Possible Proximity to Ferromagnetism in the V$_2$Ga$_5$ Superconductor

TL;DR

This work presents a concerted experimental and theoretical investigation of V$_2$Ga$_5$, a bulk type-II superconductor with a quasi-one-dimensional V chain structure, to assess its proximity to ferromagnetism. Magnetic susceptibility, M(H) loops, resistivity, specific heat, and STM measurements reveal weak ferromagnetic correlations emerging around $T\sim 10$ K that do not establish long-range order due to superconductivity, reinforced by DFT results showing a Fermi-level DOS peak and a small, intrinsic magnetic moment. The resistivity upturn and field-enhanced specific heat at low temperatures are interpreted as signatures of spin fluctuations near a ferromagnetic instability, consistent with a Stoner-type tendency. Overall, V$_2$Ga$_5$ emerges as a rare, clean d-electron system where superconductivity coexists with proximity to itinerant ferromagnetism, offering a platform to explore quantum critical behavior and the symmetry of the superconducting order parameter under ferromagnetic fluctuations.

Abstract

Superconductivity and ferromagnetism are generally competing ground states in $d$-electron systems, making their interplay of fundamental interest. We report a comprehensive study of high-quality single- and polycrystalline V$_2$Ga$_5$, a bulk type-II superconductor ($T_c = 3.54 \ K$) with a quasi-one-dimensional crystal structure, supplemented with density functional theory (DFT) calculations, suggesting possible proximity to ferromagnetic order. Below $T \approx 10 \ K$, magnetic susceptibility shows ZFC/FC splitting, along with saturation and hysteresis in $M(H)$. Moreover, electrical transport measurements reveal a magnetic-field-dependent resistivity upturn, while specific heat is enhanced in magnetic fields. DFT calculations show that the Fermi level in V$_2$Ga$_5$ is located at a peak in the density of states, with a small magnetic moment per unit cell comparable to the experimental value. Together, these results indicate the possibility that ferromagnetic correlations develop below $T \approx 10 \ K$, well above $T_c$, with long-range ferromagnetic order suppressed by the superconducting transition.

Figures (8)

• Figure 1: Powder X-ray diffraction pattern of crushed V2G5 single crystals. Black circles represent measured data, the red line is the Rietveld refinement fit, green ticks indicate calculated Bragg peak positions, and the blue line is the difference curve. The inset illustrates the P4/mbm (s.g. no 127) crystal structure model used in the refinement, with Ga and V atoms represented by green and red balls, respectively.
• Figure 2: (a) Zero-field-cooled (black) and field-cooled (red) magnetic susceptibility of V2Ga5 single crystal, corrected for the demagnetization factor, measured in field H = 10 Oe parallel to the c-axis. T$_c$ onset and T$_c$ offset were calculated using the 5%/95% criterion, and T$_c$ was determined from the maximal-slope criterion (green). (b) Temperature dependence of the lower critical field H$_{c1}$ determined for H $\parallel$ c, along a linear fit to the data (red). In the inset, M$_V$(H) curves for several temperatures are presented.
• Figure 3: (a) $M(H)$ loop in the $\pm 9$ T field range measured at 2 K, showing clear high-field saturation and quantum oscillations. (b) Expanded view of the low-field $M(H)$ curve, revealing hysteresis that persists for $H>H_{c2}$. The bottom dataset is denser, as it is both the virgin curve (red squares) and the curve measured after the full $\pm 9$ T loop. (c) Low-field $M(H)$ highlighting the diamagnetism present at 2 K, which vanishes at 5 K, with hysteresis in the superconducting state. (d) Temperature dependence of magnetic susceptibility $\chi(T)$ for V2Ga5 single crystals, measured under field-cooled (FC, red) and zero-field-cooled (ZFC, black) conditions in the field $\mu_{0}H=0.1T$. Clear ZFC/FC splitting is observed above $T_c$; inset shows the superconducting transition. All measurements were performed for $H \parallel c$.
• Figure 4: Temperature dependence of the resistivity for a V2Ga5 single crystal in the 1.9 - 300 K temperature range, showing metallic behavior with a high residual resistivity ratio $RRR = 32$. Inset (a) presents the temperature dependence of the upper critical field H$_{c2}$, determined from the midpoint of the resistive transition under applied magnetic field. The extrapolated zero-temperature value is $\mu_{0}H_{c2}(0) = 0.643(5)$ T. Inset (b) shows normalized resistivity $\rho/\rho_{0}$ as a function of temperature under various magnetic fields, illustrating the systematic suppression of the superconducting transition with increasing field. The transition remains sharp, with minimal field-induced broadening up to $\mu_0$H = 0.4 T.
• Figure 5: (a) Resistivity of V2Ga5 batch #1, crystal #2 (see text for details) at 0 T, 0.5 T, and 9 T, showing an upturn below $T \approx 10$ K, enhanced by a small field of 0.5 T and suppressed by the 9 T field. The 0.5 T and 9 T datasets are shifted downwards to correct for the positive magnetoresistance. (b) Resistivity of the same crystal, with a $\mathrm{\rho(T) = \rho_0 + AT^5}$ fit performed in the 10 - 15 K temperature range presented in red.