New Limits on Variation of the Fine-Structure Constant Using Atomic Dysprosium

We report on the spectroscopy of radio-frequency transitions between nearly degenerate, opposite-parity excited states in atomic dysprosium (Dy). Theoretical calculations predict that these states are very sensitive to variation of the fine-structure constant alpha owing to large relativistic corrections of opposite sign for the opposite-parity levels. The near degeneracy reduces the relative precision necessary to place constraints on variation of alpha, competitive with results obtained from the best atomic clocks in the world. Additionally, the existence of several abundant isotopes of Dy allows isotopic comparisons that suppress common-mode systematic errors. The frequencies of the 754-MHz transition in Dy-164 and 235-MHz transition in Dy-162 are measured over the span of two years. The linear variation of alpha is (alpha) over dot/alpha = (-5.8 +/- 6.9([1 sigma])) x 10(-17) yr(-1), consistent with zero. The same data are used to constrain the dimensionless parameter k(alpha) characterizing a possible coupling of alpha to a changing gravitational potential. We find that k(alpha) = (-5.5 +/- 5.2([1 sigma])) x 10(-7), essentially consistent with zero and the best constraint to date.