Tunnel Diode Oscillator Technique Research Articles

Superconducting properties and gap structure of the topological superconductor candidate Ti3Sb

We present a study of the superconducting properties of the candidate topological superconductor Ti_(3)Sb. Electrical transport measurements show zero resistance with a T_(c,onset) of ~ 5.9 K with a transition width {\Delta}T_c~ 0.6 K. The superconducting phase boundaries as derived from magneto-transport and magnetic susceptibility measurements agree well. We estimate an upper critical field Bc2(0)~4.5 T. A Ginzburg-Landau (GL) analysis yields values of the coherence length and penetration depth of {\zeta} = 6.2 nm and {\lambda} = 340 nm, respectively, and a GL parameter ~ 55, indicating extreme type-II behavior. Furthermore, we observed a step height in the specific heat ({\Delta}C_e)/({\gamma}T_c )~1.61, a value larger than the Bardeen-Cooper-Schrieffer (BCS) value of 1.43, suggesting modest coupling. Measurements of the temperature dependence of the London penetration depth via the tunnel-diode oscillator (TDO) technique down to ~ 450 mK show a full superconducting gap, consistent with a conventional s-wave gap structure.

Discovery of quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2 under extreme conditions of field and pressure

The 2-dimensional layered oxide material SrCu2(BO3)2, long studied as a realization of the Shastry-Sutherland spin topology, exhibits a range of intriguing physics as a function of both hydrostatic pressure and magnetic field, with a still debated intermediate plaquette phase appearing at approximately 20 kbar and a possible deconfined critical point at higher pressure. Here, we employ a tunnel diode oscillator (TDO) technique to probe the behavior in the combined extreme conditions of high pressure, high magnetic field, and low temperature. We reveal an extensive phase space consisting of multiple magnetic analogs of the elusive supersolid phase and a magnetization plateau. In particular, a 10 × 2 supersolid and a 1/5 plateau, identified by infinite Projected Entangled Pair States (iPEPS) calculations, are found to rely on the presence of both magnetic and non-magnetic particles in the sea of dimer singlets. These states are best understood as descendants of the full-plaquette phase, the leading candidate for the intermediate phase of SrCu2(BO3)2.

Proximity enhanced magnetism at NiFe2O4/Graphene interface

Here, we explore the change in effective magnetic anisotropy of the ferrimagnetic (FM) insulator nickel ferrite (NFO) thin film due to the inclusion of monolayer graphene (MLG) grown on top of the NFO layer. This was done by performing radio frequency (RF) transverse susceptibility (TS) measurements on bare NFO and NFO/MLG bilayer samples for both in-plane (IP) and out-of-plane (OOP) configurations utilizing a tunnel diode oscillator technique. Our magnetometry measurements indicated an enhancement in the overall saturation magnetization of the NFO/MLG bilayer with respect to the bare NFO film. The TS measurements reveal that the inclusion of MLG reduces the effective magnetic anisotropy for both IP and OOP configurations drastically, by up to a factor of 2 over the temperature range 40 K ≤ T ≤ 280 K. Since NFO is a magnetic substrate, it is possible that NFO could induce magnetic ordering in MLG at the NFO/MLG interface via the magnetic proximity effect. Furthermore, since NFO is insulating and MLG is a semimetal, there likely exists a large conductivity difference at the interface, making charge transfer plausible. These two effects could modify the interfacial magnetism leading to a change in the effective magnetic anisotropy. These results highlight the importance of understanding the interfacial magnetism of FM/MLG heterostructures.

Nontrivial Fermi surface topology of the kagome superconductor CsV3Sb5 probed by de Haas–van Alphen oscillations

We have investigated the normal state Fermi-surface properties of the kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ using torque magnetometry with applied fields ($H$) up to 35 T. The torque signal shows clear de Haas--van Alphen (dHvA) oscillations above 15 T. The oscillations are smooth and consist of seven distinct frequencies with values from $\ensuremath{\sim}$ 18 T to 2135 T. The presence of higher frequencies in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ is further confirmed by carrying out additional measurements using the tunnel diode oscillator technique. All frequencies measured at different tilt angles ($\ensuremath{\theta}$) of the field direction with respect to the c axis show a $1/\mathrm{cos}\ensuremath{\theta}$ dependence, implying that the Fermi surfaces corresponding to these frequencies are two dimensional (2D). The absence of dHvA oscillations at $\ensuremath{\theta}$ = ${90}^{o}$ further supports the presence of 2D Fermi surfaces. The Berry phase ($\ensuremath{\phi}$) determined from the Landau level fan diagram for all frequencies is $\ensuremath{\sim}$ 0.4. This value is close to the theoretical value of $\ensuremath{\phi}$ = 0.5 for a nontrivial system, which strongly supports the nontrivial topology of the Fermi surfaces of these frequencies. Several quantities characterizing the Fermi surface are calculated employing the Lifshitz-Kosevich theory. These findings are crucial for exploring the interplay between nontrivial band topology, charge-density wave, and unconventional superconductivity of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.

Campbell penetration depth in low carrier density superconductor YPtBi

Magnetic penetration depth, $\lambda_{m}$, was measured as a function of temperature and magnetic field in single crystals of low carrier density superconductor YPtBi by using a tunnel-diode oscillator technique. Measurements in zero DC magnetic field yield London penetration depth, $\lambda_{L}\left(T\right)$, but in the applied field the signal includes the Campbell penetration depth, $\lambda_{C}\left(T\right)$, which is the characteristic length of the attenuation of small excitation field, $H_{ac}$, into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent $\lambda_C$ exhibit $\lambda_{C}\sim B^{p}$ with $p=1/2$ in most of the conventional and unconventional superconductors, we found that $p\approx 0.23\ll1/2$ in YPtBi due to rapid suppression of the pinning strength. From the measured $\lambda_{C}(T,H)$, the critical current density is $j_{c}\approx40\,\mathrm{A}/\mathrm{cm^{2}}$ at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable $T_{c}$. Since the pinning centers (lattice defects) and vortex structure are not expected to be much different in YPtBi, this observation is direct evidence of the low density of the Cooper pairs because $j_{c}\propto n_s$.

Local characterization of a heavy-fermion superconductor via sub-Kelvin magnetic force microscopy

Using magnetic force microscopy operating at sub-Kelvin temperatures, we characterize the heavy fermion superconductor CeCoIn5. We pinpoint the absolute London penetration depth of λ(0)=435 ± 20 nm and report its temperature dependence, which is closely linked to the symmetry of the superconducting gap. In addition, we directly measure the pinning force of individual Abrikosov vortices and estimate the critical current density of jc=9×104 A/cm2. In contrast to the related, well-established tunnel diode oscillator technique, our method is capable of resolving inhomogeneities locally on the micrometer scale at ultra-low temperature.

Penetration depth study of the type-I superconductor PdTe2

Superconductivity in the topological non-trivial Dirac semimetal PdTe2 was recently shown to be type-I. We hereby report measurements of the relative magnetic penetration depth, , on several single crystals using a high precision tunnel diode oscillator technique. The temperature variation follows an exponential function for , consistent with a fully-gapped superconducting state and weak or moderately coupling superconductivity. By fitting the data we extract a -value of ∼500 nm. The normalized superfluid density is in good agreement with the computed curve for a type-I superconductor with nonlocal electrodynamics. Small steps are observed in , which possibly relates to a locally lower due to defects in the single crystalline sample.

Magnetic and thermodynamic properties of CuxTiSe2 single crystals

We present a detailed study of the phase diagram of copper intercalated TiSe$_2$ single crystals, combining local Hall-probe magnetometry, tunnel diode oscillator technique (TDO), specific-heat, and angle-resolved photoemission spectroscopy measurements. A series of the Cu$_x$TiSe$_2$ samples from three different sources with various copper content $x$ and superconducting critical temperatures $T_c$ have been investigated. We first show that the vortex penetration mechanism is dominated by geometrical barriers enabling a precise determination of the lower critical field, $H_{c1}$. We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both $H_{c1}$ and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength $2\Delta_s \sim [2.4-2.8]k_BT_c$, regardless of the copper content, in puzzling contradiction with specific heat measurements which can be well described by one single large gap $2\Delta_l \sim [3.7-3.9]k_BT_c$. Finally, our measurements reveal a non-trivial doping dependence of the condensation energy, which remains to be understood.

Evidence of nodes in the order parameter of the superconducting doped topological insulator NbxBi2Se3 via penetration depth measurements

The low-temperature variation of the London penetration depth $\ensuremath{\lambda}(T)$ in the candidate topological superconductor ${\mathrm{Nb}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3} (x=0.25)$ is reported for several crystals. The measurements were carried out by means of a tunnel-diode oscillator technique in both field orientations (${H}_{\mathrm{rf}}\ensuremath{\parallel}c$ and ${H}_{\mathrm{rf}}\ensuremath{\parallel}ab$ planes). All samples exhibited power-law behavior at low temperatures ($\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}\ensuremath{\sim}{T}^{2}$) clearly indicating the presence of point nodes in the superconducting order parameter. The results presented here are consistent with a nematic odd-parity spin-triplet ${E}_{u}$ pairing state in ${\mathrm{Nb}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.

Field-induced magnetic transitions inCa10(Pt3As8)((Fe1−xPtx)2As2)5compounds

We report a high magnetic field study up to 55 T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$) ((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ (x=0.078(6)) with a Tc 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22 T, when the magnetic field is applied perpendicular to the ab planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition not of long-range ordered moments, but of nematic-like antiferromagnetic fluctuations.

Possible nodal superconducting gap in Fe1+y(Te1−xSex) single crystals from ultralow temperature penetration depth measurements

Using a radio frequency tunnel diode oscillator technique, we measured the temperature dependence of the in-plane London penetration depth $\Delta\lambda_{ab}(T)$ in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ single crystals, down to temperatures as low as 50 mK. A significant number of samples, with nominal Se concentration $x$=0.36, 0.40, 0.43 and 0.45 respectively, were studied and in many cases we found that $\Delta\lambda_{ab}(T)$ shows an upturn below 0.7 K, indicative of a paramagnetic type contribution. After subtracting the magnetic background, the low temperature behavior of penetration depth is best described by a power law with exponent $n\approx2$ and with no systematic dependence on the Se concentration. Most importantly, in the limit of T$\rightarrow$0, in some samples we observed a narrow region of linear temperature dependence of penetration depth, suggestive of nodes in the superconducting gap of Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$.

Evidence for fully gapped strong coupling s-wave superconductivity in Bi4O4S3

We report on the superconducting gap and pairing symmetry in the layered superconductor Bi4O4S3. The measurement of temperature dependence of magnetic penetration depth was carried out using the tunnel diode oscillator technique. It is observed that Bi4O4S3 is a conventional s-wave type superconductor with a fully developed gap. The zero-temperature value of the superconducting energy gap Δ0 was found to be 1.54 meV, corresponding to the ratio 2Δ0/kBTc = 7.2 which is much higher than the BCS value of 3.53. In the superconducting range, superfluid density is very well described by the single gap s-wave model.

Penetration depth study of LaOs4Sb12: Multibands-wave superconductivity

We measured the magnetic penetration depth $\lambda(T)$ in single crystals of LaOs$_{4}$Sb$_{12}$ ($T_c$=0.74 K) down to 85 mK using a tunnel diode oscillator technique. The observed low-temperature exponential dependence indicates a s-wave gap. Fitting the low temperature data to BCS s-wave expression gives the zero temperature gap value $\Delta (0)= (1.34 \pm 0.07) k_B T_c$ which is significantly smaller than the BCS value of 1.76$k_B T_c$. In addition, the normalized superfluid density $\rho(T)$ shows an unusually long suppression near $T_c$, and are best fit by a two-band s-wave model.

Fabrication and magnetic response probed by RF transverse susceptibility in La 0.67Ca 0.33MnO 3 nanowires

The temperature and magnetic field dependence of the radio-frequency (RF) transverse susceptibility ( χ T ) of La 0.67Ca 0.33MnO 3 crystalline nanowires has been studied using a very sensitive self-resonant tunnel-diode oscillator (TDO) technique. The nanowires were synthesized using porous templates of anodized alumina by chemical solution deposition technique, and the crystalline nature of the nanowires with the average diameter of 70 nm was confirmed by TEM, SAED, and HREM. RF transverse susceptibility experiments reveal the presence of a double-peak structure at T≤245 K (the Curie temperature) but a single peak at T>245 K. This distinguishes the low temperature ferromagnetic state from the high temperature paramagnetic state. The effective magnetic anisotropy field ( H K ), which corresponds to the peak location of χ T , has been found to increase with decrease in temperature from the Curie temperature.

Magnetic switching and magnetic transitions in ErCo2 probed by radio-frequency transverse susceptibility

ErCo2 represents a typical example of magnetism of itinerant electron systems and metamagnetic processes and has been the subject of extensive research. We present here the first study of radio-frequency transverse susceptibility (TS) of bulk ErCo2 using a self-resonant tunnel-diode oscillator technique. TS measurements reveal the collective magnetic switching of the Er moments at temperatures below the ferrimagnetic transition temperature, Tc∼32 K, and the existence of Co nanoclusters with short-range correlations at Tc<T<Tf (Tf denoted as the flipping temperature). The difference in the magnetic configuration between the ferrimagnetic, parimagnetic, and paramagnetic states, as well as the change from the paramagnetic to parimagnetic regime upon varying dc magnetic fields are also probed by TS experiments. These findings are discussed in the context of our previous investigations using other different techniques which provide further insights into the magnetism and the so-called parimagnetism phenomenon in ErCo2.

Destruction of chain superconductivity inYBa2Cu4O8in a weak magnetic field

We report measurements of the temperature-dependent components of the magnetic penetration depth $\ensuremath{\lambda}(T)$ in single-crystal samples of ${\text{YBa}}_{2}{\text{Cu}}_{4}{\text{O}}_{8}$ using a radio-frequency tunnel diode oscillator technique. We observe a downturn in $\ensuremath{\lambda}(T)$ at low temperatures for currents flowing along the $b$ and $c$ axes but not along the $a$ axis. The downturn in ${\ensuremath{\lambda}}_{b}$ is suppressed by a small dc field of $\ensuremath{\sim}0.25\text{ }\text{T}$. This and the zero field anisotropy of $\ensuremath{\lambda}(T)$ likely result from proximity induced superconducting on the CuO chains. However we also discuss the possibility that a significant part of the anisotropy might originate from the ${\text{CuO}}_{2}$ planes.

Radio-frequency spectroscopy of the low-energy spectrum of the magnetic moleculeCr12Cu2

We present tunnel diode oscillator (TDO) measurements of dynamic magnetic susceptibility, inelastic neutron-scattering (INS) measurements, and theoretical predictions based on quantum Monte Carlo (QMC) calculations for a magnetic molecule system, ${\text{Cr}}_{12}{\text{Cu}}_{2}$. The TDO measurements show not only ground-state level crossings (as can also be observed in low-temperature dc magnetization measurements) but also clear evidence of crossings between certain excited energy levels. These TDO results are in excellent agreement with our theoretical predictions for a Heisenberg Hamiltonian and are further confirmed by our INS measurements. Our present findings demonstrate that the TDO technique is a valuable magnetic spectroscopic tool for studying magnetic molecules, and that the QMC method is a valuable tool for predicting properties of computationally demanding systems such as ${\text{Cr}}_{12}{\text{Cu}}_{2}$.

Upper critical field in the organic superconductorβ″−(ET)2SF5CH2CF2SO3: Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state

We report upper critical-field measurements in the metal-free-all-organic superconductor ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{ET})}_{2}{\text{SF}}_{5}{\text{CH}}_{2}{\text{CF}}_{2}{\text{SO}}_{3}$ obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low-temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semiempirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.

S-wave superconductivity probed by measuring magnetic penetration depth and lower critical field ofMgCNi3single crystals

The magnetic penetration depth $\lambda$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $\delta\lambda$(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $\lambda(0)=230 $nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease of the critical temperature at the sample surface.

Quantum Oscillations in the Underdoped CuprateYBa2Cu4O8

We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85 T. There is a clear signal, periodic in inverse field, with frequency 660+/-15 T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m(*)=3.0+/-0.3m(e). In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.