tc. Thus, planar Cu nuclear relaxation is governed by a simple, dominant mechanism that couples the nuclei with varying anisotropy to a rather ubiquitous bath of electronic excitations that appear Fermi liquid-like irrespective of doping and family. In particular, there is no significant enhancement of the relaxation due to electronic spin fluctuations, different from earlier conclusions. Only the La2−xSrxCuO4 family appears to be an outlier as additional relaxation is present; however, the anisotropy remains temperature independent. Also systems with very low doping levels, for which there is a lack of data, may behave differently.

Properties of the Electronic Fluid of Superconducting Cuprates from 63Cu NMR Shift and Relaxation

Nuclear magnetic resonance (NMR) provides local, bulk information about the electronic properties of materials, and it has been influential for theories of high-temperature superconducting cuprates. NMR reported early that nuclear relaxation is much faster than what one expects from coupling to fermionic excitations above the critical temperature for superconductivity (Tc), i.e., what one estimates from the Knight shift with the Korringa law. As a consequence, special electronic spin fluctuations have been invoked. Here, based on literature relaxation data, it is shown that the electronic excitations, to which the nuclei couple with a material and doping-dependent anisotropy, are rather ubiquitous and Fermi liquid-like, i.e., they are only affected by Tc not the pseudogap. A suppressed NMR spin shift rather than an enhanced relaxation leads to the failure of the Korringa law for most materials. Shift and relaxation below Tc support the view of suppressed shifts, as well. A simple model of two coupled electronic spin components, one with 3d(x2 − y2) orbital symmetry and the other with an isotropic s-like interaction, can explain the data. The coupling between the two components is found to be negative, and it must be related to the pseudogap behavior of the cuprates. We can also explain the negative shift conundrum and the long-standing orbital shift discrepancy for NMR in the cuprates.

Microscopic investigation of local structural and electronic properties of tungsten tetraboride: a superhard metallic material

Tungsten borides, such as tungsten tetraboride ( WB4) exhibit a wide range of appealing physical properties, including superhardness, chemical inertness and electronic conductivity. Among the various tungsten borides, the most puzzling remains WB4, with its crystal structure to linger in question for over half a century ( Lech et al. in Proc Natl Acad Sci USA 112: 3223- 3228, 2015). In the present investigation, polycrystalline WB4 samples have been synthesized with two different methods and characterized at the atomic level by combining X- ray diffraction, scanning electron microscopy and nuclear magnetic resonance spectroscopy. The 11 B multiple quantum MAS experiment revealed a range of boron sites that were not resolved within the experiment. This result is in contrast to the 11 B MAS spectrum of WB2 with four resolved, discernible boron resonances. However, despite the structural complexity and boron- site variety in WB4, the detection of a single exponential of 11 B spin- lattice relaxation recovery suggested that all of the boron sites relaxed with a single time constant. The Knight shift ( K) was found to be independent of temperature while the T 1 1 was governed by the Korringa law with a Korringa product T1T = 350 sK across the entire temperature range ( 168- 437 K) of this study. The measured Korringa product was small, indicating substantial spin- lattice relaxation resulting from coupling with the conduction carriers. The abovementioned experimental results not only clearly rule out structures, such as the `` MoB4- type phase'' of WB4, with the resulting Fermi level in the pseudo- gap as has previously been predicted theoretically; but they also provide a comprehensible and valuable insight into the structural and electronic properties of WB4 at the atomic level.

Exploring the anisotropic Kondo model in and out of equilibrium with alkaline-earth atoms

We propose a scheme to realize the Kondo model with tunable anisotropy using alkaline-earth atoms in an optical lattice. The new feature of our setup is Floquet engineering of interactions using time-dependent Zeeman shifts, that can be realized either using state-dependent optical Stark shifts or magnetic fields. The properties of the resulting Kondo model strongly depend on the anisotropy of the ferromagnetic interactions. In particular, easy-plane couplings give rise to Kondo singlet formation even though microscopic interactions are all ferromagnetic. We discuss both equilibrium and dynamical properties of the system that can be measured with ultracold atoms, including the impurity spin susceptibility, the impurity spin relaxation rate, as well as the equilibrium and dynamical spin correlations between the impurity and the ferromagnetic bath atoms. We analyze the non-equilibrium time evolution of the system using a variational non-Gaussian approach, which allows us to explore coherent dynamics over both short and long timescales, as set by the bandwidth and the Kondo singlet formation, respectively. In the quench-type experiments, when the Kondo interaction is suddenly switched on, we find that real-time dynamics shows crossovers reminiscent of poor man's renormalization group flow used to describe equilibrium systems. For bare easy-plane ferromagnetic couplings, this allows us to follow the formation of the Kondo screening cloud as the dynamics crosses over from ferromagnetic to antiferromagnetic behavior. On the other side of the phase diagram, our scheme makes it possible to measure quantum corrections to the well-known Korringa law describing the temperature dependence of the impurity spin relaxation rate. Theoretical results discussed in our paper can be measured using currently available experimental techniques.

Resistively Detected Nuclear Magnetic Resonance in Gated Insb QW at Elevated Temperature for Quantum Information Processing

M. M. Uddin,1,2* H. W. Liu,3,4 K. F. Yang,3 T. D. Mishima,5 M. B. Santos,5 K. Nagase,3 and Y. Hirayama1,3,6 1 Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan 2 Department of Physics, Chittagong University of Engineering & Technology, Chittagong-4349, Bangladesh 3 ERATO Nuclear Spin Electronics Project, Sendai, Miyagi 980-8578, Japan 4 State Key Laboratory of Superhard Materials and Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, People’s Republic of China 5 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks, Norman, Oklahoma 73019-2061, USA 6 WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan *E-mail: mohi_cuet@yahoo.com Narrow band-gap semiconductor InSb has long been considered as potentially exciting materials for next-generation high-speed electronics, quantum devices and spintronics applications due to its high room-temperature mobility (77000 cm2/Vs), significant ballistic transport length, large Lande g factor, high nuclear spin (In=9/2, 121Sb I = 5/2, 123Sb I = 7/2) and high spin-orbit coupling. Nuclear spins in low-dimensional semiconductor structures have attracted growing interest because the long spin coherence time of nuclei facilitates the implementation of quantum information processing (QIP). The quadrupole splitting of 115In have been reported in a single InSb two dimensional electron gas (2DEG) that are of practical importance for the coherent control of the ten nuclear-spin quantum levels and for the implementation of multiple NMR-based quantum bits (≥2) [1, 2]. The success of gate controlled InSb quantum wells [3, 4] allows us to perform pump and probe technique to investigate temperature dependent nuclear spin relaxation time T 1 at different filling factors (ν ). Collective spin excitations in the domain walls enhance dynamic nuclear polarization in the quantum Hall ferromagnet around 2 characterized by a short (~ 60 sec) and temperature-independent T 1. On the other hand, relatively long (~ 400 sec) and temperature dependent T 1with following Korringa law have been demonstrated in the absence of the domains as well as collective spin texture at around 3. In addition, the large Zeeman, cyclotron and exchange energy scales of the InSb 2DEG favour to demonstrate the resistively detected NMR (RDNMR) signal at elevated temperature up to 6 K. Our results clearly show that the InSb 2DEG is a suitable candidate for implementation of the high temperature nuclear-spin based QIP. Ref: [1] H. W. Liu et al. Phys. Rev. B 82, 241304(R) (2010). [2] K. F. Yang et al., Appl. Phys. Lett. 98, 142109 (2011). [3] M. M. Uddin et al., Appl. Phys. Lett. 101, 233503 (2012). [4] M. M. Uddin et al., Appl. Phys. Lett. 103, 123502 (2013). Fig. : Filling factors (ν) dependence of nuclear spin relaxation time T 1 at different temperature. Top axis shows the filling factors are calculated by gate bias (V g) in bottom axis. Figure 1

The electron spin resonance study of heavily nitrogen doped 6H SiC crystals

The magnetic and electronic properties of heavily doped n-type 6H SiC samples with a nitrogen concentration of 1019 and 4 × 1019 cm−3 were studied with electron spin resonance (ESR) at 5–150 K. The observed ESR line with a Dysonian lineshape was attributed to the conduction electrons (CE). The CE ESR (CESR) line was fitted by Lorentzian (insulating phase) (T &amp;lt; 40 K) and by Dysonian lineshape (metallic phase) above 40 K, demonstrating that Mott insulator-metal (IM) transition takes place at ∼40 K, accompanied by significant change in the microwave conductivity. The temperature dependence of CESR linewidth follows the linear Korringa law below 40 K, caused by the coupling of the localized electrons (LE) and CE, and is described by the exponential law above 40 K related to the direct relaxation of the LE magnetic moments via excited levels driven by the exchange interaction of LE with CE. The g-factor of the CESR line (g‖ = 2.0047(3), g⊥ = 2.0034(3)) is governed by the coupling of the LE of nitrogen donors at hexagonal and quasi-cubic sites with the CE. The sharp drop in CESR line intensity (25–30 K) was explained by the formation of antiferromagnetic ordering in the spin system close to the IM transition. The second broad ESR line overlapped with CESR signal (5–25 K) was attributed to the exchange line caused by the hopping motion of electrons between occupied and non-occupied positions of the nitrogen donors. Two mechanisms of conduction, hopping and band conduction, were distinguished in the range of T = 10–25 K and T &amp;gt; 50 K, respectively.

Spin relaxation times of single-wall carbon nanotubes

We have measured temperature ($T$)- and power-dependent electron spin resonance in bulk single-wall carbon nanotubes to determine both the spin-lattice and the spin-spin relaxation times, ${T}_{1}$ and ${T}_{2}$. We observe that ${T}_{1}^{\ensuremath{-}1}$ increases linearly with $T$ from 4 K to 100 K, whereas ${T}_{2}^{\ensuremath{-}1}$ decreases by over a factor of two when $T$ is increased from 3 K to 300 K. We interpret the ${T}_{1}^{\ensuremath{-}1}\ensuremath{\propto}T$ trend as spin-lattice relaxation via interaction with conduction electrons (Korringa law) and the decreasing $T$ dependence of ${T}_{2}^{\ensuremath{-}1}$ as motional narrowing. By analyzing the latter, we find the spin hopping frequency to be 285 GHz. Last, we show that the Dysonian line shape asymmetry follows a three-dimensional variable-range hopping behavior from 3 K to 20 K; from this scaling relation, we extract a localization length of the hopping spins to be $\ensuremath{\sim}$100 nm.

Static and dynamic susceptibility in the putative ferromagnetic quantum critical system YbNi4P2 probed by 31P NMR

AbstractThe ferromagnetic (FM) quantum critical system YbNi4P2 has been investigated by means of 31P‐NMR. This is a newly discovered heavy fermion system which shows a FM transition at a very low TC = 170 mK. The present reported result is the extension of our previous work published in Ref. 11. 31P NMR results suggest the validity of the Korringa law in the presence of strong FM correlations. 31P NMR spectra have shown the anisotropic nature. The local magnetization derived from our NMR spectra is presented as a function of field at selected temperatures. The obtained spin‐lattice relaxation rate 31(1/T1T) follows a weak power‐law as T−0.75 in a wide field range down to T = 1.8 K. Possible scenarios for this unusual power‐law behavior are discussed. Additionally, 31(1/T1T) is nicely scaled with the observed (C/T)2 ∼ T−0.8 power law in the temperature range 1.8–10 K within the framework of renormalized Landau Fermi Liquid theory.

NMR Probe of Metallic States in Nanoscale Topological Insulators

A 125Te NMR study of bismuth telluride nanoparticles as a function of particle size revealed that the spin-lattice relaxation is enhanced below 33 nm, accompanied by a transition of NMR spectra from the single to the bimodal regime. The satellite peak features a negative Knight shift and higher relaxivity, consistent with core polarization from p-band carriers. Whereas nanocrystals follow a Korringa law in the range 140-420 K, micrometer particles do so only below 200 K. The results reveal increased metallicity of these nanoscale topological insulators in the limit of higher surface-to-volume ratios.

NMR studies of CDW transition in the spinel-type Cu(Ir1−xVx)2S4

The spinel CuV2S4 exhibits two-step anomalies at about 92 K (Tt1) and 56 K (Tt2), reflecting the CDW state. In order to investigate the CDW state on V rich side in Cu(Ir1−xVx)2S4 microscopically, NMR measurements of 51V for Cu(Ir1−xVx)2S4 (x = 1.00, 0.98 and 0.95) have been carried out between 4.2 and 300 K. The line width of 51V for x = 1.00 is very narrow above Tt1 and become wider with decreasing x. For x = 1.00 and 0.98, the line width is suddenly broadened at Tt1 due to an electrical quadruple interaction between 51V nucleus and CDW charge modulation, but for x = 0.95, the line width is continuously increased around Tt1. The Knight shifts of 51V for x = 1.00, 0.98 and 0.95 show almost the same negative shift due to the influence of d-electrons above Tt1. At Tt1, the values of the Knight shift for x = 1.00 and 0.98 suddenly change to positive values, but for x = 0.95 continuously shifts to a positive one. The 1/T1T's of 51V for x = 1.00, 0.98 and 0.95 obey the Korringa law above Tt1. Below Tt1, the 1/T1T values of 51V for x = 1.00, 0.98 and 0.95 take constant values, which shows the Korringa law resulting from Pauli paramagnetic state.

Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs

We present nuclear spin relaxation measurements in GaAs epilayers using a new pump-probe technique in all-electrical, lateral spin-valve devices. The measured T(1) times agree very well with NMR data available for T>1 K. However, the nuclear spin relaxation rate clearly deviates from the well-established Korringa law expected in metallic samples and follows a sublinear temperature dependence T(1)(-1) is proportional to T(0.6) for 0.1 K≤T≤10 K. Further, we investigate nuclear spin inhomogeneities.

Spin–Lattice Relaxation in Bismuth Chalcogenides

Bismuth chalcogenides Bi2Se3 and Bi2Te3 are semiconductors that can be both thermoelectric materials (TEs) and topological insulators (TIs). Lattice defects arising from vacancies, impurities, or dopants in these materials are important in that they provide the charge carriers in TE applications and compromise the performance of these materials as TIs. We present the first solid-state nuclear magnetic resonance (NMR) study of the 77Se and 125Te NMR resonances in polycrystalline powders of Bi2Se3 and Bi2Te3, respectively. The spin–lattice (T1) relaxation is modeled by at most two exponentials. Within the framework of this model, the NMR measurement is sensitive to the distribution of native defects within these materials. One component corresponds to a stoichiometric fraction, an insulator with a very long T1, whereas the other component is attributed to a sample fraction with high defect content with a short T1 resulting from interaction with the conduction carriers. The absence of a very long T1 in the bismuth telluride suggests defects throughout the sample. For the bismuth selenide, defect regions segregate into domains. We also find a substantial difference in the short T1 component for 125Te nuclei (76 ms) and 77Se (0.63 s) despite the fact that these materials have nearly identical lattice structures and chemical and physical properties. Investigations of the NMR shift and Korringa law indicate that the coupling to the conduction band electrons at the chalcogenide sites is much stronger in the telluride. The results are consistent with a stronger spin–orbit coupling (SOC) to the p-band electrons in the telluride. If most parameters of a given material are kept equal, then this type of experiment could provide a useful probe of SOC in engineered TI materials.

31P NMR investigations on the ferromagnetic quantum critical system YbNi4P2

We studied the new heavy-fermion system YbNi$_{4}$P$_{2}$, which presents strong ferromagnetic correlations, using the local $^{31}$P NMR probe over a wide field (0.2-8.6 \textsf{T}) and temperature (1.8-200 K) range. $^{31}$P NMR Knight shift provides the static spin susceptibility which tracks the bulk susceptibility whereas the spin-lattice relaxation rate $^{31}(1/T_{1}$) provide information about the fluctuations of the Yb 4f moment. The Korringa law is valid over a wide range in temperature and field. The Korringa product $^{31}(1/T_{1}TK^{2}$S$_{0}$) $\ll$ 1 gives evidence for the presence of strong ferromagnetic correlations. Over two decades in temperature a $^{31}(1/T_{1}T) \sim T^{-3/4}$ behaviour was found.

27Al-NQR/NMR Study of Kondo Semiconductor CeFe2Al10

27 Al-NQR/NMR measurements have been performed on CeFe 2 Al 10 in relevance to the novel phase below T 0 =27 K of isostructural CeRu 2 Al 10 . NQR peaks are assigned to five crystallographically inequivalent Al sites. No splitting of the NQR spectra down to 1.5 K confirms the lack of phase transition in this compound. The gaplike decrease in the spin–lattice relaxation rate 1/ T 1 above about 20 K, following the Korringa law ( T 1 T = const.) below 20 K, indicates a pseudogap opening near the Fermi level. The gap magnitude of 70 K with a mid-gap state of 42% is estimated based on a rectangular density of states. The gap magnitude is much larger than the gap of 15 K evaluated previously from the electrical resistivity, which suggests CeFe 2 Al 10 to be a Kondo semiconductor with a Kondo temperature much higher than 300 K.

Low-temperature properties ofCeRu4Sn6from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state

The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $\Delta E_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient $\gamma = C(T)/T$ : In zero field, $\gamma$ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at $\gamma = 0.6$ J/molK$^2$. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap $\Delta E_g2$. A simple model, based on two narrow quasiparticle bands located at the Fermi level - which cross the Fermi level in zero field at 0.022 states/meV f.u. - can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting our data of both specific heat and NMR to the model, incorporating a Ce magnetic moment of $\mu = \Delta E_g1/\mu_{0H} \simeq 1 \mu_B$, leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced $\gamma$ in CeRu$_4$Sn$_6$ and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of system

Magnetic-field-induced reentrance of Fermi-liquid behavior and spin-lattice relaxation rates in [formula omitted

A strong departure from Landau–Fermi liquid (LFL) behavior have been recently revealed in observed anomalies in both the magnetic susceptibility χ and the muon and 63Cu nuclear spin-lattice relaxation rates 1 / T 1 of YbCu 5 − x Au x ( x = 0.6 ). We show that the above anomalies along with magnetic-field-induced reentrance of LFL properties are indeed determined by the dependence of the quasiparticle effective mass M ∗ on magnetic field B and temperature T and demonstrate that violations of the Korringa law also come from M ∗ ( B , T ) dependence. We obtain this dependence theoretically utilizing our approach based on fermion condensation quantum phase transition (FCQPT) notion. Our theoretical analysis of experimental data on the base of FCQPT approach permits not only to explain the above two experimental facts in a unified manner, but to unveil their universal properties, relating the peculiar features of both longitudinal magnetoresistance and specific heat in YbRh 2Si 2 to the behavior of spin-lattice relaxation rates.

Probing theYb3+spin relaxation inY0.98Yb0.02Ba2Cu3Oxby electron paramagnetic resonance

The relaxation of ${\text{Yb}}^{3+}$ in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ $(6&lt;x&lt;7)$ was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the ${\text{Yb}}^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy $(\ensuremath{\sim}500\text{ }\text{K})$ optical phonons or an Orbach-type process via the excited vibronic levels of the ${\text{Cu}}^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x=6.98$, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below ${T}_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of ${\text{Yb}}^{3+}$ relaxation in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.98}$. It was shown that the electronic part of the ${\text{Yb}}^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}\text{C}\text{u}$ and $^{17}\text{O}$ nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

Cluster dynamical mean-field study of the Hubbard model on a 3D frustrated hyperkagome lattice

We study the Hubbard model on a geometrically-frustrated hyperkagome lattice by a cluster extension of the dynamical mean field theory. We calculate the temperature (T) dependences of the specific heat (C) and the spin-lattice relaxation time (T1) in correlated metallic region. C/T shows a peak at T = Tp1 and rapidly decreases as T → 0. On the other hand, 1/T1T has a peak at a higher temperature Tp2 than Tp1, and largely decreases below Tp2, followed by the Korringa law 1/T1 ∝ T as T → 0. Both peak temperatures are suppressed and the peaks become sharper as electron correlation is increased. These behaviors originate from strong renormalization of the energy scales in the peculiar electronic structure in this frustrated system; a pseudo-gap like feature, the van-Hove singularity, and the flat band. The results are discussed in comparison with the experimental data in the hyperkagome material, Na4Ir3O8.